Coalbed Biogenic Methane: Insights on the “Blind Spots” in Mitigation of Emissions

Abstract

Biogenic or microbial methane (CH₄) emissions, believed to be the main driver of the recent surge in global atmospheric CH₄ emissions, have altered monitoring, measurement, and mitigation of fossil-fuel emissions. As of 1981, over 20% of the world’s natural gas reserves were biogenic in origin. Additional biogenic CH₄ reserves from coal have been discovered since 1981 mixed (40–80%) with thermogenic CH₄. Biogenic CH₄ accumulates up to 100% in coal reservoirs in the Powder River Basin (PRB), USA. Biogenic CH₄ is generated by microbial breakdown of fossil organic matter as an early-stage (primary) type during burial over geologic time and is rarely preserved. Also, biogenic CH₄ is generated as a late-stage (secondary) type from recent geologic to present times and is commonly preserved. Late-stage biogenic CH₄ is sustained by nutrients and microbes in meteoric/surface waters discharged into coal aquifers. Groundwater is pumped from wells in coal aquifers to desorb and produce CH₄ and dewater coal mines. The co-produced water with dissolved CH₄ is discharged into diverse surface aquatic systems. The emission factors (EFs) of co-produced water are 2.0522 × 10⁻⁹ Gg CH₄/gal of water in the PRB and 2.0694 × 10⁻³ Gg CH₄/well in the Black Warrior Basin, U.S. Accurate data on biogenic CH₄ emissions from coal sources is a major gap in the accounting of current global groundwater-driven CH₄ whose average flux is estimated to be 3.9 ± 6.2 mmol/m²/day or accounting for up to 70% of CH₄ emissions from surface aquatic systems. Biogenic CH₄ emissions from coal mining and coalbed gas extractions and related infrastructures are overlooked because the focus has been on coalmine methane (CMM) emissions. CMM data from ground-based measurements is highly variable and used by the Intergovernmental Panel on Climate Change three-tier system to estimate EFs for national inventories. However, 90% of CMM emissions are attributable to a small group of the most coal-consuming-and-producing countries but fails to capture other coal sources worldwide. This created gaps and “blind spots” in “unstructured” low-concentration, diffused biogenic CH₄ emission data. These key “blind spots” include sources from flooded, abandoned coal mines; coalbed methane (CBM) co-produced water with dissolved CH₄ and infrastructures/facilities; and groundwater drawdown from water withdrawals during coal mining and CBM extraction. Also, a critical “blind spot” is the mixing of biogenic CH₄ emissions from subsurface coals with biogenic CH₄ generated at the surface from wetlands, agriculture, and landfills/wastes, which grew 85% from 2008 to 2020. Limited understanding of the mixing of biogenic CH₄ from diverse sources and their contributions to global methane requires accurate attribution of overlapping isotopic signatures (δ¹³C_CH4 and δD). This paper addresses knowledge gaps in coalbed biogenic CH₄ emissions by a systematic review of the literature and specific study cases, which provided insights on key “blind spots” in their mitigation.

1. Introduction

Methane (CH₄), a major contributor to global greenhouse gas (GHG) emissions, was elevated to prominence at the 2021 UN Climate Change Conference (COP26) in Glasgow after many decades of being overshadowed by carbon dioxide (CO₂) [1]. Although CH₄ was recognized in the 1997 Kyoto Protocol as a major GHG under the United Nations Framework Convention on Climate Change (UNFCCC) [2], its short-lived yet potent atmospheric impact was overshadowed by carbon dioxide (CO₂). CO₂ is less potent but has a longer-term warming effect on climate. The climate policy shift toward CH₄ was driven by the European Union and the United States to achieve a 30% reduction in GHG emissions by 2030 and net-zero emissions by 2050, as called for in the 2015 Paris Agreement. About 150 countries committed to the Global Methane Pledge launched at the COP26 to reduce CH₄ emissions and submit national CH₄-emission inventories to the UNFCCC.

A driver of the shift in the importance of atmospheric CH₄ is the record-breaking increase since 2007, presumably due to the onset of natural gas development from shale formation in the United States (U.S.) [3,4,5]. However, CH₄ emissions from natural gas have declined from 8% to 2% over the past 3 decades [6]. Instead, the rise in atmospheric CH₄ from 2007 to 2014 was attributed to isotopic shifts (δ¹³C_CH4) with increasing emissions of biogenic or microbial CH₄ depleted in ¹³C [7]. Furthermore, this observation was supported by a record-high atmospheric CH₄ growth rate during 2020–2022, driven by increased emissions from wetlands, waste, and agriculture [8]. This study [8], focusing on the rapid shift in CH₄ carbon isotopes from 2008 to 2022, is complicated, as shown by box modeling that balances the mass of microbial CH₄ emissions, adjusted for increased fossil-fuel CH₄ emissions, with decreased hydroxyl (OH) radical or oxidation reactions. The model shows incremental increases in microbial CH₄ emissions of 14 Tg yr⁻¹ in 2008, concurrent with a 10 Tg yr⁻¹ increase in fossil-fuel CH₄ emissions, followed in 2014 by an additional 22 Tg yr⁻¹ in microbial CH₄ emissions and a simultaneous 3 Tg yr⁻¹ increase in fossil-fuel CH₄ emissions [8]. This observation is succeeded by an increase in microbial CH₄ emissions of 32 Tg yr⁻¹ in 2020, with no increase in fossil-fuel CH₄ emissions. So about 85% of the atmospheric CH₄ growth from 2008 to 2020 was driven by increased microbial CH₄ emissions [8].

Biogenic or microbial CH₄ is generated by microorganisms through methanogenesis by anaerobic decomposition of organic matter in peats found in most latitudinal wetlands largely (85%) located in the pantropical regions (tropics and subtropics) where annual average CH₄ fluxes from 2001 to 2020 are the highest (>200 g m⁻² yr⁻¹) (Figure 1) [9,10]. This study [10] indicates the primary mechanism for biogenic CH₄ transmission from the wetland peats to the atmosphere is plant-mediated transport (>70%), e.g., through plant internal air spaces (aerenchyma), followed by molecular diffusion (passive movement of gas molecules from high- to low-concentration areas), and ebullition (e.g., gas bubbles from tropical peats). Methane emissions are controlled by seasonality, especially in the temperate and boreal regions while in the tropics, CH₄ emissions tend to stabilize (~11 Tg mon⁻¹) [9]. New research on atmospheric CH₄ emissions found high methanogenic activity in the boreal-arctic wetlands across North America, Europe, and Asia, giving rise to biogenic CH₄ emissions [9,11]. Global warming-induced increases in groundwater temperature, permafrost thawing, and shifting precipitation accelerate microbial conversion of carbon to CH₄. These processes trigger a positive feedback loop with increased GHG emissions driving further atmospheric warming and perpetuating the degradation of frozen-stored carbon. That is, continuous microbial breakdown of carbon into CH₄ creates self-enforcing cycle such as warming, which leads to more thawing and drives more CH₄ generation that causes more warming! Atmospheric CH₄ levels increased 2.6 times above pre-Industrial Revolution levels [12].

Biogenic CH₄ emissions also arise from human activities (anthropogenic), including agriculture (cultivation of organic-rich soils), cattle digestion (enteric fermentation), waste decomposition in landfills and wastewater treatment plants, biomass burning, and the storage of livestock manure [13,14,15,16]. These dispersed anthropogenic biogenic CH₄ emissions are mixed with wetland biogenic CH₄, creating uncertainty in the attribution of CH₄ fluxes and confounding bottom-up (small-scale, local sites) and top-down (large-scale, regional sites) measurements. Also, biogenic CH₄ emissions are highly variable in wetlands because not all accumulate peat in bogs/mires/swamps [17]. About 65% of wetlands are peatlands or peat-forming environments and accumulate peat deposits [1,10,17]. Moreover, wetlands and peatlands are influenced by human activities (e.g., agriculture), and increased emissions from these natural systems are in part anthropogenic [1]. The complexity and variability of wetlands are exemplified along the Amazon River, where floodplain “shallow peats” (swamps) are influenced by water flow and release one-third as much CH₄ as the adjoining “deep peats” (bogs/mires) [1,17,18]. The spatial and temporal variability of biogenic CH₄ generation in the Amazonian peats reflect the difficulty of mitigating wetland gas emissions.

In 1981, over 20% of the discovered natural gas reserves worldwide were reported as biogenic gas, and 80% as thermogenic or mixed (thermogenic/biogenic) gases [19,20]. Since the 1980s, new biogenic gas reserves in coal and shale beds have been discovered and production has increased worldwide [1]. This newly discovered biogenic gas, often mixed with thermogenic gas, is known as coalbed methane (CBM), coalseam gas (CSG), coalmine methane (CMM), and abandoned mine methane (AMM). Biogenic gas typically consists of 95–99% pure CH₄ [21]. So, for all intents and purposes, the biogenic gas component in coal is predominantly biogenic CH₄. In contrast to thermogenic gas, biogenic gas contains minor amounts of CO₂ and nitrogen, and trace amounts of hydrocarbon gases (e.g., ethane, propane, butane, and pentane), which are low to rare, respectively [22]. Because biogenic gas is often mixed with thermogenic gas, certain coal basins in China (e.g., Ordos Basin) have average proportions of biogenic CH₄ ranging from 62% to 74% between coal beds [23]. In another part of the Ordos Basin, the same coal beds contain biogenic CH₄ at 48–50% [24]. In contrast, all 10 coal beds in the Powder River Basin, USA, produced as much as 100% biogenic CH₄ (CO₂ reduction) not mixed with thermogenic gas from >29,000 wells [1,17,25,26,27]. Other studies suggest production in specific shale gas is up to 30% biogenic CH₄ [28,29]. The variable occurrence of biogenic CH₄ between coal beds and coal basins exemplifies the complexity of technically mitigating biogenic CH₄ emissions worldwide.

About 90% of global CMM emissions are attributable to 7 major coal producing and consuming countries [Figure 2] [30]. China accounts for about 50% of the total global CMM emissions. Active underground coal mine operations are major sources of CMM, accounting for 70% of global emissions, while surface mines account for the remainder [31]. These estimates of global CMM emissions do not include AMM emissions from abandoned coal mines. Traditionally, the primary mitigation technologies for CH₄ emissions have been the capture of CMM by pre-drainage boreholes and ventilation air methane (VAM) systems [32,33]. These technologies are effective because they target 60–70% of underground coal mine emissions [30,31]. The technology includes boreholes directionally drilled within coal beds (in-seam) to pre-drain high-concentration CMM prior to mining, which is then captured, collected, and used for coal mine electricity generation, heating, ventilation, coal drying, and fueling boilers [33]. The VAM technology creates a dilute mixture of CH₄ (<1%) and air, which is exhausted from underground coal mines. The VAM is mitigated by thermal oxidizers, which burn the low-concentration CH₄ into carbon dioxide and water vapor, reducing its warming impact by >95% [31]. This study [31] suggests VAM is the single most important mitigation measure, which would reduce CMM by 30% on a global level. Satellite trackers such as GHGSat have uncovered small (>100 kg/hr) CH₄ emissions while Sentinel-5P detect larger (5 t/hr) CH₄ emissions from coal mines [31]. But these CH₄ emissions originate from active coal-mine point sources compared to lower-concentration and diffused CH₄ emissions from abandoned coal mines.

Globally, the International Energy Agency [31] estimates that the abatement potential of CMM emissions from steam coal (subbituminous/bituminous rank) and lignite (low rank) is about 50%, while that of coking coal (high rank) is about 60%. Abatement potentials for CMM emissions vary with mining methods, with about 70% in underground mines and 20% in surface mines. Abatement potentials of CMM emissions also vary between countries, exemplified by China producing mainly high rank coals (anthracite, bituminous) in underground mines while Indonesia producing mainly low rank coals (subbituminous) in surface mines [31]. These coal variables reveal different abatement strategies for CMM emissions in China and Indonesia. China’s underground coal mine abatement strategy for CH₄ emissions is the traditional technical capture of CMM utilizing pre-drainage boreholes and VAM. In Indonesia, the abatement strategy for CMM emissions from surface coal mines is direct release into the atmosphere, as the emissions are diffuse and at low concentrations, making capture technically challenging. Here, economic barriers, lack of regulation, and low CH₄ concentrations relative to those in underground coal mines make mitigation expensive and unfeasible, unlike high-concentration pre-drainage boreholes in China [31].

The discovery of biogenic CH₄ in coal beds mined in China and Indonesia from 2007–2014 [34,35,36,37,38] has complicated mitigation of CH₄ emissions because these countries are among the small group of major CMM-emitting countries (Figure 2). In Indonesia, extraction of subbituminous coals from surface mines adjacent to wetlands (e.g., Kalimantan, Borneo) allows the mixing of anthropogenic and natural biogenic CH₄ [1,17]. Here, [31] it is estimated that about 25% of the Indonesian surface-mine CMM could be mitigated using pre-mine drainage borehole technology drilled from the surface. This mitigation technology has been used for decades in Australia, Canada, Russia, and the United States, where biogenic CH₄ in mineable coal beds was discovered about 4 decades ago [1,17]. Most of these coal-producing countries account for about 85% of the total global CMM emissions (see Figure 1). As in Indonesia, many surface coal mines in these countries are on farmlands and ranchlands, resulting in similar mixing of anthropogenic biogenic CH₄ from diverse surficial sources, which is a “blind spot” in biogenic CH₄ emissions accounting.

The finding of biogenic CH₄ in coal beds worldwide presented a complex mix of economic opportunities, environmental risks, and mitigation challenges, but it also revealed a major “blind spot” in emissions mitigation. One critical “blind spot” is the discovery of biogenic CH₄ in flooded, abandoned coal mines in the Ruhr Basin [39,40]. The presence of living methanogens in the coal-mine water and recent generation of biogenic CH₄ based on isotopic composition analysis indicate the coals are active “bioreactors” in the Ruhr flooded, abandoned coal mines. For four decades, technology for pre-draining and capturing CMM in underground coal mines has been used to extract CBM/CSG biogenic CH₄ from deep unmineable coal beds beyond the coal mines within the same coal basin [1,17]. This intrabasin coal and gas development is made possible by the dual-function (dual-role) coal beds, where CBM/CSG and solid fuel are extracted simultaneously in coal basins worldwide [1,17]. The regional stratigraphic connectivity of dual-role coal beds has enlarged and complicated the abatement potential for biogenic CH₄ emissions basinwide. These regionalized CH₄ emissions have transformed traditional local point-source emissions into unintended “unstructured,” widespread, fugitive CH₄ emissions. That is, localized diffuse CH₄ emissions from gas wells, pipelines, and co-produced water discharged points in the basin center, and coal mine pits and facilities in the basin margin, create a highly ineffective abatement strategy. Also, simultaneous basinwide development of CMM and CBM/CSG initiate environmental risks by the encroachment of coal operations into areas with other sources (e.g., ranchlands, agricultural lands, landfills, wetlands) of biogenic CH₄. This permits the mixing and overlap of biogenic CH₄ emissions from shallow to deep coal sources with surficial, natural, and anthropogenic sources, complicating scientific, technical, and economic abatement strategies.

This study systematically reviews and synthesizes the current literature and research to identify knowledge gaps regarding shallow- and deep-sourced coalbed biogenic CH₄ emissions relative to surficial-sourced biogenic CH₄ emissions from wetlands, agriculture, and landfills. Also, the study provides synthesis and insights on key “blind spots” in biogenic CH₄ emissions from groundwater CH₄ co-produced during heavy dewatering of coal aquifers by coal mining and coalbed gas extractions. Thus, this paper explores the role of groundwater drawdown in terms of long-lasting CH₄ fluxes after extraction ceases. Moreover, this article provides feedback on legacy biogenic CH₄ emissions from flooded abandoned coal mines that are omitted from national inventory estimates. Finally, the impacts of “unstructured” coalbed biogenic CH₄ emissions on mitigation is assessed by understanding sources, measurements, uncertainties, and generations in ensuing sections.

2. Understanding Sources, Measurements, and Uncertainties of Biogenic CH₄ Emissions

Understanding the differences between atmospheric biogenic CH₄ emissions from natural (wetlands) and anthropogenic (agriculture, waste, biomass) sources, and from mainly coal and natural gas (Energy) sources is crucial for obtaining accurate ground-based measurements for estimations of national inventories (Figure 3) [30]. Knowledge of the similarities and differences among these diverse sources of biogenic CH₄ can inform more effective mitigation strategies. This often results in mixing of biogenic CH₄ sourced from human activities, natural processes, coal, and natural gas.

Biogenic CH₄ is unique as a sustainable, low-temperature energy source produced by the microbial decomposition of organic matter in oxygen-deprived environments (e.g., wetlands, landfills, digestive systems). Its diverse sources stem from its varied origins (acetoclastic vs. hydrogenotrophic metabolic pathways of methanogenesis), diverse environments (freshwater/brackish wetlands, landfills, farmlands, digestive system, marine sediments), and distinct formation types (early-stage or late-stage generation) [1,17,20,22,23,24,25]. Acetoclastic (fermentation) methanogenesis involves the breakdown of acetate (CH₃COOH) into CH₄ and carbon dioxide (CO₂) by methanogens. Hydrogenotrophic (CO₂ reduction) methanogenesis reduces CO₂ using hydrogen (H₂) to generate CH₄. The biogenic methanogenesis fractionation is shown in

Table 1. Methanogenesis fractionation of biogenic methane (CH₄) according to metabolic pathways, substrates, common microbes, key environments, intermediates, and depths observed in coal worldwide. Depths * of occurrence of biogenic coalbed gas are generally reported to occur from >100 to >1000 m. Acetoclastic and hydrogenotrophic methanogenesis generally occur in shallow and deep coal beds controlled by geology and hydrology, but it varies between coal basins worldwide. For example, in the Powder River Basin, USA, hydrogenotrophic (CO₂ reduction) occurs in stratigraphically correlated Wyodak-Anderson coal zone from 59 to 488 m (from basin margin to center). H, hydrogen; C, carbon; CO₂, carbon dioxide; pH, acidity. Compiled from Flores and Moore [1]; Rice and Claypool [20]; Rice [22]; Bao et al. [23]; Flores et al. [25], Rice et al. [26]; Tao et al. [34]; Tang et al. [35]; Guo et al. [36]; Susilawati et al. [37]; Guo et al. [38]; Beckmann et al. [40]; Wormald et al. [41]; Gründger et al. [42]; Bao et al. [43]; Cheung et al. [44].

Parameters	Acetoclastic Methanogenesis	Hydrogenotrophic Methanogenesis
Metabolic Pathways	Acetate (CH3COOH)	(H2) and (CO2)
Substrates	Mainly low rank (lignite and subbituminous) coals. Coal matrix dominated by macropores.	Low rank (subbituminous) and high rank (bituminous/anthracite) coals. Coal matrix dominated by micropores.
Isotope Fractionation	Utilize lighter isotopes (12C, 1H) during CH4 production resulting in isotopically light CH4	Produce CH4 significantly depleted in 13C relative to the CO2 source
Common Microbes	Methanosarcina, Methanosaeta	Methanomicobiales, Methanocalculus, Methanobacterium, Methanothermobacter
Key Environments	Often dominant in neutral pH (6.5–7.5), nutrient-rich systems	Often dominant in high acidic and alkaline, or low-acetate conditions
Intermediates	Direct biodegradation	Syntrophic (H2) transfer
Depths *	Shallow (varies between basins)	Deep (varies between basins)
Temperatures (Mitigation Strategies)	Low temperatures (20–45 °C) enhance metabolic pathways	High temperatures (35–55 °C) enhance metabolic pathways

[1,20,22,23,25,26,34,35,36,37,38,39,40,41,42,43,44]. Biogenic CH₄ originated in low-temperature (from <50 °C to 100 °C) environments, while thermogenic CH₄, commonly formed in coal and oil/gas, is generated under high pressure and temperature (from >157 °C to 221 °C) [42,43]. Biogenic and thermogenic CH₄ in coal are distinguished by their stable carbon (δ¹³C) and hydrogen (δD or δ²H) compositions. Biogenic CH₄ is characterized by very light δ¹³C values and typically ranges from −90‰ to −55‰. Hydrogen isotopes are typically between −400‰ and −150‰. In contrast, thermogenic CH₄ is characterized by heavier δ¹³C values and typically ranges from −55‰ to −35‰. Hydrogen isotopes are typically >−200‰ [42,43].

Uncertainty arises from misattribution of the sources of biogenic CH₄ due to the dynamic interchange of CH₄ fluxes between various sources. Natural processes in wetlands are the largest natural source, accounting for 35% of atmospheric CH₄ emissions [13]. This recent study indicates that 65% of top-down estimates of 2020 CH₄ emissions are attributable to human activities (anthropogenic). The largest anthropogenic sources of CH₄ emissions are agriculture, fossil fuels, and landfill decomposition. Bioaugmentation and bio- stimulation of coal to re-generate CH₄ for renewable energy and accompanying emissions, can be classified as anthropogenic in origin [1,17]. Human-induced microbial methanogenesis in depleted coal reservoirs by injecting co-produced water, microbes, and nutrients or amendments to stimulate and accelerate metabolic activities in the subsurface ecosystem is an emerging and innovative technology. Supercharging microbial activity in coal reservoirs has been successful in bench- and field-scale applications regenerating new CBM/CGS for economic and sustainable energy extraction [1,17].

Thus, understanding the sources of biogenic CH₄ emissions is essential as they are a major driver of recent increases in atmospheric CH₄, especially following the discovery of new biogenic gas reserves in coal and shale. Accurately identifying and measuring these sources, especially from all types of coal operations, is critical for effective mitigation targets.

2.1. Understanding Sources of Biogenic CH₄ Emissions

Understanding CH₄ sources is crucial because it helps identify who (humans/nature) and what (energy, agriculture, peatlands/wetlands, landfills) is emitting this potent GHG (see Figure 3). Also, knowledge enables targeted emission reduction strategies, monitoring of climate impacts (such as ozone formation), and assessment of mitigation potential, especially because CH₄ is potent but short-lived. This means that possible mitigation/reduction measures offer quick benefits in reducing climate warming. Also, pinpointing sources (e.g., super-emitters or super-leaks in fossil-fuel operations versus seasonal wetland releases) enables effective deployment of technology and addresses both climate change policies and local air quality. The biogenic CH₄ from both sources is critical for attributing atmospheric emissions because the energy-rich gas is derived from fossilized organic matter with isotopic signatures distinct from those of recent wetland organic matter. Key reasons for understanding sources include targeted mitigation, policy and accountability, climate-modeling accuracy, detection of super-emitters, air quality and health, understanding natural cycles, and rapid reductions in warming.

Different sources require different solutions, such as identifying whether it’s cows, landfills, oil/gas, or coal mine leaks, to direct mitigation efforts (e.g., capturing/using gas from landfills or coal mines, reducing fossil-fuel flaring). Differentiating between human-caused (anthropogenic) and natural sources is essential for setting realistic climate goals and policies. Also, this increases the accuracy of national inventories for reporting to the UNFCCC. Accurate source data improves climate models that predict future warming trajectories, and assessing methane’s role as the second-most important GHG after CO₂. Identifying sudden, large CH₄ releases (super-emitters) from oil/gas fields, coal mines, or landfills allows for rapid intervention and mitigation. Methane contributes to ground-level ozone, so understanding its sources helps protect public health and reduce pollution [45]. Identifying natural sources (e.g., peatlands, wetlands, termites), helps distinguish them from human-caused increases, and helps clarify climate feedback loops, such as warming peatlands/wetlands releasing more CH₄ emissions. Because CH₄ has a short atmospheric life (around 12 years) [45], reducing its emissions yields faster climate benefits than reducing long-lived gases like CO₂, making source identification critical for near-term warming control.

Methane from fossil fuel operations (e.g., oil/gas wells, processing plants, pipeline infrastructure, coal mining) accounts for about 40% of human-caused CH₄ emissions [30]. Coal mining operations account for approximately 38% of CH₄ emissions worldwide compared with 33% for natural gas and 29% for oil operations. For coal mining operations, mitigation is more challenging than in oil and gas due to lower concentrations and the dispersed, varied nature of facilities (e.g., coal mine highwalls, conveyor belts or transportation trucks, silos, coal trains, preparation/processing plants, etc.) from which CH₄ emissions originate. Abandoned coal mines include underground shaft mines, drift mines, and surface mines, as well as quarries, and associated silting or tailing ponds where CH₄ is emitted at variable concentrations. Surface coal mine spoil piles serve as reservoirs of microbial communities to degrade lignin in coal fragments in the spoils [46,47,48]. These mining waste acting as “bioreactors” [39,40,49,50] support microbial communities that produce CH₄, or the methanogenesis process that emits high biogenic CH₄ concentrations. Methane concentrations range from 33 to 95 vol% with higher concentrations at coal mines closed decades ago [48]. Coal mine silting ponds or tailings/settling ponds are crucial containment areas where water mixed with fine coal waste (tailings) and wastewater are held, allowing with metals and minerals to settle out. Minerals such as pyrite (FeS₂), gypsum (CaSO₄·2H₂O), siderite (FeCO₃), apatite (Ca₅(PO₄)₃F) etc., contain elements that can support and nourish microbial metabolic activity. The tailings ponds or mine sludge also serve as “bioreactors” [50,51,52,53,54] in which the vast majority of coaly material and metals/minerals are submerged, creating an anoxic environment necessary for anaerobic microorganisms, including methanogens, to thrive and generate biogenic CH₄ that is emitted [50,54]. So, flooded abandoned mines and tailings ponds as “bioreactors” can serve as long-term CH₄ emitters, potentially acting as super-emitters for a considerable time, especially if not properly managed or mitigated. The metabolic pathway that dominates in an abandoned mine or coal tailing pond is heavily influenced by substrate availability, pH and alkalinity, syntrophic acetate oxidation, and microbial community structure (see Table 1). Metabolic pathways of flooded abandoned coal mines, exemplified in the Ruhr Basin, include both acetoclastic (fermentation) and hydrogenotrophic (CO₂ reduction) methanogenesis [39,40].

Other potential CH₄ super-emitters, often ignored as a “blind spot”, include abandoned CBM wells and associated co-produced water impoundments and aquatic environments. Unlike working conventional natural gas wells, abandoned coalbed gas wells are closely spaced (8 wells/mi² or 2.589 km²) and have a 10–15-year lifespan [17]. These abandoned CBM wells can continue to leak biogenic CH₄ for decades, or even for “eternity,” if not properly plugged. For example, ten abandoned or orphaned shut-in coalbed gas wells in the Powder River Basin emitted biogenic gas ranging from 1.6 to 4530 mg CH₄/hr (mean emissions), with an average of 653 mg CH₄/hr as of 12–16 December 2020 [55]. This study [55] disregarded these coalbed gas wells as potential real-time “bioreactors” that produced biogenic gas for 17 years from the most productive Big George coal aquifer in the basin [25]. This article [55] interprets that conditions in these abandoned wells are such that continued groundwater flow in the wellbores connected to meteoric/surface-water recharge stopped gas desorption and emission. This recharge process introduces timing (temporal) and distance (spatial) uncertainties of biogenic CH emissions. Also, groundwater recharge replenishes nutrients and microbial communities, thereby facilitating methanogenesis and the generation of new CH₄ that is released into the atmosphere [17]. This cyclical generation and emission of new biogenic CH₄ in abandoned CBM wells could take place for decades. The existence of these potentially long-lasting, high-CH₄-emitting abandoned coalbed gas wells, along with related co-produced groundwater impoundments and aquatic environments underscores the importance of properly mitigating their significant GHG impacts. No proper methods are currently being used to mitigate this co-produced groundwater containing dissolved CH₄.

2.2. Measurement and Tracking Approaches of Methane Emissions

Approaches for measuring CH₄ emissions include various spatial scales, both local sources (small-scale, bottom-up) and atmospheric platforms (large-scale, top-down) [56]. Combining the strengths of both approaches presumably yields accurate and reliable national emission inventories that inform effective mitigation strategies. The top-down approach applies inverse modeling methods to infer atmospheric CH₄ emissions using prior data from the bottom-up approach, providing improved national inventories [57]. Foremost are satellite observations like Greenhouse Gases Observing Satellite (GOSAT), Tropospheric Monitoring Instrument (TROPOMI), and PRecursore IperSpettrale della Missione Applicativa (PRISMA) that detect and quantify CH₄ emissions from local sources, particularly from use of coal, oil, and natural gas [57,58,59]. The International Energy Agency (IEA) Global Methane Tracker (GMT) uses the latest satellite data and aircraft observations to track CH₄ emissions, focusing on fossil fuels, bioenergy, and non-energy sectors [60]. The GMT raises awareness of mitigation models and informs policymakers of CH₄-emission reduction, particularly from fossil fuels and coals’ biogenic CH₄.

The most critical measurement by satellite sensors is distinguishing diffused emissions of biogenic CH₄ from coal, oil/gas, wetland, landfill and farmland sources. Modern satellite sensors can distinguish biogenic CH₄ emissions (e.g., landfills and wetlands) from thermogenic CH₄ sources (e.g., coal mines, oil/gas facilities) by using high-resolution, pixel-level mapping to identify “super-emitters” [61,62,63]. Satellite sensors (e.g., TROPOMI, GHGSat, NASA EMIT) can distinguish biogenic CH₄ emissions (e.g., wetlands vs. livestock) from anthropogenic sources (e.g., coal mine, oil/gas, and groundwater CH₄ surface aquatic systems). Analysis of unique spectral fingerprints in the shortwave infrared range allows the instruments to identify specific absorption patterns of CH₄ in reflected sunlight [62]. Using advanced imaging spectrometers, satellites identify specific emission signatures, allowing them to differentiate compounds, map hotspots, and estimate emission rates, as seen with GHGSat’s and PRISMA’s high-resolution technology [59,63]. However, current satellite sensors cannot directly measure the isotopic signatures (e.g., δ¹³C) of methane to distinguish between sources. But sensors can separate biogenic CH₄ from wetland and coal emissions by combining high-resolution spatial mapping, regional atmospheric modeling, and known spatial-temporal locations of these sources [64,65].

Airborne measurements and visualization of CH₄ emissions come from aircraft equipped with specialized sensors (e.g., lidar, spectrometers) that fly through discrete plumes from diverse sources [66]. Quantification employs methods such as the mass-balance approach to estimate CH₄ releases from sources including oil/gas fields, landfills, and wetlands. This approach provides higher-resolution data, often revealing much higher emissions and offering more precise measurements than reported inventories. The mass-balance method measures the difference in CH₄ concentration upwind and downwind of a source area (e.g., surface coal mine) to calculate total emissions, often using Gaussian dispersion models. Planes carry sophisticated instruments, including thermal infrared (TIR) imagers, spectrometers (AVIRIS), and lidar systems (like Bridger Photonics), to detect and map CH₄ plumes. Aircraft fly repeated patterns, often in a grid or around a source, measuring CH₄ concentrations and wind speed/direction to calculate the rate of emission (flux) [56,66]. The challenge for an airborne platform is that its reliability depends on calm wind conditions, as convective conditions can lift plumes out of the measurement layer, potentially causing an underestimation of emissions [56,66].

The GMT provides country-level historical data and inventories of CH₄ emissions from fossil-fuel facilities, which account for about one-third of the global CH₄ emissions [60]. However, most national inventories of CH₄ emissions are underreported to the UNFCCC relative to satellite observations, which are about 80% higher than the totals reported by countries (Figure 4) [66]. For example, studies reveal inconsistent trends of significant gaps and uncertainties between bottom-up and top-down estimates of CH₄ emissions from the oil and gas sectors in the U.S. [67,68,69]. These studies show that bottom-up estimates of CH₄ emissions from oil and gas basins are significantly lower than top-down estimates in the Permian Basin, Texas. Satellite estimates from May 2018 to March 2019 recorded the largest CH₄ emissions in the U.S., twice as high as bottom-up estimates in the basin [69,70,71]. The discrepancy is probably from intentional massive venting and flaring point sources due to inadequate gas processing and transportation infrastructures.

The quantification algorithms applied to bottom-up estimates of CH₄ emissions require input from a solitary point source of CH₄ plumes [72]. Overlapping CH₄ plumes from multiple point sources worsen quantification errors when plumes are not separated or isolated. Separation splits and isolates overlapping CH₄ plumes into single-source points, enabling the application of high-precision quantification models to achieve fine spatial resolution in plume images [72].

This bottom-up approach is exemplified by monitoring (2020 and 2023) coal mine ventilation shaft point sources for CMM emissions using top-down PRISMA hyperspectral satellite data for quantification (Figure 5) [59]. The study shows that top-down estimates of mean CH₄ fluxes are generally lower than bottom-up estimates, which is attributed to difficulties in quantification due to dense vegetation and the area’s terrain. Despite discrepancies in mean emissions, robust statistical measures (e.g., the interquartile range) that are less affected by outliers are useful for interpreting data variability, spread, and skewed distributions [59]. However, the lack of isotopic signatures in CH₄ emissions from the studied mine, although presumed to be thermogenic, introduces uncertainty about the local source of the bottom-up data. Bottom-up data collection may be required to create national inventories of gas origin in coal mines, especially in regions where coal is reported to host biogenic gas.

To create CH₄ emissions inventories at national, regional, and local levels, an estimation method was developed to provide a numerical basis for setting and tracking emission mitigation goals. The value that represents the quantity of a CH₄ emissions released to the atmosphere by a specific activity from a source (e.g., coal mine) is expressed as an emission factor [73]. The emission factor is typically expressed as the weight of the CH₄ emissions per unit weight, volume, distance, or duration of the activity. This formula provides a standardized way to quantify and estimate emissions from various activities or sources. Source-specific testing to confirm the accuracy of emissions data and periodic retesting are required to confirm the emission factor [73]. Emission factors are used as a basis for developing reduction and control programs, appropriate mitigation strategies, and national or global emission reports submitted to the UNFCCC. They are also used for accounting, managing, understanding the impact, and meeting regulatory requirements related to CH₄ emissions. Most of all, the standardized approach enables governments and global organizations to set emissions limits and mitigation targets.

The IPCC tier system is a hierarchy of methodological complexity, where the lower tier generally yield higher uncertainty due to reliance on default data, while higher tiers aim to reduce uncertainty through country-specific measurements and detailed local site or ground-based data. The source uncertainties in the higher tiers, driven by the complexity of coal geology and properties, are explained in the following section.

2.3. Sources of Uncertainties and Tier Systems of Emission Factors

Sources of uncertainty in CH₄ emissions include the wide variability of their sources, limited knowledge of their origin, analytical and measurement limitations, and technical differences in collecting bottom-up data among countries. More specifically, significant differences in CH₄ emission factors for a given source, such as a coal mine and/or coalbed methane well in a coal basin, coalfield, or coal beds, lead to large discrepancies and uncertainties among reporting countries. For example, CMM emissions from underground or surface coal mines are estimated using bottom-up approaches that rely on measurements from safety sensors or control devices. But these measurements involve calculating CH₄ emissions using emission factors derived from parameters, such as the gas content of the coal beds, coal bed depths, and coal production volumes in coal mines [74]. However, the gas content of coal is controlled by other properties and variables (e.g., porosity/permeability, maceral composition, moisture, ash content, rank) to determine CMM emissions. For instance, the gas content of high-rank coals (e.g., anthracite) is usually higher than that of low-rank coals (e.g., lignite) (

Table 2. Coal properties, variables, and uncertainty drivers. The uncertainties are significantly affected by the limited data on the coal properties, geology and hydraulic parameters. These uncertainties require robust modeling techniques simulation to quantify risk, estimate methane emissions, and optimize mitigation strategies. CH₄, methane; m, meter; mD, millidarcy. Compiled from Flores [17]; Flores and Moore [1].

Coal Property, Geology, and Hydrology	Variable	Level of Uncertainty	Key Parameters Influencing Uncertainty
Depth	Coal burial	High	Deep coals (>1000 m) have reduced porosity/permeability and unpredictable gas content. Critical depths (700–800 m) can trigger changes where gas content decreases.
Thickness	Coal bed	Medium	Thin or discontinuous coal beds are harder to characterize stratigraphically; net-to-gross ratio uncertainty on gas volume.
Maturation	Coal rank	Medium	High rank coals (e.g., anthracite, bituminous) usually means higher CH4 content due to greater thermal maturity and more developed pores. Lower rank coals (e.g., lignite, subbituminous) typically have a lower CH4 content tied to coalification.
Deformation	Coal structure	High	Broken/mylonitic coal limits permeability and creates high-uncertainty zones.
Gas Content	Volume	High	Measurement errors (lost gas) and spatial variations in saturation. Driven by coal burial depth, thickness, quality, rank, maceral composition, and pores.
Quality	Mineral matter (Ash/moisture)	Medium	High ash/moisture reduces sorption capacity, reducing gas content. Mineral matter (ash) content reduces gas content by filling pores and fractures.
Composition	Organic matter (Macerals)	Medium	Vitrinite macerals (woody plant tissue) in higher rank coals generally adsorb more methane due to abundance of micropores. Inertinite macerals (oxidized plant matter) with visible cellular structures form macropores. Liptinite macerals (spores/pollen) low porosity.
Permeability	Cleats/Fractures	High	Exponential decrease with depth; high variability (0.1–100+ mD).
Porosity	Size (Micro-, meso- and macro-pores)	High	Porosity is variable by coal rank (generally increase in low-rank lignite/subbituminous coals; decrease in medium-rank bituminous coals; and increase in high-rank anthracite coal). Macropore size in low rank coals and mesopore/micropore size in high-rank coals.
Geology	Coal stratigraphy, sedimentology, and tectonism.	High	Coal stratigraphy, sedimentology, and depositional environments determine lateral, vertical, and accumulation patterns of coal beds within a coalfield/coal basin. These variables control continuity, thickness, and volume of coals within these areas, which in turn, influence gas volume. Uncertainty in these geological variables arises from limited data and natural heterogeneity of coal beds leading to significant risks in local gas estimates and extrapolation of regional models. Most importantly, geology controls all the coal properties.
Hydrology	Coal hydrostratigraphy (aquifers/aquitards) and hydraulic properties.	High	Coal aquifers’ ability to store/transmit groundwater and generate/accumulate biogenic gas is based on hydraulic properties, which vary by several orders of magnitude based on coal geology/properties. Uncertainty in hydrogeological variables like groundwater level fluctuations (drawdown), movement/flow, porosity/permeability, and water composition in coal aquifers are crucial to microbial pathways and methanogenesis of biogenic CH4. These parameters are highly variable in space and time leading to major uncertainties in modeling biogenic CH4 emissions from coal aquifers. The above parameters are controlled by the coal geology and properties.

).

The interrelationships of the coal properties and variables driving uncertainty levels are exemplified by burial depths of coal beds, which control the formation of coal porosity (pores/voids and cleats/fractures) and coal permeability (connectivity of pores and cleats/fractures) developed in coal matrix/macerals (organic matter) [1,17]. CH₄ molecules are adsorbed on the surface of coal pores/fractures and mainly held by physical adsorption (Van der Waals forces), which determines the gas content and volume of the coal. In many coal aquifers where biogenic CH₄ accumulates, the gas is adsorbed (physical adsorption), and the hydrostatic pressure of the formation water (groundwater) holds the gas in place within the coal pores and cleats/fractures. Also, coal pores are semi-closed, which, along with low permeability or pore connectivity, creates a high-porosity/low-permeability systems that traps biogenic CH₄, often hindering its desorption or release. Thus, poor coal pore connectivity makes it difficult for the gas to desorb, migrate, or diffuse, prolonging gas residence in the coal reservoir and mitigation. Addressing unmitigated emissions from the residual CH₄ in coal reservoirs requires the application of enhanced coalbed methane (ECBM) recovery technique using CO₂ injection into the coal to displace/replace the CH₄ molecules for sequestration) [1,17]. The success or failure of ECBM sequestration depends not only depends on coal porosity/permeability but also relies on coal shrink/swell properties. Coal shrinkage and swelling are critical sorption-induced phenomena in which the coal matrix changes volume as it adsorbs (takes in) or desorbs (releases) CH₄ and CO₂ molecules) [1,17]. Moisture content of wet coals is less influenced by swelling than dry coals, making them more suitable for CO₂ sequestration. Thus, these specific coal properties and variables must be accurately analyzed and included in the database at the coal mine sites for national inventories and reporting CH₄ emissions. The uncertainty levels of CMM emissions mainly from underground and surface mines must be determined from the coal bed properties and mitigation practices.

Ground-based data collection often overlooks the highly localized nuances between coal properties (coal rank, gas content, and bed thickness) and their specific extraction sites (mines or basins) [74]. This knowledge gap, which results in use of generic or default values, is exemplified in the Ordos Basin (OB) in China and Bowen Basin (BB), Australia. The bituminous and anthracite coals in OB/BB contain more gas (6–20 m^3/t; 3–16 m³/t, respectively) than the subbituminous coals (ave. 1–2 m³/t) in the Powder River Basin (PRB) in the U.S. [1,17]. But the thicker (<68 m) coal beds at 50–600 m deep in the PRB compensate for the gas content of the higher rank coal beds in OB (1–20 m thick; 100–>2000 deep) and BB (<45 m; 100–900 m deep) [1,17]. The level and influence of uncertainty (Table 2) are judge by coal depth in which porosity/permeability is reduced at >1000 m and gas content is unpredictable. Coal gas content could be reduced at 700–800 m deep. These ground-based data should serve to calibrate the IPCC tier-system framework.

Existing IPCC guidelines on CH₄ emissions accounting methods include three-tiered systems for estimating and reporting GHG emissions [75]. Each tier, from Tier 1 to Tier 3, requires progressively more data and produces more accurate results [73,74,75]. Countries choose the most appropriate reporting levels for emission factors, categorized into low, medium, and high ranges of uncertainty, to estimate, for example, CMM emissions for a given coal region, basin, or mine [75]. The highest uncertainty is given to the Tier 1 emission factor, which provides a rough estimate and is less accurate due to limited data availability. Reported uncertainty is reduced to medium in Tier 2, which is more accurate and supported by more data than Tier 1 and is designed for country-specific refinement to better reflect national, regional, coal basin, or coal mine conditions. The lowest level of uncertainty is Tier 3 assessment, which uses the most accurate and precise estimates from measured data, such as from specific coal mines for national inventories and reporting. The progression from Tier 1 to Tier 3 reflects decreasing uncertainty in CH₄ emissions estimates, with greater data granularity and increased precision of the CH₄ emissions factor for a given source type (e.g., surface, underground, or abandoned mine). Data accuracy is important because it determines the absolute CH₄ emissions for coal mine, coalfield, coal basin, and country region considered. The variability of uncertainty, especially at the Tier 3 level, creates inconsistencies between economically developed and underdeveloped countries. Moreover, this difference is exacerbated between technologically developed and less developed countries.

The top-down approach uses atmospheric CH₄ concentration data at scales ranging from local to broad geographic areas to assess total emissions from aggregated multiple sources [56]. A bottom-up approach to CH₄ emissions from point sources (local areas) extrapolates findings to a larger scale or broader (conterminous) area. For example, measuring the ratio of stable carbon isotopes in atmospheric CH₄ enables models to estimate the proportions of thermogenic gas emissions from fossil fuel sources and biogenic CH₄ emissions from wetlands, agriculture, and waste/landfill sources. But the discovery of large biogenic CH₄ or natural gas reserves, including coalbed gas and CMM, requires better isotopic coverage to conduct bottom-up estimates. Microbial-driven CH₄ emissions from coal exhibit spatiotemporal variability due to the interplay of geological, tectonic, hydrological, and groundwater factors, as well as microbiological factors [1,17]. That is, CH₄ is generated in coal beds by anaerobic microorganisms, and the rate and amount of gas release change over time, differ from one location to another, and vary with depth and coal characteristics. Also, the mining of coal beds at various depths and the development of coalbed gas, which require the massive withdrawal of groundwater containing CH₄ from coal aquifers, present local and regional uncertainties of CH₄ emissions due to seasonal and climatic conditions [1,17]. These observations require specialized and sophisticated analytical methods, the availability of which varies between countries; thus, creating relative uncertainty in reporting systems.

Understanding the sources, measurements, and uncertainties of biogenic CH₄ is crucial for identifying its generation and accumulation in coal. These factors facilitate the identification and distinction of the biogenic CH₄ sources elaborated in the next section.

3. Coalbed Biogenic CH₄ Stages of Generation and Accumulation

Simply put, biogenic CH₄ forms from the anaerobic microbial decomposition of organic matter in peat, often in organic-rich sediments/sedimentary rocks, as well as in coal beds [20,22]. Gas generation in these organic-rich rocks involves CO₂ reduction (hydrogenotrophic) by hydrogen (H₂) produced by anaerobic microorganisms during the breakdown of organic matter [22]. After oxygen (O₂) is consumed, sulfate (SO₄) reduction becomes a dominant microbial activity. Once SO₄ is depleted, CH₄ generation becomes the dominant process via hydrogenotrophic methanogenesis, in which methanogens (methanogenic archaea) use H₂ to reduce CO₂ to CH₄ [25]. This process requires low concentrations of sulfate to avoid thermodynamic competition with sulfate-reducing bacteria (SRB).

Early-stage (primary) and late-stage (secondary) biogenic CH₄ in coal beds represent two distinct generations of microbial CH₄, differing mainly in the timing of generation, geological/tectonic settings, and the source of microbial activity [1,20,22]. Early-stage (primary) biogenic CH₄ forms during the early sedimentation, deposition, and burial history of low-rank coals (peats to subbituminous rank; vitrinite reflectance or R_o values <0.5%) [20,22]. It’s generation and preservation are promoted by rapid deposition with enclosed coastal and fluvial sediments, influenced by high rates of basin subsidence, and permitting rapid burial. Late-stage (secondary) biogenic CH₄ is generated during recent geologic time (tens of thousands to a few million years ago) and is associated with active groundwater systems [20,22]. Coal beds are commonly aquifers in which late-stage biogenic CH₄ generation can occur at any rank. The uplift and erosion of ancient coal-bearing rocks facilitated influx of meteoric water bringing new microbes and nutrients into the degraded organic matter of the coal beds. This requires the coal-bearing rocks found along basin margins or in areas where uplift has allowed freshwater to flush through old, previously compacted and lithified sedimentary rocks [1].

3.1. Stages of Biogenic CH₄ Generation

Biogenic CH₄ in coal beds can be generated during the early stage of burial history from peat to brown coal or lignite but continued intensification of heat and pressure during coalification nullifies its retention [20,22]. However, the notion of non-retention of early-stage biogenic CH₄ during early coalification was disproved by the discovery of deeply buried microbial communities including hydrogenotrophic methanogens, at subsea depths (2466 m below the seafloor or b.s.f.) in Miocene lignites at the IODP C0020 drill site off the Shimokita Peninsula, Japan [76]. The main reaction of carbon dioxide (CO₂) in coal methanogenesis is hydrogenotrophic methanogenesis, in which hydrogen (H₂) acts as an electron donor to reduce CO₂ to methane (CH₄) (see Table 1) [1,17,24]. This microbial process occurs in anoxic environments within coal beds, typically expressed as CO₂ + 4H₂ → CH₄ +2H₂O.

The lignites and associated sediments host microbial life and metabolic potential inherited from freshwater (tidal-terrestrial) peat precursor, which generates CH₄ through methylotrophic methanogenesis, and were trapped and survived burial for 20 million years (Figure 6) [76,77]. The methylotrophic (acetoclastic) pathway is less dependent on syntrophy than hydrogenotrophic pathway, which explains the methanogens’ survival in geological isolation for 20 million years. Cell count analysis from the lignites and sediments below 1500 m b.s.f. ranged from ~10² to 10³ cells cm⁻³, which peaks in the lignite beds [76]. The temperature for biogenic CH₄ generation in the lignite beds (R_o(vitrinite reflectance) = 0.5–0.7%) is at ∼70 °C, which is lower than the temperature for thermogenic CH₄ generation of >150 °C [76,77,78]. The Shimokita Peninsula is a forearc basin that has undergone both subsidence and uplift related to plate tectonics. The biogenic gas- and microbial-bearing Miocene lignites and sediments are interpreted to be rapidly deposited during periods of subsidence preceding uplift.

As coal matures, thermogenic CH₄ is generated by the thermal alteration of the organic matter, which occurs at great depths and intense heat and pressure in the basin. However, following uplift of the coal-bearing basin, late-stage or secondary biogenic CH₄ generation can be triggered in the shallower coal beds by the intrusion of meteoric surface water, which brings in fresh nutrients and microbes. Late-stage biogenic CH₄ in coal aquifers can be generated from recent geologic time (tens of thousands to several million years ago) to modern times. CH₄ generation is associated with active groundwater ecosystems, surface water recharge, and the uplift and exposure of coal-bearing rocks at the surface [22,79,80,81]. Groundwater systems played significant roles in generating biogenic CH₄ in coal aquifers, in producing from these gas reservoirs, and in CH₄ emissions in coal basins worldwide [1,17]. Groundwater provides the necessary chemical and hydrodynamic (hydro-chemical) environment for CH₄ to form and accumulate in the coal. Once CH₄ forms and accumulates in the coal, groundwater traps it within the coal-matrix fractures/pores by providing hydrostatic pressure that keeps the CH₄ bonded or adsorbed on the surfaces of these voids. To release CH₄, producers must lower the hydrostatic pressure in the coal matrix by pumping large volumes of groundwater out of the well. As water is removed, the pressure drops, and CH₄ desorbs from the coal, flows to the wellbore, and reaches the wellhead on the surface.

The decline curves of gas production from coalbed gas and natural gas wells, shown in Figure 7 [82], can be used to forecast differences in CH₄ emission volumes throughout the wells’ lifetimes. The coalbed gas wells show water production is initially high (phase 1) and then declines exponentially, while CH₄ production increases over time as the coal bed dewaters, eventually reaching the peak (phase 2) before declining late (phase 3) in the well’s life. On the other hand, the natural gas wells reach peak production rate relatively quickly and then enter a phase of decline. Also, large volumes of groundwater are produced from the coalbed gas wells, which decline as gas production increases, reaching a later peak. In natural gas wells, lower formation water volumes are incidental to gas extraction. Thus, the potential for large volumes of CH₄ emissions from coalbed gas wells occurs during later peak production, compared to the early peak production in natural gas wells.

Generation of late-stage (secondary) biogenic CH₄ in coal aquifers (e.g., subbituminous and bituminous ranks) is well documented in major coalbed gas-producing U.S. basins [24,79,80,81]. Also, late-stage biogenic CH₄ was recorded in major coalbed gas-producing basins in Australia (Bowen, Surat, and Sydney) [83,84,85,86], Canada (Western Canada Sedimentary Basin) [87], and China (Qinshui, Ordos, and Junggar Basins) [88,89,90]. The coal aquifers in these global coal basins include subbituminous, bituminous, and anthracite ranks. Late-stage biogenic CH₄ can occur in coal beds of any rank, provided the following parameters are met. The initial microbial process in the coal aquifer is probably aerobic oxidation, which provides a new food supply for the anaerobic community, which can be brought to life when dissolved O₂ in the water depleted [22]. Late-stage biogenic CH₄ generation is due mainly to reduction of CO₂ produced in the previous oxidation phase by electrons (hydrogen or H₂) or generated by fermentation of oxidized substrates produced by earlier oxidation (Figure 8) [17,22,24,25,91]. New biogenic CH₄ generated by microbes is an ongoing process in real time, particularly in shallow, wet coal beds actively being fed by fresh groundwater recharge, such as in abandoned coal mines and coalbed gas wells [1,17].

3.2. Groundwater Is Key to Biogenic CH₄ Accumulation and Generation

Groundwater ecosystems in coal aquifers connected to meteoric/surface water recharge are crucial parameters in initiating and sustaining methanogenic pathways in the coal aquifers by introducing microorganisms, nutrients, electron acceptors, and altering the geochemical environment to generate and accumulate biogenic CH₄ as well as affecting the chemical composition of coalbed gas (see Figure 8) [1,17,24,25,92]. Surface runoff and meteoric water carry active microbial communities from the surface into shallow coal beds particularly along coal basin margins. This process introduces the necessary hydrolytic bacteria and methanogenic archaea that can degrade the complex organic matter in coal beds. Water flows from the surface to subsurface coal aquifers and carries essential nutrients, terminal electron acceptors, and dissolved atmospheric gases into the coal’s subsurface environment [22,24,25,80]. Changes in solute concentrations influence the microbial communities present and dominant methanogenic pathways [93].

Meteoric water intrusion is a critical mechanism for deep coal biosphere reactivation by breaking the metabolic dormancy of long-isolated microorganisms, primarily through the delivery of surface-derived energy sources and the drastic alteration of subsurface redox potentials [94]. This process transforms stable, energy-limited subsurface ecosystems into active biogeochemical hotspots by altering the thermodynamic state of the environment. Thus, the meteoric water intrusion performs as an “input” that relieves nutrient limitations and shifts the environmental redox potential from low-energy anaerobic states to high-energy environments, triggering the metabolic reactivation of dormant microbes in the coal aquifer [95].

Biodegradation of solid coal requires a water-hydrocarbon interface (e.g., coal cleats, fractures, pores) where microorganisms can access the organic matter. Surface water flushed into the groundwater (formation water) in coal aquifers creates this necessary environment, and the size of the interface influences the rate of coal degradation. Surface water primes the groundwater ecosystem by injecting water that, at times, contains amendments or nutrients, such as algal extracts that can stimulate resident microbial communities to generate new CH₄ resources [96,97]. Specific water composition derived mainly from water/rock interactions can influence which methanogenic pathway becomes dominant [25]. Studies suggest that hydrogenotrophic methanogenesis (CO₂ reduction) is dominant while acetoclastic methanogenesis also occurs in certain coal basins [83,98,99]. Methanogenesis relies on the cooperation of microbial consortia, such as where fermentative bacteria break down complex coal polymers into simpler organic compounds. Methanogenic archaea then use these organic products for energy to generate CH₄ in the coal. Ultimately, introducing surface water recharge sustains these consortia by providing essential ingredients, geochemical environments with groundwater flow and water/rock interactions dissolving rock minerals, and altering the composition of water types basinward, fundamentally driving and enhancing coal bioconversion into CH₄ (see Figure 8) [1,17,24].

A critical factor limiting microbial conversion of coal into CH₄ is bioavailability [1,17]. Bioavailability in coal bioconversion to CH₄ refers to the accessibility and susceptibility of coal’s complex organic structure to microbial biodegradation [100]. The structural complexity, often described as a 3D network of aromatic and aliphatic compounds, inherently resists microbial degradation. Thus, the initial solubilization and depolymerization steps are critical for unlocking carbon for microbial use [101]. This process is coal-rank dependent, exemplified by high-rank coals containing highly condensed aromatic structures. This makes it extremely resistant (recalcitrant) to microbial degradation. In contrast, low-rank coals are more susceptible to biodegradation due to higher oxygen content and smaller aromatic rings, which provide “biological doorways” for microbial attack [102]. To overcome low natural availability in coal, methods such as chemical (e.g., hydrogen peroxide), physical, and thermal pre-treatments are used to increase the surface area and generate more soluble compounds [103,104].

Biogenic CH₄ is commonly mixed with thermogenic CH₄ in coal aquifers across coal basins, including those in the United States (San Juan and Black Warrior Basins), China (Huainan, Ordos Basins), and Germany (Ruhr Basin). This mixing is primarily controlled by basin hydrodynamics, coal rank, and the geological history of burial, uplift, and erosion, which allows groundwater to intrude into previously deeply buried coal beds, bringing microbes and nutrients. Gases in coalbed CH₄ wells in the north-central part of the San Juan Basin are a mixture of thermogenic (25–50%), secondary biogenic (15–30%), and migrated thermogenic gases, driven by regional overpressure and meteoric recharge [79]. In coalbed gas wells in the Black Warrior Basin, thermogenic gas is the primary source (about 70–80%), but the actual proportion in each area depends highly on the local geological and hydrological conditions [80]. Isotope data from the Panxie coal mining area of the Huainan coalfield in China confirm that the coalbed CH₄ consists of both biogenic and thermogenic gases, with the generation process occurring in distinct stages: primary thermogenic gas followed by late-stage biogenic gas generated by CO₂ reduction [105]. In the Liulin Block of the eastern Ordos Basin, China, the proportions of mixed secondary biogenic gas and thermogenic gas are 48.0% to 49.7%, respectively [24]. Mixed thermogenic and biogenic gases generated by CO₂ reduction are widely observed in active, and more pronounced in abandoned, coal mines in the Ruhr Basin [39,40]. These studies [39,40] indicate a significant portion of the microbial CH₄ might be generated “today by living methanogenic microbes in mine waters” in flooded abandoned coal mines by acetoclastic methanogenesis [39,40].

With deeper burial and increased coalification, which result in higher temperatures and pressures, coals are enriched in carbon as large amounts of volatile matter rich in H₂ and O₂ are released [1,17,22]. Methane, CO₂, and water are the most important byproducts of this devolatilization process [106]. The generation of CH₄ from residual kerogen and thermal cracking of previously formed heavier hydrocarbons is thermogenic in origin [20,22]. Cracking mainly occurs in high-volatile bituminous and low-volatile anthracite(R_o values > 0.6%) under high-pressure, high-temperature conditions. The process of coalification is characterized by the chemical evolution of different types of kerogen, with the elimination of oxygen-containing volatiles, such as CO₂ and water, and the generation and subsequent elimination of hydrogen-rich volatiles, notably CH₄ and oil, during later stages of thermal generation [106]. As a result of thermal cracking, isotopically lighter CH₄ is prevalent in hydrogen-rich kerogen, whereas isotopically heavier CH₄ is predominant in oxygen-rich kerogen [22].

Distinguishing biogenic CH₄ from thermogenic CH₄4H₄ in coals is easier than differentiating biogenic CH₄ from coal and other anthropogenic and natural sources (e.g., wetlands, agriculture, landfills). Biogenic and thermogenic CH₄ originate from distinct generation processes (low temperature microbiology versus high temperature geology) that yield distinct, non-overlapping isotopic signatures [1,17,107]. In contrast to biogenic CH₄, thermogenic CH₄ is distinguished by the common presence of heavier hydrocarbons (>C₂₊ values). For example, in bituminous coal, enrichment of heavy isotope ¹³C in CH₄ and ethane increase with coal rank, and enrichment of deuterium (CH₄ δD values more positive than −250‰) increase with coal rank [22]. Biogenic CH₄ from different sources (e.g., coal vs. wetlands) often overlaps in isotopic signatures. Also, thermogenic CH₄ can be altered to appear biogenic, complicating its identification [78,107]. The key to effective mitigation strategies is distinguishing the isotopic signatures of biogenic CH₄ from those of coal formed geologically in the subsurface and from other sources formed recently on the surface.

4. Distinguishing Isotopic Signatures of Biogenic CH₄ from Coal and Other Sources

Distinguishing the isotopic signatures of biogenic CH₄ from fossil fuels, natural, and anthropogenic sources using δ¹³C (δ¹³C_CH4) is difficult due to complicating factors. These challenges are mainly due to overlapping isotopic ranges among sources, variability in reaction conditions within a single source, between sources, and across source materials, and the effects of secondary processes such as oxidation.

For example, early-mature thermogenic CH₄ can have a δ¹³C_CH4 value similar to primary microbial or biogenic CH₄ within the range from −65 to −55‰ [108]. This study [108] indicates that thermogenic CH₄ undergoes chemical fractionation due to differences in the diffusion rates of CH₄ relative to ethane and propane during migration into shallow subsurface intervals (e.g., higher-porosity reservoirs). Most importantly, microbial processes and isotopic signatures are complex and inconsistent [109,110]. These studies [109,110] indicate that isotopic signatures vary with specific microbes, metabolic pathway diversity (e.g., hydrogenotrophic or CO₂ reduction, methylotrophic/acetoclastic or acetate fermentation), and environmental parameters, such as available substrates and energy levels. Acetoclastic methanogenesis is ¹³C-enriched in CH₄ relative to hydrogenotrophic methanogenesis [111]. Also, the isotopic signature values can vary based on the original composition of the carbon (e.g., organic matter from parent plant material; C₃/C₄ photosynthetic pathways) and hydrogen (e.g., groundwater or formation water) sources, so a single isotopic signature is not always definitive. Distinguishing source material of coal from ancient C3 plants (“old carbon”) and modern C4 plants (“new carbon”), which both show low-to-high δ¹³C signatures, respectively, and creates a profound analytical “blind spot” [112,113,114,115,116,117,118,119,120,121,122] (

Table 3. Summary of difference between “old carbon” and “new carbon” based on geologic time, dominant plant type, photosynthesis, atmospheric CO₂, isotopic signature, and analytical issue. Ma, mega-annum (one million years); ppm V, parts per million by volume; ppm, parts per million; C, carbon; CO₂, carbon dioxide; VPDB, Vienna Pee Dee Belemnite, the international standard for reporting stable carbon isotope ratios (¹³C/¹²C). Carbon isotopic signature based on Meyers [112] study of marine organic matter in black shales. Atmospheric CO₂ based on carbonate fossil soils (paleosols) study by * Breecker et al. [113] and by ** The Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium [114]. Also, compiled from Osborne et al. [115]; Edwards et al. [116]; Gowik and Westhoff [117]; Osborne et al. [118]; Wang et al. [119]; Hare et al. [120]; Polissar et al. [121]; Kirkels et al. [122].

Feature	“Old Carbon” (Paleozoic-Mesozoic Era)	“New Carbon” (Oligocene-Holocene Epoch)
Geologic time	>300 Ma to 65 Ma	~34 Ma to ~11,700 years ago; Late Miocene (~12 to ~5 Ma)
Dominant Plant Type	C3 Plants (Cool/wet; Temperate)	C4 Plants (Hot, sunny, and arid environments); Late Miocene establishment of grasslands
Process of photosynthesis	Highly efficient in cool, wet climate; Stomata open, CO2 enters, and enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes CO2 directly into a 3-carbon compound phosphoglycerate (hence the name C3)	Highly efficient in high light, hot, dry, sunny climate; initial fixation occurs in mesophyll cells CO2 through the leaf using enzyme PEP (phosphoenolpyruvate) carboxylase to 3-carbon compound, creating 4-carbon compound oxaloacetate (hence the name C4).
Atmospheric CO2	~1000 ppmV *	Low <400 ppm **
δ13C signature (‰ VPDB)	Low (Range) −33‰ to −22‰	High (Range) −21‰ to −18‰
Analytical issue	High background noise	Difficult to identify early C4 plants

).

Additional complications arise after emissions into the atmosphere, due to oxidation of CH₄ fluxes [123]. The primary sink for atmospheric CH₄ is its oxidation by the hydroxyl (OH) radical, which causes isotopic fractionation, preferentially consuming the lighter isotopes of carbon and hydrogen and enriching the residual CH₄ in the heavier isotopes such as ¹³C and ²H (deuterium, D) [123]. This reaction, which reduces and limits atmospheric CH₄ concentration, is affected by solar radiation, ozone, and water vapor. When the OH radical reacts with CH₄, the radical does not disappear; instead, it is recycled, and the broadening tropical zones over time provide more moist air, which creates hydroxyl molecules. Increasing OH concentration will accelerate CH₄ oxidation and reduce atmospheric CH₄ concentration [124]. The degree of CH₄ oxidation by OH can vary regionally due to differences in sunlight intensity, making simple signature comparisons unreliable.

The intermixing of CH₄ fluxes from various sources complicates the accurate measurement and attribution of atmospheric CH₄ emissions. This complexity presents significant challenges for monitoring and assessing uncertainty in the development of effective mitigation strategies. One of the most significant impacts of intermixed CH₄ fluxes is the effect on the atmosphere’s oxidizing capacity [123]. The primary sink for CH₄ in the troposphere is the hydroxyl radical (OH). When CH₄ concentrations increase significantly, a chemical feedback loop reduces the abundance of OH [123]. This increases the atmospheric lifespan of CH₄ and other pollutants, leading to more climate warming. Since different sources require distinct reduction and mitigation strategies, knowledge of source types permits the formulation of national, regional, and local inventories [123]. Many potential strategies to facilitate the implementation of these measures are exemplified by separate bottom-up and top-down strategies for the fossil fuel, agriculture, and waste sectors, particularly between biogenic gas sources [56]. The reliance on standard isotopic signatures (e.g., δ¹³C) for CH₄ source attribution is increasingly creating source attribution errors in national inventories because “biogenic fossil methane” from coal beds can be mistaken for modern anthropogenic and natural biogenic CH₄ sources (e.g., wetlands, landfills) [125,126] (see Table 3). This confusion obscures the true source of emissions, leading to mitigation strategies that may incorrectly target sectors such as agriculture or landfills while underestimating fugitive emissions from the fossil fuel industry.

Biogenic CH₄ emissions from CBM/CSG and coal mining extractions occur at a basin scale that includes farmlands and ranchlands with a variety of anthropogenic biogenic CH₄ sources (e.g., agriculture, livestock, landfills, wetlands). So, their attribution has large uncertainties. Measurement of radioactive isotope carbon-14 (¹⁴C) analysis is a critical diagnostic tool for tracing CH₄ sources [127,128,129]. Because fossil fuels are millions of years old, CH₄ derived from coal—whether it is thermogenic or biogenic CH₄—is considered “dead radiocarbon” because all ¹⁴C has decayed (¹⁴C has relatively short half-life 5730 years) [127,128,129]. This characteristic enables scientists to clearly distinguish biogenic coal CH₄ from younger sources, such as landfill gas and livestock emissions, which contain modern, active ¹⁴C.

4.1. Distinguishing Isotopic Signatures of Biogenic CH₄

Analysis of the carbon isotopic composition of CH₄ (δ¹³C_CH4) is the most effective means of tracking its sources and sinks [8,130]. Different sources of biogenic CH₄ produce distinctive δ¹³C_CH4 signatures and values (‰ VPDB) [7,8,123,131,132,133]. Microbial-processed CH₄ emissions from natural wetland, livestock, and landfill sources have lower δ¹³C_CH4 values (global mean of −62‰) than anthropogenic biomass and biofuel burning (global mean of −24‰) and fossil fuel CH₄ emissions (global mean of −45‰) (

Table 4. Differences in values of isotopic signatures of biogenic methane relative to VPDB from coal, anthropogenic, and natural sources. VPDB, Vienna Pee Dee Belemnite, the international standard for reporting stable carbon isotope ratios (¹³C/¹²C); VSMOW Vienna Standard Mean Ocean Water, the international standard for hydrogen (deuterium or D) isotope; δ¹³C, stable carbon isotope; ‰, per mil; D, deuterium; CH₄, methane. C3 and C4 refer to two distinct photosynthetic pathways plants use to fix carbon dioxide, differing in the initial carbon molecule produced (3-carbon for C3, 4-carbon for C4) and in their environmental adaptations. C3 plants thrive in cooler, wetter conditions (like rice and wheat) while C4 plants excel in hot, dry environments (like corn and sugarcane). Compiled from Flores and Moore [1]; Nisbet et al. [7]; Michel et al. [8]; Flores et al. [25]; Zazzeri et al. [111]; Riddell-Young et al. [131]; Bakkaloglu et al. [133]; Basu et al. [134]; Dong et al. [135]; Oh et al. [136]; Lu et al. [137].

Source	δ13CCH4 Values (‰ VPDB)	δ13DCH4 Values (‰ VSMOW)	Key Characteristics
Coal	Less depleted (heavier) 13C than modern biogenic gas, δ13CCH4 generally range from −40‰ to −83‰. Overlap with other microbial methane sources (see below).	Less depleted in D with δ13DCH4 than modern biogenic gas, approximately −160‰ to −310‰.	Generally lighter (more depleted in 13C and D) than thermogenic gas. Isotopic signature varies with coal rank and depth. Mainly C3 plants with transition from gymnosperm to angiosperm may have introduced structural and chemical differences (e.g., lignin) affecting isotopic signature.
Agriculture	Highly depleted (lighter or more negative than fossils) in 13C with δ13CCH4, often in the −50‰ to −70‰ range. Soils: from −71‰ to −54‰ Root associated: from −50‰ to −60‰ Cattle: from −60‰ to −50‰ Rice: from −60‰ to −50‰	Highly depleted in D with δ13DCH4 generally range from −280‰ to −350‰.	Depletion in heavier isotopes (13C and D) compared to thermogenic gas. Signatures depend on animal diets (C3 versus C4 plants) and manure management. Dominated by plants using C3 photosynthesis.
Landfills/Wastes	Depleted (lighter) in 13C with δ13CCH4 in the range of −44‰ to −67‰.	Highly depleted in D with δ13DCH4 like agricultural sources, range from −270‰ to −350‰.	Range is slightly more enriched in 13C than some wetlands, affected by waste composition and microbial oxidation in soil.
Wetlands	Highly variable, mainly very depleted (lighter) in 13C with δ13CCH4 from −37‰ to −70‰. Boreal wetlands have values about −71‰ and tropical wetlands about −60‰.	Variable but highly depleted in D with δ13DCH4 value influenced by the local water composition.	Significant variation based on latitude (tropical versus boreal) or regional and seasonal microbial oxidation. Dominated by C3 or C4 plants depending on geographic locations.

) [1,7,8,24,111,131,134,135,136,137].

However, methanogenic pathways complicate the isotopic signatures of biogenic CH₄ generated from the acetate fermentation pathway (δ¹³C-CH₄: range from −90 to −45‰) [133,138] (Figure 9). Also they are more enriched in ¹³C and less enriched in ²H than biogenic CH₄ generated from CO₂ reduction or the hydrogenotrophic pathway (δ¹³C-CH₄: range from −90 to −60‰; δ²H-CH₄) [133,138] (Figure 9). Thus, the global mean isotopic signatures overlap between biogenic CH₄ from wetlands, livestock, landfills, and fossil fuels.

The isotopic signatures of biogenic CH₄ for each source are determined by examining the variations in ratios of stable carbon-12 (¹²C) to slightly heavier carbon-13 (¹³C) isotopes as well as hydrogen isotopes, specifically the deuterium δD (²H) to hydrogen (¹H) [133,140]. The (¹³C/¹²C) ratio is expressed as a delta value (δ¹³C). Combining carbon isotope analysis with hydrogen isotope analysis (δD) helps differentiate between specific biogenic CH₄ and other sources. A more refined tracing of sources is proposed by Bakkaloglu et al. and Haghnegahdar et al. [133,141] using methane’s “clumped isotopologues” (e.g., ¹³CH₃D, ¹²CH₂D₂) to distinguish between microbial and fossil sources. Clustered (or “clumped”) isotopes of CH₄, specifically the isotopologues ¹³CH₃D, ¹²CH₂D₂, provide a definitive record of CH₄ formation temperature [78,142,143,144]. Biogenic CH₄ typically forms at temperatures below ~80 °C, while thermogenic CH₄ forms at higher temperatures (>60 °C to >200 °C) [78]. Thus, clumped isotopes allow calculation of formation temperature, separating low-temperature biogenic CH₄ from high-temperature thermogenic CH₄ regardless of whether the gas has migrated. During migration or secondary processes, thermogenic CH₄ can lose heavy isotopes (carbon-13 and deuterium), making it appear “falsely light or biogenic” using conventional isotope analysis [22]. In a CBM/CSG mixed gas system, clumped isotopes have been used to identify secondary microbial CH₄ generated by biodegradation previously mistaken for thermogenic CH₄ [144]. However, these carbon-isotope analyses of CH₄ account only for thermogenic, not biogenic, CH₄ in natural and coalbed gases. Although there is some overlap between these fossil fuel gases, the thermogenic CH₄ is generally enriched in ¹³C and deuterium (D, ²H) with δ¹³C_CH4 values ranging from −45‰ to −15‰ [133]. Methane produced by microorganisms in wetlands, landfills, and agriculture has very little δ¹³C, so the signature of microbial CH₄ is “lighter” than that produced by fossil fuels, such as natural gas [133]. The biogenic CH₄ generated in coal beds is derived from ancient plant remains (fossil carbon; see Table 3). So the CH₄ produced by microorganisms from old carbon sources yields δ¹³C values in coals that often range from approximately −40‰ to −60‰, though these values can vary based on coal rank or maturation (coalification), depth, and methanogenic processes [133].

The transition zone is characterized by a shift from highly depleted (lighter) ¹³C and ²H (deuterium) values in biogenic CH₄ to enriched (heavier) values in thermogenic CH₄. The biogenic CH₄ typically shows δ ¹³C values that range from >−70‰ to −50‰, while the thermogenic CH₄ range from −50‰ to −20‰ (see Figure 10). The deuterium isotope (δD or δ²H) values in the transition zone (orange, see Figure 10) between hydrogenotrophic (CO₂ reduction) and methylotrophic (Fermentation) pathways frequently range from −300‰ to −250‰ relative to VSMOW. These transition zones result in overlapping gas types that are difficult to separate from distinctive isotopic signatures of biogenic CH₄. However, “clumped isotopologue” can aid in identifying CH₄ emissions by analyzing the “clumping” of heavy isotopes within CH₄ molecules, thereby distinguishing between thermogenic and microbial sources, which often have overlapping “bulk” isotopic signatures [141,142,143,144].

The isotopic compositional fields related to methanogenic pathways for biogenic CH₄ (e.g., CO₂ reduction or hydrogenotrophic vs. acetate fermentation or methylotrophic) or the thermal maturity of the source rock for thermogenic gas are best shown in Figure 10 [17,145]. Combining these isotopic data with the molecular composition (e.g., δ¹³C-CH₄; δ²H-CH₄ or D, ²H) is a powerful tool for robust genetic characterization of gas types. Each gas type occupies a distinct, empirically defined field based on its unique formation processes. However, the transition zone (blue, see Figure 10) between biogenic and thermogenic gas fields exist because natural gas accumulations are frequently mixtures of methane from both microbial and thermal sources.

4.2. Variations of Coalbed Biogenic CH₄ Isotopic Signatures

The isotopic signature of CH₄ produced during coalification of organic matter in coal is controlled by the CH₄ origin pathway [111]. Coalification correlated with burial depth intensification of temperature and pressure, which transforms the fossil organic matter in precursor peat over geologic time, affects the isotopic composition of coal-generated CH₄.

Also, the isotopic signature is influenced by the stages of biogenic CH₄ generation (early- and late-stages) during coalification. Post-generation processes such as desorption, diffusion, migration, mixing, and oxidation in coal aquifers contribute to variations in the isotopic signature of biogenic CH₄. Isotopic fractionation during biogenic CH₄ migration over a long time or distance, or has undergone desorption and diffusion, becomes isotopically heavier because the lighter isotopes are preferentially released or moved faster [146]. The oxidation of biogenic CH₄ occurs during migration into more oxidized coal aquifers, in which preferential consumption of lighter isotopes leaves the remaining CH₄ pool isotopically heavier (enriched in ¹³C and ²H or D) [22,146].

The above isotopic-fractionation process within coal aquifers is probably exemplified by the biogenic CH₄ in the Powder River Basin with highly variable δ¹³C_CH4 signatures based on 165 gas and water samples collected from 10 major/minor producing subbituminous C–A rank coal beds ranging from 51 to 700 m deep [25]. Here, the δ¹³C _CH4 signatures range from >−53.5‰ to −64.9‰ for the isotopically heavier biogenic CH₄ distributed in the deep central part (653–198 m in depth) of the basin and from −65.4‰ to <−83.4‰ for isotopically lighter biogenic CH₄ distributed in selected areas of the shallow (97–197 m) margins of the basin. The trends in δ¹³C_CH4 indicate that the heaviest CH₄ isotopes are in the central part of the basin, where conditions are more anoxic and hydrogen is available, which is typically associated with CO₂ reduction or hydrogenotrophic methanogenesis [25,26]. In contrast, during migration into more oxidized zones (typically shallower aquifers), CH₄-oxidizing microbes preferentially consume lighter isotopes (¹²C and H¹) and are dominated by fermentative (acetoclastic) methanogenesis. Coincidentally, the coal ranks at the deep central part of the Powder River Basin are subbituminous A and B (higher end of the low-rank coal with more carbon content), and at the shallow margins are subbituminous C (lower end of the low rank-coal or less carbon content). More importantly, biogenic CH₄ generation in the deep central part of the Powder River Basin is dominated by CO₂ reduction, a specific type of hydrogenotrophic methanogenesis, and minor methyl-type fermentation (acetoclastic) methanogenesis sustained by nutrients from surface water recharge and occurs in the shallow eastern and northwestern basin margins [25,26].

The concept of isotopic fractionation during gas desorption in CBM production is more pronounced in high-rank coals and depends on the “speed of pressure reduction” [147,148]. When pressure (e.g., hydrostatic) is reduced the CH₄ adsorbed in the coal matrix desorbs and diffuses. The isotopically lighter CH₄, which contains a lighter carbon isotope (¹²C), desorbs and diffuses more rapidly than the heavier CH₄, which contains a heavier isotope (¹³C) [147,148]. Because ¹²CH₄ escapes faster, the initial gas released during early stages of CBM/CSG production or coal mining dewatering is depleted in ¹³C, making it appear more “biogenic” (lighter or false biogenic) than the bulk composition of the source. As production continues, the coal reservoir becomes depleted in the lighter isotope (¹²C). Consequently, the remaining CH₄ desorbs later and becomes increasingly enriched in ¹³C, resulting in a heavier isotope signature [147,148]. The key implication of this phenomenon is that isotopic analysis of CH₄, particularly during early CBM/CSG production, may not accurately represent the true maturity or genetic origin of the CH₄ [148]. Also, for coal mines, this means CH₄ leaking from long-closed, abandoned, or newly opened, low-flow areas might appear artificially light. Thus, this requires careful analysis to determine if it is true biogenic CH₄ or just fractionated thermogenic CH₄. Lastly, analysis between truly biogenic and thermogenic CH₄ focuses on other indicators such as ethane-to-methane ratio [22,25]. The dynamic fractionation during CBM/CSG production and coal mining dewatering reflects the challenges in measuring fugitive CH₄ and unreliable and accurate emission tracking, which is a significant “blind spot” in mitigation.

Similar methanogenic pathways toward the generation and accumulation of biogenic CH₄ are observed in various rank coals (e.g., subbituminous, bituminous, anthracite) in coal basins in Australia [83,84,149,150], Canada [44,87], China [89,151], India [152], Poland [153], and the U.S. [25,80,81,154]. Australia and China have replaced the U.S., Canada, and the European Union as major producers of coalbed CH₄ since 2008 [1]. In the Australian and Chinese coal basins, the main methanogenic pathway for biogenic CH₄ generation is CO₂ reduction (hydrogenotrophic), and the minor methanogenic pathway is acetate fermentation. In the Australian Bowen, Surat, and Sydney Basins, with coal ranks ranging from subbituminous to anthracite, the δ¹³C_CH4 signatures range from −60‰ to −40‰, −57.3‰ to −54.2‰, and −76.8‰ to −50.1‰, respectively [83,84,155,156]. These δ¹³C_CH4 signatures indicate that late-stage biogenic CH₄ generated in the Australian basins is mixed with a thermogenic CH₄ component that is produced in CSG wells.

In the Chinese Ordos and Qinshui Basins with coal ranks from bituminous to anthracite, the δ¹³C_CH4 signatures range from −81.9‰ to −9.1‰, with most falling between −65‰ and −30‰, [157]. The biogenic CH₄ generated by methylotrophic (acetate fermentation) methanogens originates from the eastern margin of the Ordos Basin [35,157,158]. In the southern Qinshui Basin, the δ¹³C_CH4 signatures are mainly <−55‰ with biogenic CH₄ generated by CO₂ reduction (hydrogenotrophic) methanogenesis [36,159]. The Chinese coal basins contain biogenic CH₄ that is mixed with a major thermogenic CH₄ component. The δ¹³C_CH4 signatures range from −32.58‰ to −26.7‰ [36,159], often in deeper parts of the basins. The mixed gases are often produced in single wells, which are difficult to isolate.

Even for the same source, such as coalmine CH₄ emissions, the isotopic signature varies depending on the geologic and biogenic processes involved. A study of δ¹³C_CH4 signatures from different coal mine types in the United Kingdom, Australia, and Poland indicates that the progression in coal rank and generation of biogenic CH₄ is due to the incursion of groundwater affecting coal-derived CH₄ signatures [111]. As a result of this study [111], bituminous coal extracted in surface (opencast) mines was analyzed to have a δ¹³C_CH4 signature average value of −65‰ compared to underground deep mines with δ¹³C_CH4 signatures ranging from −65‰ to −55‰. Also, anthracite coal in opencast and underground deep mines has ¹³C_CH4 signatures of −40‰ and −30‰, respectively. Shallow coal mines extracting various coal ranks are exposed to meteoric water, most likely associated with the microbial generation of lighter biogenic CH₄. The study concluded that the isotopic signatures in global atmospheric models of coalmine CH₄ emissions are region- or nation-specific, requiring more detailed research, given the wide global variation in coal rank [111].

Thermogenic gas, such as CH₄, contains more ¹³C and has a distinct ²H/¹H isotopic signature compared to biogenic gas, which is characterized by a ¹³C-depleted signature and broader ²H values (VSMOW) [78]. The molecular composition of thermogenic gas typically contains other hydrocarbons like ethane, propane, and butane, in addition to CH₄, in contrast to biogenic gas, which is primarily CH₄ [20]. Thermogenic gas forms at high temperatures and pressures, while biogenic CH₄ forms at low temperatures generated by microbes sustained by surface water recharge. By extrapolation, deep coals generally contain thermogenic gas, and shallow coals comprise biogenic CH₄. These gas-generation processes in coals essentially suggest that deep underground mines are composed of thermogenic gas, whereas shallow surface mines contain biogenic gas. However, the biogenic CH₄ potential in a coal mine producing thermogenic gas in Hubei, China, could be stimulated by indigenous microorganisms [160]. The highly diverse fermentative bacteria and acetotrophic, hydrogenotrophic, and methylotrophic methanogens in the Triassic coal of the coal mine could be enriched to regenerate biogenic CH₄. Mixed thermogenic and biogenic gases are widely observed in coals in coal mines in the Ruhr Basin in Germany [39,40] as well as in the eastern Ordos Basin [157], Huaibei Coalfield [161,162], and Qinshui Basin [151,163] in China. In the Chinese coal mine examples, the mixed thermogenic and biogenic gases, which formed at depths of 250 to 1000 m [162], are impossible to separate, but both gases can be mitigated by drilling and capturing in advance of mining, as well as by VAM [32,33].

5. Sources of CH₄ Emissions in Coal Mine and Coalbed Gas Operations

The total CH₄ emissions from coal sector operations originate from two diversified areas of mining operations: mineable shallow coal beds and drilling for commercial gas production of deep unmineable coal beds (

Table 5. Summarizes the key types of CH₄ emissions and gas types in coal sector operations. Also, it differentiates between point sources and diffuse/disperse flow types along with their primary mitigation challenges. CH₄, methane, CMM, coalmine methane; VAM, ventilation air methane; AMM, abandoned mine methane; CBM, coalbed methane; CSG, coalseam gas methane; %, percent. Compiled from Flores and Moore [1]; USEPA [16]; Flores [17]; IEA [30,31]; Beckmann et al. [40]; Thielmann et al. [39]; Karacan et al. [59,164]; Karacan [165].

Coal Sector Operations	Type of CH4 Emissions	Gas Types	Sources of CH4 Flow	Main CH4 Mitigation Challenges
Underground Coal Mine	CMM	Thermogenic, biogenic, and mixed gas.	Point source and diffuse from inseam and surface drainage boreholes, coal pillars, roadways, roof collapsed areas, and abandoned pipe/ventilation shafts. Also, from post-mining handling, processing, and storage sites.	Economic viability (e.g., low/high gas prices), infrastructure to transport gas, and safety requirements to drain gas, and gas quality. Gob well flares often used for low quality gas to be used commercially.
	VAM	Thermogenic, biogenic, and mixed gas.	Point source and diffuse high volume to low concentration CH4 in ventilation shafts (<1%), exhaust points, belt portals, and bleeder shafts (up to 2%).	Low concentration (<1%) makes capture technically difficult and expensive and requires specialized thermal oxidizers.
Surface (Open-cast) Coal Mine		Thermogenic and biogenic gas. Commonly biogenic gas due to shallow conditions.	Point source and disperse from abandoned drainage wells in advance of coal mining, old blastholes in the coal, highwalls composed of coal, interburden, and overburden, open pit floor, spoil piles, coal silos/stockpiles, coal loading points, preparation and treatment or handling plants, and tailing ponds.	Low concentration and highly dispersed nature of CH4 emitted, which makes capturing uneconomic. Often lacks stringent regulation for operators to mitigate CH4 emissions.
Abandoned Coal Mine	AMM	Thermogenic and biogenic gas. Generation of new, real-time microbial gas in flooded mines.	Point source and disperse from gob wells, old drainage boreholes, old (sealed) ventilation shafts, portals (sealed), rock (overburden) and surface fissures, fractures, and old (sealed) vents.	Identifying responsible parties for inactive or orphaned sites, technically unviable measurement of low concentration and diffused CH4 emissions, and high costs of site rehabilitation/clean up. Gob well flares often used for low quality gas to be used commercially.
Coalbed Gas Development	CBM/CSG	Thermogenic, biogenic, and mixed gas. Commonly biogenic in shallow coal aquifers.	Point source and low concentration CH4 from production wells, gas gathering and compression facilities, gas treatment plants, and gas pipeline infrastructure.	Ensuring well and pipeline integrity to prevent gas leakage. Gas gathering facilities emit dispersed gas. Abandoned/orphaned wells gas leakages and biogenic gas wells are perpetual CH4 emitters.
	Co-produced Water	Commonly biogenic gas in coal aquifers with dissolved CH4 in groundwater.	Diffuse and disperse sources of low concentration CH4 from human-made impoundments and outfalls as well as natural ponds, lakes, creeks, streams, rivers, and wetlands around ponds/lakes. Also, from water storage tanks and drilling mud pits.	Managing water spread regionally and discharged in all kinds of aquatic systems with dissolved CH4, which are economically and technically difficult to mitigate.

) [1,16,17,30,31,59,164,165]. The key problem in mitigating CMM from mining operations is that the CH₄ concentration in emissions is low and from diffused/dispersed point sources, such as in underground, surface and abandoned coal mines (Table 5). The lower the CH₄ concentration, the more difficult it is to abate the emissions technically and economically [166].

So, the dominant and singular focus of the global source of CH₄ emissions is on CMM from underground coal mine operations and production, surface (open-cut) mining, and post-mine processing and transportation. However, indirect (fugitive) emissions come from CMM and from the energy required to process and transport the coal (Figure 11) [30,166]. According to IEA [166] shown in Figure 11 the lowest 10% coal production (Mtce) has a total average emissions intensity of 80 kg (kg) of CO₂-eq/tce. In contrast, the highest 10% coal production (Mtce) has emissions intensity of 1000 kg CO₂-eq/tce. Thus, the most emissions intensive coal produces greater than ten times more indirect emissions than the least emissions [166].

The least-known source of CH₄ or CMM emissions from coal mining is from operations during the degasification of mineable coal using boreholes in advance of mining and the production of natural gas (CBM/CSG) from associated deep unmineable (“intensive coal”) coal beds. Often, the gas captured by this pre-drainage process is utilized for electricity generation to run equipment and factifies in the mine site. Depending on the quality of the gas (>95% CH₄) and the availability of nearby pipeline infrastructure, the captured gas is injected into the local natural gas grid and marketed as CBM/CSG. Thus, prior to coal mine operations, it is necessary to invest capital for drainage technologies, pipeline networks, and monitoring equipment [166].

Worldwide (e.g., U.S., China, India, Russia), there is a blurring of lines in the common usage of CMM and natural gas (CBM/CSG) development [32]. While CMM is traditionally viewed as a safety hazard [32,164,165], these and other top-emitting countries (see Figure 2) are moving toward treating it as a valuable source of clean energy, effectively merging it into the natural gas (CBM/CSG) market [33].

Simply put, the geological origin of CH₄ from coal beds does not support separating GHG inventories, as in the U.S. EPA’s catalogs of CMM and coalbed methane (CBM). In China, CH₄ (CMM), which is recovered from coal beds in coal mines in the shallow part of the coal basins, is developed in the same coal beds in the deep basin as a part of natural gas (coalbed gas or CBM/CSG), focusing heavily on utilizing this resource for energy and safety as mandated by the government’s Five-Year Plans [33]. As exemplified in the Huainan mining area (Anhui Province) CBM pre-drainage system is essential for extracting coal safely and efficiently [167,168]. Because Huainan’s deep coal beds have low permeability, high gas pressure, and a high risk of outbursts, operators implement regional CBM pre-mining drainage methods to mitigate explosions and capture the gas for energy use. These methods include first mining outburst-prone coal to relieve stress and gas pressure in adjacent outburst-prone coal, making CBM extraction more efficient. Also, surface vertical wells and underground-to-surface target “sweet spots” around longwall mining goafs prevent fugitive CMM emissions. Although surface/ground and underground drilling methods are effective, CBM extractions fail due to borehole instability and blockages due to tectonically sheared coal beds. Thus, technical and operational conflicts between CBM production and coal mining in the Huainan mining area are driven by complex geological conditions, technical incompatibilities, and rights overlap [167,168]. The separation between CBM exploration/mining rights from traditional coal mining rights has led to coordination issues. To mitigate these conflicts, the industry uses the “Huainan Model” of co-exploitation through stress-relief mining to prevent outbursts.

In India, Coal India Limited, which accounts for 80% of the country’s coal production, along with its subsidiaries, was granted permission by the government to explore and develop CBM/CMM from mineable coals and all other coal-bearing rocks within their coal-mining leases [1]. In U.S. coal basins where both gases (CMM and coalbed gas) are produced from the same coal beds, coal mines (in the shallow margins of the basin) and natural gas companies (in the deeper part of the basin) have filed lawsuits about lost gas. In the Powder River Basin, natural gas companies sued coal mine operators for lost gas reserves resulting from decades of dewatering of coal beds in coal mines leased by both companies. Dewatering lowered the basin groundwater level (drawdown), which in turn reduced the hydrostatic pressure in the coal reservoir, causing gas desorption and emissions down-basin of the coal mines [17]. Coal mines drill shallow coalbed gas wells from the surface to drain CH₄ (CMM) and groundwater from the mineable coal beds in advance of mining. This drilling technique allows the capture of the gas for energy use in the mines’ facilities but also contributes to CH₄ emissions from abandoned boreholes. This mode of draining CMM serves as a precursor to the production of coalbed gas from deeper unmineable coal beds [165]. The key to both coal operations is the shared coal aquifers in which the groundwater system is co-produced during development.

The shared groundwater system refers to the lateral stratigraphic connectivity of the groundwater within the same coal aquifer (dual-role coal), which is both mined on the surface along the basin margin and developed for gas in the basin center [1,17,80]. Active pumping of water wells in the surface coal mine area causes localized cone of depression drawing groundwater towards the wells from all surrounding directions. This localized drawdown, or lowering of the groundwater table/level, creates a low-pressure area and a hydraulic gradient, promoting gas flow in the coal aquifer. The gas migrates updip through the coal beds, faults, fractures, and fissures in the overlying rock (overburden) above the coal aquifers and invisibly escapes to the atmosphere. This process is analogous to “transverse hydraulic conductivity” in multi-seam mining of the upper coal bed, creating a fractured zone in the overburden that extends to the lower coal bed [169,170]. The fractures and fissures provide pathways for gas migration and invisible emissions.

5.1. Sources of CH₄ Emissions from Coal Mines

Mixed biogenic and thermogenic methane is released by active surface (open-cut or opencast) and underground coal mines as CMM, abandoned coal mines as AMM, and coal preparation/processing, and coal storage and transport as fugitive methane [1,17,73,164,165,169]. The difference between these sources lies in the degree of CH₄ emissions’ diffusivity, with surface coal mines emitting more than abandoned coal mines and less than underground coal mines. That is, surface coal mines emit CMM over large areas (e.g., open pits, highwalls with exposed coal beds, extended facilities such as spoil piles and coal storage areas) rather than from the VAM mitigation point source in underground coal mines. Most U.S. CMM emissions are from VAM systems (60.2%), followed by minor emissions from abandoned mines (11.8%), post-mining surface operations (11.8%), surface mines (11.2%), and drainage or pre-drainage systems (4.3%) [171,172,173]. As of 2023, coal mining accounts for 31% of the CH₄ emissions in the energy sector worldwide [171,172,173].

In surface coal mines, the extraction of shallow subsurface coal beds over large areas involves blasting to fragment the overlying rocks (overburden) by blasting, thereby releasing trapped CH₄ into the atmosphere. Blasting enables large-scale surface mining and the removal of large volumes of soil and rock through mechanical excavation (e.g., trucks and shovels), causing CMM to seep over a broader surface area. Also, blast-fractured and freshly exposed coal beds in the highwalls serve as point sources of high concentrations of CMM emissions controlled by coal’s properties. But properties such as gas content, rank, permeability, pressure, desorption rate, water/moisture content, and pace of advancement of the coal face or highwall vary over time [32,33]. These mine-site-specific data, such as gas contents of mined or in-situ coal beds analyzed from cores over time, are traditional standardized data for bottom-up Tier 2/3 emission factors estimations. Thus, updated CH₄ emissions from surface coal mines are calculated over time using the emissions factors of the USEPA guidelines [73]. Normally, an isolation flux chamber is used by placing an enclosure on the mine surface, flowing known air through it, and measuring CH₄ concentration in the outlet air to find the flux (kgCH₄/km²/day). However, this technique is inherently problematic due to poor spatial and temporal representation across the large surface area of a coal mine.

While remote sensing offers expansive, time-series data, it often fails to capture fine-scale, ground-level details, creating a critical gap between spectral pixels and meteorological processes, such as in surface coal mines. Satellite-based monitoring of CH₄ emissions in coal mining faces significant challenges due to episodic leakages driven by variable microclimatic conditions, creating a “blind spot” in mitigation. Although satellites provide broad, long-term observation of CH₄ emissions, they are predominantly passive sensors that require specific conditions, which can lead to missed detection of emission peaks [171]. Satellites like TROPOMI (near-daily) or GHGSat (targeted) pass over a site only at certain times of the day. If a peak emission event occurs outside this observation window (e.g., at night or on the weekend) it will not be captured [173]. Detection thresholds indicate that high-resolution satellites such as GHGSat can detect individual CH₄ plumes as small as ~100 kg/hr with precision. However, many smaller or more diffuse CH₄ leaks from coal mining operations may remain below the detection limit [172]. The intersection of surface reflectivity (albedo) and complex topography in or around surface and underground mines creates a highly variable setting that challenges traditional modeling techniques [173,174,175]. For example, surface coal mine highwalls and spoil piles (30–100 m relief) act as topographic barriers, creating turbulence, cold-air pooling, stagnant air pockets, or channeling wind, which can cause unnatural microclimate to form [173]. Surface coal mines expose various rock layers, overburden, and infrastructure, each with distinct albedo values. Dark, freshly excavated rock absorbs more solar radiation, increasing localized heating, while light-colored and weathered, exposed rock reflects it [171]. These surface coal mine unnatural microclimatic, albedo, and topographic conditions make CH₄ dispersion models difficult to measure and mitigate due to the lack of reliable baseline data.

Historically, it was assumed that surface (or opencast) mines released very little CH₄ because the shallower coal beds they access contain less gas than deeper seams. Also, the shallower coal beds, usually found in basin margins, occur in zones of low hydrostatic pressure that adsorb or hold CH₄ in the coal’s pore systems and are controlled by fluctuations of groundwater level in the zones. However, recent studies utilizing advanced remote-sensing satellite data and airborne instruments have challenged this assumption and confirmed that some surface mines are significant, concentrated point sources of CH₄. These super-emitter events are often caused by concentrated gas releases, such as those at the Hail Creek open-cut mine in the Bowen Basin in Queensland, Australia [175]. The diffuse nature of CMM emissions in the opencast coal mine requires advanced techniques, such as aircraft-based lidar to map mine topography and remote sensing instruments to measure top-down CH₄ concentration gradients (Figure 12) [175]. The CMM emissions emanate from bituminous, hard coking coal typically containing thermogenic gas, but potential biogenic gas in Bowen Basin Permian coal measures is suggested by high hydraulic conductivity and groundwater recharge [176]. Also, the within-pit source of CMM emissions does not account for potential in-situ biological gas production from water management ponds, disturbed soils, and exposed coal stockpiles [175]. Thus, the uncertainty of the biogenic origin of the gas in the Hail Creek opencast mine contributes to potential under-reporting of the origin of CMM emissions.

A common strategy during mining operations to prevent large-scale CMM emissions from surface coal mines (e.g., Hail Creek coal mine) is to drill surface boreholes into the coal beds to drain them before mining and capture CH₄ for beneficial use [17]. The pre-drainage system includes drilling boreholes in front of or in advance of mining and continues as coal extraction moves into adjoining or deeper areas where more concentrated gas is encountered. So, they are documented sources of CH₄ emissions. Gas pre-drainage in surface coal mines is determined by site-specific geological conditions, mining methods, and operational requirements. Geological conditions include coal-bed depth (e.g., gas content increases with depth) and geological structures (e.g., faults, folds) [32,33,177]. Operational requirements include vertical drilling from the surface into the coal, horizontal drilling from the surface or directional drilling for more precise targeting of gas-rich areas, and maintaining boreholes within the bed [178,179,180,181]. Pre-drainage boreholes drilled from the surface-to-inseam coal, are a primary method for mitigating CH₄ emissions in coal mining and are designed to convert uncontrolled, diffuse CH₄ releases into controlled, captured, and usable gas. Also, this method significantly reduces the amount of gas that would otherwise be released into the underground coal mine’s VAM.

In contrast to surface coal mines where the CH₄ is dispersed, underground coal mines are more contained, controlled, and managed. Also, methane emissions from underground coal mines are usually more than those from surface coal mines, as deeper coal beds tend to contain higher CH₄ content [31,32,33,172]. The ventilation systems in underground coal mines are the single most important source of CMM emissions, which accounts for about 30% of global CMM emissions [31,172]. VAM comes from coal beds, where the trapped CH₄ is released during mining operations, such as coal excavation, exposure of coal faces, and drilling for gas in front of longwall mining. Large fans move air through the mine to keep CH₄ levels below explosive limits (5–15%). Diluted CH₄ is typically less than 1% and is released to the atmosphere via VAM by moving fresh air into the mine [17,59,172]. Because coal mine exhaust flow rates are high, VAM is a major source of CH₄ emissions in active coal mines [73]. On-site recovery and use of VAM can be used to heat mine facilities or to dry coal [172]. An alternative method to reduce CMM emissions uses thermal oxidation to destroy VAM, even at very low concentrations. To reduce reliance on VAM systems, an underground coal mine degasification system is used to remove trapped CMM by drilling in-seam boreholes in advance of longwall mining. Underground coal mine pre-drainage systems include directional, horizontal, and cross-layer drilling [33,179,180,181]. Multiple boreholes from the same site and cross-measure drilling at an angle to drain CH₄ above and below working areas maximize space efficiency and CMM recovery in underground coal mines [178,179,180,181].

Abandoned coal mines, especially underground mines, are a significant source of CH₄ emissions because they continue to release AMM that was trapped as residual CH₄ in the coals and surrounding rocks, even after a mine has closed. AMM continues to be released from abandoned underground coal mines for decades and, presumably, when flooded with groundwater, stops leaking after 7 years (Figure 13) [182]. The figure shows a decline curve for flooded abandoned coal mines, indicating a 50% decrease in CH₄ emissions in about 1 year after abandonment. In contrast, it takes about 5.5 years to reduce about 50% of CH₄ emissions for non-flooded (dry) abandoned coal mines. Additionally, it takes about 7–8 years to reduce CH₄ emissions to 0% in flooded abandoned coal mines, whereas during the same period, CH₄ emissions decrease by about 40% in non-flooded abandoned coal mines. This process is known as the “piston effect,” in which flooding a coal mine creates hydrostatic pressure that can trap residual CH₄ within the coal. However, continued gas generation from the submerged coal bed can increase pressure in the unflooded voids above the water level, forcing gas to the surface and into the surface atmosphere. As groundwater levels rise in an abandoned, flooded coal mine, the rising groundwater behaves like a piston, displacing and forcing CH₄ trapped in the coal bed, mine voids, and gobs towards the surface through fractures or mine shafts [180]. Simultaneously, the influx of water in the coal bed creates a hydrostatic pressure effect. This water fills pores, fissures, and fractures within the coal bed, significantly reducing CH₄ desorption from the coal, trapping much of the gas within the coal matrix and unexploited coal beds. However, in the coal bed above the groundwater level, the residual CH₄ (thermogenic) in the coal matrix desorbs through a reduction in gas pressure that exceeds the adhesive forces holding the gas in micropores.

If the groundwater table continues to rise due to persistent flooding, the gas-free micropores will be filled, creating a favorable habitat for microbial life. The succeeding methanogenic process (see Table 1) perpetuates the emission cycle in the flooded abandoned coal mine. That is, the flushing of groundwater through the coal bed can reintroduce microbes and nutrients, helping to initiate and sustain biogenic CH₄ production in abandoned underground coal mines [39,40]. Acetoclastic methanogenesis as a primary source of biogenic CH₄ was discovered in groundwater from flooded abandoned underground coal mines and drainage water [39]. However, acetoclastic and hydrogenotrophic microbes often work in flooded abandoned coal mine. The biogenic CH₄ generated in the flooded coal mine can escape to the surface over time through natural fissures, fractures, and abandoned boreholes and ventilation shafts in the overlying rock strata. Largely flooded abandoned underground coal mines have associated reduced gas reservoirs where small barometric pressure fluctuations can cause the mine to “breathe,” meaning air flows into the mine during periods of high barometric pressure, which promotes AMM emissions [183]. In contrast, dry or partially flooded abandoned underground coal mines will continue to emit biogenic CH₄ to the atmosphere at rates that depend on the remaining coal, the coal’s gas content, water level, and climatic conditions [184]. Biogenic CH₄ emissions from abandoned underground coal mines will increase faster than those from active underground coal mines as more mines are closed over time. For example, China has closed 12,000 to 13,000 underground coal mines from 2011 to 2019 and is projected to reach 15,000 closures by 2030 as coal supply is reduced [185]. The contribution of biogenic CH₄ emissions from flooded abandoned coal mines warrants further study of the potential long-term impacts of these biogenic gas emissions and their technical/scientific mitigation.

Fugitive CH₄ emissions from coal mining operations are generated by coal preparation, processing, storage, and transport facilities [1,73]. Dispersed, low-concentration fugitive CH₄ emissions are released during coal handling and cleaning during preparation and transportation to market. Unintentional releases of residual CH₄ from sources such as preparation plants, storage yards, bunkers or silos, and transport systems account for the total of fugitive emissions. Coal preparation is a process of crushing and cleaning raw coal to remove impurities, improving its quality and making it more suitable for market and industrial use [1,73]. Preparation includes reducing coal size through crushing and screening and removing unwanted materials such as rock, ash, and some sulfur using wet and dry methods. Wet methods involve using a liquid medium, such as washing with water, hydrocyclones to sort coal by density, and froth flotation to recover fine particles. Dry methods use airflow and vigorous shaking to separate materials, often performed before washing. The cleaned coal is dried to remove excess water. Throughout these processes at various stages of preparation, residual CH₄ is released as fugitive emissions and is difficult to mitigate. Also, the organic-rich wastewater from these processes is discharged into settling or silting ponds, which are primarily physical sedimentation basins. These ponds/basins can act as “passive bioreactors” by promoting anaerobic digestion of organic matter in the accumulated sludge, which slowly emit biogenic CH₄.

5.2. Sources from Coalbed Gas Development and Co-Produced Groundwater

Coalbed gas (CBM/CSG) development involves extracting natural gas from coal beds by drilling wells into the coal and pumping out groundwater to lower the hydrostatic pressure [17]. This intensive groundwater withdrawal reduces hydrostatic pressure, allowing the CH₄ adsorbed onto the coal’s matrix pore and fracture systems, to be desorbed and to flow into the gas well and to the wellhead on the surface. This process creates unintended potentially large atmospheric CH₄ emissions at the wellhead and co-produced water collection sites because CBM/CSG water is at peak production initially when the well starts to desorb gas (see Figure 6). The hydrostatic pressure must drop below a specific threshold (the critical desorption pressure) before CH₄ can begin to detach or desorb (Langmuir model) from the coal matrix and fractures. The process requires significant groundwater withdrawal and management, as large volumes of water are co-produced. This process raises environmental concerns including impacts on groundwater-dissolved CH₄ and on atmospheric CH₄ emissions from surface land [17,186,187,188,189,190,191]. The co-produced groundwater is often pumped and collected into diverse, interconnected surface aquatic systems such as natural ponds or lakes, impoundments or reservoirs along alluvial plains, creeks, streams, and rivers. Major coal-producing regions, notably Qinshui and Ordos Basin (China), the Black Warrior and Powder River Basins (U.S.), and Bowen/Surat Basins (Australia), extract large volumes of co-produced water related to CBM/CSG production. China’s CBM co-produced water extraction range from 10 to 271,280 L per well per day as of 2014 [191]. Australia’s CSG-associated co-produced water extraction has stabilized to about 52,000 megaliters per year with 82% from Surat Basin as of 2025 [192]. The Powder River Basin yielded a total 1.4 billion m³ of CBM co-produced water from about 29,000 wells during the 1987–2020 period [1]. The Black Warrior Basin yield over 1.6 billion barrels CBM co-produced water from more than 2900 wells as of 2011 [193]. In the Powder River Basin, co-produced groundwater from up to 10 closely-spaced coalbed gas wells is connected to an outfall (manifold system) where water is discharged and collected in a nearby pond or impoundment (Figure 14) [17]. Here, during the 1987–2020 period, 1.08 billion m^# of co-produced water with coal fines and microbiota were discharged into diverse surface aquatic environments from 29,026 CBM wells [17]. These surface aquatic environments probably became active bioreactors where distinct microbial guilds decomposed organic matter into biogenic CH₄ and CO₂ that were emitted. Groundwater pumped out into countless ponds/impoundments and into aquatic environments during the co-production of CH₄ from coal beds worldwide is a potential source of global atmospheric CH₄ emissions [186,187]. This phenomenon is highlighted by a study based on global groundwater CH₄ concentrations data, which shows that sites with low-to-high methane concentrations require days to decades for unconsumed CH₄ to leak into local streams, where it can escape into the atmosphere and contribute to global warming [186].

In the Surat Basin (Walloon Coal Measures), Australia, the maximum estimated CH₄ emission from coalseam gas (CSG) co-produced water is 1.88 × 10⁻³ teragrams per year (Tg/yr) using the closed-sampling method (isoflask) [186]. However, using the open-sampling method (vial), the maximum estimated CH₄ emission is only 9.56 × 10⁻⁷ Tg/yr, with the difference attributed to gas loss during sampling [186]. Considering only CSG in the Surat Basin in Australia, the maximum estimated CH₄ emission from co-produced water is 8.94 × 10⁻⁴ Tg/yr or 893,522 kg/yr. This study [186] suggests a maximum emission factor for CSG co-produced water based on actual dissolved gas (CH₄) measurements of 0.031 tonnes per Megalitre (ML) of co-produced water. Also, this study [186] contrasted the CSG emission factor in the Surat Basin with an EPA Memorandum report that indicated co-produced water emissions for the 2020 Greenhouse Gas Intensity (GHGI) emission factors specifically for the Powder River Basin (PRB), U.S., is 0.22 lb/bbl (0.63 kg/m³) based on an assumed CH₄ concentration of 542.9 mg/L at 700 ft (~213 m) well depth. This CH₄ emission factor for coalbed gas (CBM) in the PRB is much higher than that for CSG co-produced water in the Surat Basin, likely due to the PRB’s freshwater origin. However, in the PRB where over 30 trillion liters were estimated to be produced, 95% of groundwater biogenic CH₄ co-produced with coalbed gas is directly emitted into the atmosphere through impoundments (64%), direct discharge into aquatic environments (treated or untreated) (20%), surface irrigation for agriculture (8%), drip irrigation for agriculture (3%), and underground or subsurface injection control (5%) [1,17,187].

Methane in groundwater is in a dissolved state, much like carbonation in soda, but can also form pure gas pockets that can be released as CO₂ or CH₄ [1,17,188]. During gas production, groundwater rises to the surface as co-produced water, which is discharged into impoundments and aquatic systems, where decreases in atmospheric pressure and temperature cause CH₄ dissolved in the water to be released. Methane emissions from groundwater pumping across the U.S., while small (0.2% of the total annual U.S. CH₄ emissions relative to total global emissions), represent an important source that should be quantified and included in the global CH₄ budgets [188,189,194]. A significant portion of the groundwater pumped in the U.S. comes from wells in natural gas fields and from groundwater wells in which the CH₄ content is biogenic in origin [195,196,197]. The biogenic CH₄ either migrated from nearby coal aquifers [196] or was partly contaminated from gas wells [197]. These studies [196,197] emphasized that biogenic CH₄ emissions from groundwater withdrawal may be significant locally where groundwater CH₄ concentrations are high, up to 98% in the Denver–Julesburg Basin of northeastern Colorado, U.S. [197]. In some areas, the reaction of CH₄ emissions with volatile organic compounds (VOCs) results in ground- or low-level ozone formation [198]. For example, in the Powder River Basin, Wyoming, U.S., groundwater CH₄ is potentially emitted from more than 3000 surface impoundments and from CH₄ leaks [55] at >29,000 CBM wells [27]. CH₄ interacts with VOCs likely released through natural outgassing from disturbed coal beds in 12–15 operating surface coal mines that supply 43% of U.S. coal [44,198,199]. Unintended consequences from mining activity include fuel evaporation from heavy machinery, dust-suppression chemicals, and coal-mine water [1,17], which potentially contribute to ground-level ozone formation [44,198,199].

During drilling for coalbed gas (CBM) using vertical drilling techniques, as well as specialized stimulation and completion techniques to improve gas production, varying amounts of CH₄ are released from developmental gas wells. In addition to pressure drops, adsorbed CH₄ in the coal is released and captured at the surface in the wellhead, where gas is also emitted. The gas is collected through a multi-tiered infrastructure network comprising gas gathering, compression, and transportation systems [17]. Potential CH₄ emissions from these gas infrastructures originate from point sources at wellheads, from small-diameter gathering pipelines entering compression stations, and from feeders into larger-diameter transmission pipelines (interstate/intrastate) that transport gas over long distances to processing plants and storage facilities [17]. At the wellheads, CH₄ emissions continue during the abandonment stage after production ceases. The typical lifespan of CBM wells in the U.S. is 10–20 years.

Abandoned coalbed gas wells have the potential to serve as real-time producers and emitters of biogenic CH₄ as has been observed in abandoned coal mines. Real-time regeneration of biogenic CH₄ in coal beds is an ongoing process in abandoned coal mines on a human timescale [39,40]. The biogenic CH₄ produced from coalbed gas wells was generated in the coal aquifers in recent geologic time [22]. But new biogenic CH₄ can be regenerated and reproduced in much shorter time in bioreactor coal aquifers [1,17]. This prospect has led some to argue that coalbed gas (CBM/CSG) can be considered as a renewable or sustainable resource [1,17]. The debate about renewability stems from the different rates and conditions involving continuous but slow regeneration (thousands to hundreds years) and the accumulation of economic deposits. However, significant research has successfully addressed how to accelerate biogenic CH₄ production in coal “bioreactors” [1]. Methods such as pairing bioaugmentation and biostimulation by introducing an appropriate mix of specialized microbial consortia (hydrolytic bacteria and methanogens) and essential trace nutrients and electron acceptors have successfully regenerated new CH₄ yields in both laboratory and field pilot tests [1]. Research shows that combining both methods maximizes the breakdown of recalcitrant carbon within coal structures, yielding the highest CH₄ concentrations and the fastest production rates [17].

The precise number of total coalbed gas wells worldwide is unknown; most were drilled between the late 1990’s and the late 2000’s with production lifespans up to 20 years [17]. The lifespan indicates that a great number of these wells have ceased production and abandoned. It’s estimated that tens of thousands of coalbed gas wells exist globally based on well counts in the most productive coal basins. They include about 29,000 in the Powder River Basin (PRB) in the U.S., about 20,000 in the Western Coal Sedimentary Basin in Canada, about 18,000 in the Qinshui, Ordos, and Junggar Basins in China, and about 7500 in the Bowen and Surat Basins in Australia [1]. All these coal basins contain coals with various amounts of biogenic gas. About 5000 wells have been abandoned in the PRB. Research on potential CH₄ regeneration and emissions from abandoned coalbed gas wells was demonstrated by Nivitanont [55]. A comparison of CH₄ emission estimates from plugged and unplugged abandoned wells of conventional natural gas (CNG) and unconventional coalbed gas (UCG) in the U.S. is shown in Figure 15 [55]. The range of mean CH₄ emissions estimates (100–1000 mg CH₄/hr) for Wyoming (PRB) abandoned UCG wells is lower than for abandoned CNG wells across the entire U.S [55]. The highest CH₄ emitting UCG well in the PRB is 4.53 g CH₄/hr. Although the estimate of CH₄ emissions from UCG abandoned wells is low, the potential for long-term emissions is high due to the biogenic origin of the gas in the PRB. Similar fugitive CH₄ emissions from an abandoned coal exploration borehole in Queensland, Australia revealed a mean emission rate of 235 tonnes per annum [200].

The diverse coal sources, coupled with continuous biogenic CH4 regeneration in disturbed aquifers and the imprecision of inventories that separate low-concertation CBM from CMM, creates a scenario where total mitigation is technically undetectable and economically prohibitive. Also, it creates “blind spots” in biogenic CH₄ emissions.

6. Insights on “Blind Spots” in Coalbed Biogenic CH₄ Emissions for Mitigation

Legacy biogenic CH₄ emissions from coal mining and coalbed gas operations are critical “blind spots” because of their long-lasting impacts and significant contributions to GHG. These key “blind spots” include biogenic emissions from abandoned coal mines; mixing of biogenic CH₄ from coal with other sources such as wetlands, agriculture, and landfills; CBM/CSG co-produced water; and groundwater drawdown from coal mining and coalbed gas (CBM/CSG) dewatering operations. What these legacy “blind spots” in biogenic CH₄ emissions from coal have in common is the gap in basic knowledge due to limited ground-based data, which is influenced by variable coal properties (see Table 2).

The measurement/data “blind spot” relies on quantitative (e.g., number of samples, wells, mines) and qualitative (e.g., CH₄ flow rates and concentrations) measurements from coal mines especially CMM emissions estimated from VAM systems for national inventories [16,30,31,55,56,59,73,74,172,199]. For some reasons this study finds that CMM emissions measurement relies primarily on its flow rates than specific coal properties in the mines. Presumably, measuring emissions requires calculating the exact mass of CH₄ flushed out by multiplying the total VAM by CH₄ concentrations [73]. Gas concentrations are influenced by various interrelated coal properties (see Table 2). Also, bottom-up measurements derived from ground-based data are overreported in top-down estimates and require improvements to ground-based datasets and the addressing of methodology gaps [30,31]. Often, the high variability of coal properties within and between coal beds and between coal mines is overlooked creating uncertainties in accurately estimating, forecasting, and modeling CH₄ emissions. This is a significant challenge for mitigation, given that coalbed CH₄ is a complex mixture of thermogenic, biogenic, and mixed gases generated from early to late stages of the geologic time. Moreover, the biogenic CH₄ has been generated during recent geologic time to the present day (see Table 1). The atmospheric mixing of biogenic CH₄ emissions from coal with other recently generated anthropogenic (e.g., agriculture such as cattle feedlots/grazing, piggeries) and natural sources (e.g., wetlands) is a key “blind spot” that further complicates mitigation [137,199,200]. Moreover, “blind spots” in biogenic CH₄ emissions from surface coal mines, flooded abandoned coal mines, coalbed gas wells, and groundwater CH₄, have been overlooked for mitigation due to being “unstructured” source of fugitive CH₄ emissions.

“Unstructured” fugitive emissions, which are key “blind spots” are unintentional diffuse leaks of low-concentration biogenic CH₄ emissions released from coal-associated operations, facilities, and infrastructure. These “unstructured” emissions are sporadic, diffuse, and scattered across all stages of the coal and coalbed gas production over large basin-scale landscapes. Many technologies like large-scale satellites and aerial imaging, which are designed to detect strong, concentrated “point source hotspots,” are less effective for “unstructured” emissions due to inherent limitations in spatial resolution, sensitivity, and operational constraints [4,172]. Thus, mitigation of “unstructured” biogenic CH₄ emissions is difficult in many regions, particularly those with mixed coal mining, coalbed gas extraction, and agricultural/waste-related activities [137]. Here, the low-concentration signals from deep coal mining and coalbed gas extractions are drowned out by the high volume of surface biogenic CH₄ (e.g., wetlands, agriculture, landfills), a phenomenon defined as Urban/Rural Isotopic Noise [198]. However, in a coal basin in Queensland, Australia, a ground-based survey of CH₄ emissions of an abandoned coal exploration hole using a Quantum Gas LiDAR system did not interfere with multiple potential CH₄ sources (e.g., gas wells, pipelines, processing plants, coal mines, groundwater pumping, and cattle) [207]. The abandoned coal hole was drilled to a depth of 102 m and contains 27 coal beds. The mean emission rate from 1756 measurements taken in 7 days is 235 t/a 26.9 kg/hr, which is classified as a “super emitter.” This is one legacy hole of an estimated 130,000 abandoned coal explorations in Queensland [207].

More importantly, measurements and monitoring of biogenic CH₄ emissions from coal are strongly eclipsed by CMM emissions from active surface and underground coal mining operations. CMM is perceived as a major source, accounting for up to 33% of the total global fossil fuel CH₄ emissions during 2008–2017 period [14]. This perception is exemplified by Figure 16 [30], which shows satellite top-down measurements of large CH₄ “super emitters” (100/kg/h according to USEPA [208]) with flow rates ranging from <20 to >100 t/hr (<20,000 to >100,000 kg/hr) from global coal mine operations. Presumably, CH₄ emissions are from coal mining and supply, but the sites monitored in coal basins also include coalbed gas development operations with abandoned wells. So, coal operations as used here represent the complexity and totality of CH₄ emissions from coal worldwide. But it does not wholly account for the source of biogenic CH₄ emissions from abandoned coal mines (AMM), which are increasing in number as active mines are closed [1].

6.1. Key “Blind Spot” in Biogenic CH₄ Emissions from Flooded Abandoned Coal Mines

AMM accounts for a significant and growing share of global fugitive CH₄ emissions, which continues for years to decades as more coal mines are closed under Global Climate change policies. Thus, AMM represents a long-lasting impact and significant legacy. Between China and the U.S., the two largest coal-producing countries have closed about 37,500 coal mines since 2015–2017 [209]. AMM is often an overlooked and forgotten source of potent CH₄ emissions, estimated to contribute about 4.7 ± 0.94 Mt of CH₄ annually [209,210,211,212]. This makes the abandoned coal mine CH₄ emissions a major, but underreported source of atmospheric GHG. Abandoned coal mines in the European Union that have been closed since 2015 could emit 298 million cubic meters (MCM) of CH₄ per year [213]. Total AMM emissions were forecast to be 17% in 2010 and increase to 24% in 2050 [182].

The CH₄ emission estimates are based on the traditional concept that once coal mining ceases, groundwater pumping, used to keep the active underground mine drained, is usually halted leading to flooding of mine coals and workings. This should lead to a progressive reduction in the available AMM and, under hydrostatic pressure, to reduced gas desorption of residual CH₄ molecules stored in the coal matrix. Thus, several research studies assumed that CH₄ generation and desorption would cease upon flooding of the coal mine [182,183,214]. However, CH₄ adsorbed in the coal cleats/fractures may be available to flow as “free gas” towards a low pressure zone, such as the atmosphere of an abandoned coal mine [1,17]. In flooded, abandoned, underground coal mines, the gas reservoir hydrostatic pressures are reduced when small barometric pressure fluctuations occur in the coal mine atmosphere. This causes the mine to “breathe,” meaning air flows into the mine during periods of high barometric pressure promoting AMM emissions [183]. This process poses a threat to coal mine safety if there is a continuous biogenic CH₄ generation and buildup reach explosive conditions [32]. Thus, under this situation, mitigation is required, particularly flaring the AMM can create conflict with gas licensees in some countries [209].

The “blind spot” in the coal mine water flooding concept is that groundwater flooding also supplies nutrients that support the methanogenesis of living methanogens in the mine water, thereby biodegrading the coal and old timbers [39,40,215]. This real-time methanogenesis transformed the closed and flooded abandoned coal mines in the Ruhr and Saar region into a legacy of active and perpetual “bioreactors.” The 2004 Thielmann et al. [39] study estimated between 38% and 90% of the CH₄ at 13 sites sampled within 14 months from the Ruhr basin closed and flooded abandoned coal mines is microbial in origin [40,215]. This study [39] found that four out of six coal mine water samples contained living methanogenic microorganisms with an isotopic composition of 64‰ δ¹³C_CH4 and −439‰ δD. The study interprets biogenic CH₄ production as the oxidation of organic substances (coal and/or mine timber) to carbon dioxide (CO₂) by bacteria, with the CO₂ reduced by archaeal methanogens. At one study site (4), CO₂ levels rose after closure of the coal mine in 1929, and it’s assumed that microbial CO₂ accumulated over 70 years. In addition, the radiocarbon age of the CO₂ is approximately 13,000a (Years Before Present). In succeeding studies, other scientists performed isotope analysis of CH₄ collected from co-generation plants in the former coal mining areas at Ruhr and Saar during the 2019–2020 period [214]. This study [214] performed isotope composition analysis of 45 samples collected from 2019 to 2020, a 1-year period, indicating that microbial CH₄ (δ¹³C_CH4, −80‰) increased by 2% at a Saar site and 4% at a Ruhr site. Also, microbial CH₄ ranges from 42% to 44% at 3 sites in the Saar area and 69% to 78% in 3 sites at the Ruhr area [214].

The discovery of active CH₄-generating microbes in flooded abandoned coal mines reveals a legacy of indefinite continuous, in-situ sources of CH₄ emissions. That is, CH₄ production does not diminish over time as previously thought by Kholod et al. [182]. This discovery shifts the flooded abandoned coal mines from passive biogenic CH₄ emission sources to active, long-term or perpetual GHG threats, necessitating immediate monitoring, mitigation (e.g., flaring/capture for power generation), and serious reassessment of national carbon inventories based on more accurate and detailed data [209]. Also, real-time generation of biogenic AMM means official that data is likely underreports the actual contribution to global CH₄ emission. Thus, updating the status of flooded abandoned coal mines emissions should require standard field and laboratory methodologies for assessing microbial methanogenic activity in the mine waters of flooded coal mines to ensure the accuracy of a national inventory of AMM. Often, it is assumed that when coal mines are flooded by groundwater and surface water, gas is produced for only a few years after flooding or about 7 years [182]. However, this concept was not followed in the 2017 update of the database on AMM emissions of U.S. abandoned coal mines for inventory of U.S. Greenhouse Gas Emissions and Sinks [216]. The national AMM emissions inventory prioritized non-flooded abandoned coal mines as candidates for recovery and utilization opportunities. So, 91 flooded abandoned coal mines that closed from 1990 to 2002, which generated 95–4458 kg/hr (0.35–4.5 mmcfd) of CH₄ were removed from the AMM opportunities list for mitigation and were ineligible for offset credits to reduce or sequester GHG emissions.

6.2. Key “Blind Spot” in Mixing Biogenic CH₄ Emissions from Subsurface Coal and Other Recent Surficial Sources

Biogenic CH₄ from deep unmineable coal beds developed for CBM/CSG and from shallow mineable coal beds is overlooked as a significant sources of biogenic CH₄ emissions. Deep CBM biogenic CH₄ is primarily generated by reduction of CO₂ with molecular hydrogen (H₂), a process known as hydrogenotrophic methanogenesis [25,26]. The isotopic signatures of deep-generated hydrogenotrophic CBM mix and overlap with traditional δ¹³C and hydrogen isotope δD (deuterium) signatures formed by acetoclastic methanogenesis in shallow mineable coal beds. Production of these coalbed biogenic CH₄ from CBM/CSG development wells and infrastructures creates complex emissions. These emissions, in turn, mix with other biogenic CH₄ emissions originating from the surface, such as agriculture, landfills, and wetlands. Overlapping of these biogenic CH₄ isotopic signatures can be distinguished by new analysis of clumped isotopes [142]. This gas mixing from different sources is a key “blind spot,” which makes universal mitigation technically challenging. This requires advanced analytical techniques such as measuring the abundance of doubly substituted CH₄ clumped isotopes [142]. The high cost and extreme analytical complexity of these techniques render them largely unfeasible for widespread, large-scale, or routine, real-time monitoring of emission sites [142].

Another complicating factor in CBM/CSG development is the production of mixed thermogenic and biogenic CH₄, specially in high-rank coals. The key to distinguishing biogenic CH₄ from thermogenic CH₄ is the stable carbon isotope ratio (¹³C/¹²C), but this tool cannot reliably distinguish among different types of biogenic CH₄. Also, for stable isotopes, the isotope fraction is static and can be significantly altered by environmental processes. This makes it challenging to distinguish between recent microbial (biogenic) and geological (thermogenic) sources. Additionally, enrichment in ¹³C can result from secondary oxidation of CH₄ by methanotrophs before emission, thereby making biogenic CH₄ “appear” thermogenic in terms of its signature [217]. The primary method of distinguishing CH₄ of different origins is stable isotope analysis, which measures the ratios of ¹³C to ¹²C and deuterium to hydrogen. Methane-generating microbes or methanogens preferentially metabolize the lighter isotope (¹²C) over the heavier isotope (¹³C) [218]. This process is the same whether the microbes are consuming carbon formed in a wetland or landfill, on agricultural land, or from coal. However, wetlands and agricultural lands contain carbon derived from recent plant remains, whereas coal carbon originated from geologically ancient plant remains with different geobiological and geochemical properties and was affected by maturation processes over geologic timescales.

An in-depth, detailed study in the Surat Basin in Queensland, Australia where CSG development is adjacent to coal mines, cattle feedlots, grazing cattle, piggeries, landfills, raw water ponds, wastewater treatment plants, and urban centers are sources of biogenic CH₄ demonstrates what is possible in attribution of isotopic signatures [137]. This study shows that source attribution is possible when both δ¹³C_CH4 and δD_CH4 are measured for CSG, cattle, waste from feedlots and piggeries, and water treatment plants. However, in most field conditions, the use of δ¹³C _CH4 alone is not possible for attribution. Measurements of both isotopic signatures will help in source apportionment where plumes are from mixed source.

Coals are composed of ancient plant remains deposited in coastal and fluvial peatlands, which were “metamorphosed” by high pressure and temperature during variable geologic and tectonic burial [1,17]. Generally, plants are classified as C3 or C4 based on how they initially fix CO₂ during photosynthesis (see Table 3). The C3 plants (e.g., wheat and rice) produce a 3-carbon molecule as their first stable product, generally favored by cool, temperate climates [116,118,121]. The C4 plants (e.g., maize and sugarcane) produce a 4-carbon molecule and are better adapted to hot, sunny, dry climates. The evolution and ecological expansion of C3 and C4 plants are closely linked to geological climate shifts, particularly atmospheric CO₂ concentrations. Ancient C3 plants (old carbon) and modern C4 plants (new carbon) both share low δ¹³C signatures, so identifying the specific, abrupt shift to C4 dominance (roughly 24–35 Ma) with C4 grassland expansion (about 5–10 M (117,119,122] can be difficult. In coal, plants are presumably more depleted in ¹³C than wetland and agricultural plants relative to the atmosphere (see Table 3) [134]. Ancient C3 plants, which formed coal during the Paleozoic and Mesozoic Eras, are even more depleted than modern plants using the C4 photosynthetic pathway [115,116,117,118,134,135]. Consequently, C3 plants are significantly depleted in ¹³C, giving them a more negative isotope ¹³C signature (a measure of ¹³C/¹²C ratio). The C4 plants in modern wetlands are less depleted in ¹³C and have more negative isotope ¹³C values than C3 plants, but this requires further investigation [115,116,117,118,134].

The “C4 revolution” is only clearly identifiable when C4 biomass is high, making the gradual rise of C4 plants difficult to distinguish from shifts in C3-backgrounds, often referred to as a “faint” signal [115,116,117,118,119,122,134,135]. This “blind spot” creates a major challenge in isolating “fresh” surface (e.g., wetlands, landfills) signatures from deep, reactivated ancient carbon deposits (e.g., coal) as “isotopic signals overlap” [118,119,122]. The overlap of isotopic signatures between hydrogenotrophic methanogenesis in deep coal beds and acetoclastic microbial CH₄ in shallow wetlands (which can overlap in the δ¹³C_CH4 from −65‰ to −50‰) creates a significant geochemical “blind spot” for source apportionment. This makes it difficult to identify the specific origin of biogenic CH₄ when these sources are nearby [219]. Furthermore, the microbial oxidation fringe in the vadose zone acts as a dynamic filter that preferentially consumes the lighter carbon isotope (¹²C), causing the residual CH₄ to become enriched in heavy isotopes (¹³C and ²H/D). This fractionation can shift the isotopic signature of biogenic CH₄ toward that of thermogenic CH₄, masking its original biogenic identity [220]. The ambiguity in source attribution complicates the measurement of the precise contribution of modern biogenic versus ancient sources, hampering effective mitigation of CH₄ emissions.

6.3. Key “Blind Spot” in Biogenic CH₄ Emissions from CBM/CSG Co-Produced Water

A significant knowledge gap in research on the source of biogenic CH₄ emission is the co-production of groundwater CH₄ from coal mines and coalbed gas operations. Groundwater CH₄ in coal-bearing basins and regions worldwide has been underestimated and is a “blind spot” as a significant source of atmospheric biogenic CH₄ emissions during coal operations [1,17,186]. Significant uncertainty surrounds the contribution of groundwater-derived biogenic CH₄ to atmospheric CH₄ emissions, largely because its extraction during coal operations has been overlooked in the global carbon budgets, which are estimated at 3.9 ± 6.2 mmol/m²/day or up to 70% of CH₄ emissions [186,221]. Moreover, measuring and quantifying groundwater CH₄ emissions are challenging due to mixing with biogenic CH₄ from anthropogenic and natural sources, diverse management strategies for co-produced water, multiple and unpredictable escape pathways, variable amounts of co-produced water, and limited data due to the difficulty of measurement in many developed gas fields of coal basins. Groundwater CH₄ discharge into surface aquatic systems is often diffuse and hard to quantify, making measurements of its concentration and discharge rate across locations and over time extremely difficult to obtain and thus difficult to mitigate. A large portion of the groundwater CH₄ is oxidized and consumed by methanotrophic bacteria before it can escape to the atmosphere [221]. Methanotrophic bacteria consume groundwater CH₄ mainly at temperatures of 15–35 °C, near neutral pH levels (6.5–7.5), and in oxidizing environments [222,223,224]. Their physiology is highly adaptable, allowing them to thrive across a broad range of subsurface conditions. Aerobic CH₄ oxidation requires the presence of dissolved oxygen (between 1.0 and 8.0 mg/L dissolved oxygen) and positive Oxidation-Reduction Potential (ORP) (typically (>100 mV) [222,223,224]. These studies suggest advanced field-monitoring approaches for accurately assessing net GHG contribution. They include closed-dynamic flux chambers over groundwater discharged zones, stable isotope analysis (δ¹³C_CH4 and δ¹³C_CO2), and tracer tests with inert gases (e.g., sulfur hexafluoride) [225]. This microbial filter is a critical control on atmospheric emissions, but its efficiency varies, adding another layer to uncertainty in net flux estimates. Groundwater can release biogenic CH₄ directly into ponds, creeks, rivers, and streams, where it is emitted into the atmosphere via diffusion and ebullition (bubbling), but both processes are highly variable [188,221]. This emission, in turn, is mixed with biogenic CH₄ sourced from anthropogenic and natural sources.

A key “blind spot” in measuring CH₄ emission factors from co-produced water in CBM/CSG development is reliance on ground-based data collection and analysis methods. This is exemplified by the study of the U.S. Environmental Protection Agency [USEPA, 220] study that established the 2020 Inventory of U.S. Greenhouse Gas Emissions and Sinks (GHGI) using CH₄ data from CBM co-produced water. The focus is on co-produced water CH₄ in the Powder River Basin (PRB), Wyoming, and Black Warrior Basin (BWB), Alabama [1,17,25,26,193,226]. For the PRB, the calculation of the 2020 GHGI used a base emission factor (EF) of 2.0522 × 10⁻⁹ Gg CH₄/gallon of water drained [226]. For the BWB, the calculation of the 2020 GHGI used a base EF of 2.0694 × 10⁻³ Gg CH₄/well. These EF values were estimated using an assumed CH₄ concentration in co-produced water of 542.9 mg/L (0.19 lbs/bbl) at a well depth of 233 m (700 ft) [226]. This sole dataset was used to estimate EFs for CH₄ emissions from co-produced water of the PRB/BWB wells from the 1900 to 2018 period. The co-produced water CH₄ emissions of the PRB (43–50,005 metric tons), which are biogenic in origin, are about 3.5 times higher than the co-produced water CH₄ emissions in the BWB (2768–12,796 metric tons), which are biogenic/thermogenic in origin, (Figure 17) [226]. Interestingly, the bar graphs of co-produced water CH₄ emissions for both coal basins indicate relative stability and consistency, with no significant change during the 13-year period (2005–2018). Whether these patterns result from using only CH₄ concentration data from water at 233 m (700 ft) well depth is unknown because details are unavailable [226]. Statistical accuracy requires a larger sample size considering that the datasets of CBM wells in the PRB include ~29,000 wells ranging from 60–914 m (198–3000 ft) in depths and in the BWB include ~5500 wells ranging from 91–726 m (300–2500 ft) in depths [25,80,218]. Thus, the dataset used to develop the 2020 U.S. GHGI may not be representative of the key co-produced water CH₄ emissions, hindering efforts to track mitigation strategies and provide accurate data for future climate policy.

6.4. Key “Blind Spot” in Biogenic CH₄ Emissions from Groundwater Drawdown

Biogenic CH₄ emissions from groundwater during CBM/CGS and coal mining extraction are a “blind spot” driven by the process of groundwater drawdown or water-level decline. The lowering of groundwater level is caused by pumping water from local well sites to gas-field well sites to desorb gas in coal aquifers [1,17]. During CBM/CSG development, large amounts of biogenic CH₄ are gathered from gas wells, processed by gas compressors and treatment plants, and transported by pipeline networks on the surface. During this coalbed gas extraction process, large volumes of co-produced water containing dissolved CH₄ are discharged into surface aquatic systems where the change in temperature and pressure causes the water to degas and CH₄ diffuses out of the water at the water-air interface.

The result of extracting CBM/CSG co-produced water is groundwater drawdown. To desorb CH₄ in the coal reservoirs into the well, reservoir hydrostatic pressure is reduced by pumping groundwater out of the coal aquifer, thereby desorbing and diffusing gas in the coal matrix and allowing it to flow through cleat/fractures into the borehole. Groundwater withdrawal from a coal reservoir or aquifer results in a cone of depression in the water level in the well. However, several hundred pumping wells in a coalbed gas field expand the cone of depression, causing groundwater levels to decline regionally as gas and water production increase. For example, in the central PRB, groundwater drawdown in the Big George coal is up to 305 m at the gas field’s center and covers areas from 350 to 1300 km² (Figure 18) [1,17]. Dewatering at the 12 surface coal mines (north, middle and south mines) along the PRB eastern margin resulted in a groundwater-level decline of up to 35 m. The area of groundwater drawdown ranges from 1350 to 1650 km² (Figure 18).

The stratigraphic connectivity of the coal zones both mined for coal and extracted for CBM (dual-role coal beds) in the PRB created a groundwater drawdown covering 52,000–56,000 km² area (Figure 18) [1]. The groundwater drawdown began with the large-scale surface coal mining and coalbed gas extraction during the 1970s to 1980s, respectively. This combined coal operations resulted in biogenic CH₄ emissions from a dense, high-impact super-emitter region characterized by significant fugitive leaks and venting. However, mitigating these intense regional biogenic CH₄ emissions is technically infeasible due to a conflict of interest between two operators. This conflict is highlighted by the overlapping groundwater drawdown (Figure 18) caused by the dewatering of the coal mines (red contour lines) and CBM wells (black contour lines). The overlapping groundwater drawdown areas create a legal nightmare when assigning responsibilities for CH₄ emissions to the coal operators. Also, mitigating CH₄ emissions from groundwater drawdown is complicated by the long timeframe required for restoring regional groundwater levels through natural surface recharge, which may be stymied by climate change.

In summary, groundwater CH₄ released during aquifer pumping such as dewatering coal beds for mining or CBM/CSG development, is often excluded from the global carbon budget. That is, it is notoriously difficult to quantify and has historically been treated as a localized water-quality or safety issue rather than a broad atmospheric emission source [13,15,192,228]. Dissolved CH₄ in water is highly variable and often goes unquantified. Emissions depend heavily on pumping rates, water usage, and the physical characteristics of depressurizing water at the surface [188]. Historically, emissions reporting has been divided strictly by sector [186]. Groundwater management falls under water resource agencies, while CBM/CSG extraction or coal mining operations fall under energy and resource departments. The volume of fugitive emissions from pumping deep, gassy aquifers was assumed to be negligible compared to large-scale fugitive emissions from operational coal and gas wells [228]. However, emerging research indicates this underestimation leaves a major “blind spot” in fossil fuel-related GHG tracking [31,60]. Current scientific consensus emphasizes that widespread groundwater pumping requires systematic quantification to fully resolve the inconsistencies in worldwide GHG accounting [188]. Researchers actively call for standardized methodologies for dissolved gas measurements to accurately fold these emissions into both bottom-up inventories and top-down atmospheric inversion methods.

7. Conclusions

Methane is a potent, short-lived GHG, and its mitigation offers a rapid and significant opportunity to slow the rate of near-term global warming. Literature review indicates human activities are responsible for about 60% of total global CH₄ emissions and have caused atmospheric concentrations to more than double since the pre-industrial period. Anthropogenic and natural sources (e.g., wetlands, agriculture, landfills) are significant contributors (85%) of the recent surge of biogenic or microbial CH₄ emissions in the atmosphere during the last decade. The recent discovery of biogenic CH₄ in coal released during various stages of coal mining and coalbed gas operations has contributed to the surge. “Unstructured” (unintended, low-concentration, and diffused fugitive emissions) sources of biogenic CH₄ emissions from coal mine and coalbed gas extractions are ignored favoring traditional mitigation of CMM. This includes capturing high CH₄ concentrations and expelling low CH₄ concentrations VAM. Since the 1980s, the discovery of biogenic CH₄ in coals has revealed that it occurs in high proportions (40–100%), dynamic, and continuous process of generation, which is mixed with thermogenic CH₄ in coal reservoirs. The biogenic CH₄ is commonly mixed with thermogenic CH₄ in high-rank coals (bituminous and anthracite). Low-rank coals (subbituminous) commonly form biogenic CH₄. The biogenic CH₄ emissions from coal sources create key “blind spots” in accurate ground-based measurement of the gas and attribution of overlap isotope signatures of anthropogenic and natural sources. Most importantly, it creates “blind spots” in legacy biogenic CH₄ emissions from active “bioreactors” in flooded abandoned coal mines, co-produced water/groundwater CH₄ surficial collection reservoirs, and coalmine/coalbed gas wet facilities. Also, decades of extensive dewatering of the same coal aquifers in coal mines and coalbed gas wells at a basin scale create legacy groundwater drawdowns (lowering of water tables). These water depleted, regional coal aquifers take centuries to restore by surface recharge, which is stymied by climate change.

A critical “blind spot” in biogenic gas CH₄ emissions is the use of ground-based measurement and analysis methods, which are affected by variable coal properties (e.g., coal rank, composition, depth, porosity/permeability), influencing gas flow rates. Measurements often are limited by the number of samples used for national inventories. The variability of coal properties in coal mines often influences the bottom-up and top-down measurements required by the UNFCCC Tier system. Bottom-up measurements frequently underestimates CH₄ emissions compared to top-down satellite observations because they rely on static emission factors, miss unpredictable “super-emitter,” omit unmonitored infrastructure, and delimited by detection capabilities. Detection limits indicate that high-resolution satellites (e.g., GHGSat, SWIR) can detect individual CH₄ plumes (<100 kg/hr) with precision but smaller or diffuse, low-concentration CH₄ leaks from coal mine and coalbed gas operations may remain below the detection limit. Another key “blind spot” is mixing conditions of thermogenic and biogenic CH₄ from deep to shallow coal beds, which also intermix with surface-generated biogenic CH₄ from anthropogenic and natural sources. Although carbon (δ¹³C) and hydrogen (δD) isotopes of CH₄ allow distinction between biogenic CH₄ from different anthropogenic and natural sources, overlapping of isotope signatures and spatial resolution constraints of current remote sensing remains a significant challenge. Orbital instruments are mainly blind to isotope types when viewed in wide-angle swaths. Thus, in coal basins where coal mines, CBM/CSG wells, livestock feedlots, and water treatment plants coexist, the biogenic CH₄ emissions are spatially blended into a single, massive “hot spot” signal.

The most important “blind spots” are “unstructured” biogenic CH₄ emissions from coal mining and coalbed gas extractions from dual-role coal beds, which create large-scale regional diffused and dispersed emissions. These basinwide biogenic CH₄ emissions from coal-based extractions intermingle with fluxes from coal mine facilities and CBM/CSG infrastructures, as well as fluxes from ranchlands and agricultural lands. CBM/CSG infrastructures (e.g., gas compressors, pipelines, treatment plants) produce dispersed and regional-scale footprints of biogenic CH₄ emissions. Additionally, biogenic CH₄ emissions are sourced from active, unplugged, and abandoned/orphaned wells. Coal mining facilities outside mine sites such as coal storage areas, spoil piles, preparation plants, and transportation systems, are common sources of fugitive CH₄ emissions. Aquatic systems associated with coal preparation plants such as silting ponds are known as “bioreactors” that generate real-time biogenic CH₄ emissions. These multiple sources of mix biogenic CH₄ emissions, which are low concentration, diffuse, and scattered basinwide, make mitigation technically and economically unrealistic. The technical constraints are physical interventions of fragmented micro-leaks, which are difficult to detect, measure, and pinpoint. Economically, the capital investment to build and maintain basinwide capture systems far outweighs the value of the recovered gas.

Other critical by-products of coal mining and CBM/CSG extractions, which are major “blind spots” in biogenic CH₄ emissions, are large volumes of groundwater or co-produced water with dissolved CH₄ discharged into diverse surface aquatic systems from wellheads. Massive dewatering of coal aquifers associated with coal mining and CBM/CSG extractions requires thousands of closely spaced wells to depressurize and degas coal reservoirs. The co-produced water contains living archaea, bacteria, nutrients and soluble organic precursors (e.g., coal fines from drilling coal and carbonaceous beds), which are transported within various surface aquatic ecosystems. The spread of microbial communities can initiate biodegradation and methanogenesis in new, non-target areas expanding the mitigation perimeter. Another “blind spot” is that groundwater withdrawal to desorb biogenic CH₄ in coal aquifers causes groundwater drawdown. The pumping of large volumes of water creates an extensive, localized drop in the water table, resulting in a cone of depression that expands over large areas and can lowering water tables in nearby agricultural or domestic water-supply wells. Also, residual biogenic CH₄ in coal beds above the water table will be slowly released to the surface through wells, faults, fissures, and fractures. Mitigating the release of dissolved CH₄ from groundwater during coal mining and CBM/CSG development may be viable by gas stripping through osmosis but could be inefficient and costly. Another critical mitigation method is plugging abandoned CBM/CSG wellbores with cement and mechanical plugs.

Finally, a major “blind spot” in biogenic CH₄ emissions is the contribution from flooded, abandoned underground coal mines, which are often ignored as potential legacy emissions. Research on abandoned coal mines reveals flooded old-mine workings as functioning massive, long-term, passive “bioreactors” where methanogens actively metabolize coal beds and old timbers in real time. Thus, flooded, abandoned coal mine “bioreactors” are a latent perpetual engine of biogenic CH₄ emissions that continue to increase as thousands more mines are closed. Repurposing biogenic CH₄ emissions in flooded abandoned coal mines is a missing policy target for developing GHGI. Emerging research indicates that flooded abandoned coal mines and other “unstructured” sources of biogenic CH₄ emissions from coal and coalbed gas extractions are largely uncounted in official national inventories. Research requires standardized measurement and analysis methodologies to accurately fold biogenic CH₄ emissions from coal resources into both bottom-up and top-down national inventories.