Search Results (35)

The Lower-Middle Jurassic Xishanyao Formation in the Nileke Sag of the Yili Basin contains substantial reserves of coal and coalbed methane (CBM). Elucidating its depositional evolution and the controlling factors of coal accumulation within a sequence-stratigraphic framework is crucial for guiding future exploration. This study integrates regional geological surveys, core observations, well-log analysis, and quantitative lithofacies statistics of the lower member to establish a sequence-stratigraphic framework and reconstruct the sedimentary paleogeography. Eleven minable coal seams are identified, exhibiting a depositionally controlled spatial thickness distribution. The coal is classified as low-rank bituminous (Rank I–II), characterized by high inertinite, low ash, medium-high volatile matter, and ultra-low sulfur content, indicating formation in a freshwater swamp influenced by seasonal droughts and floods. Three third-order sequences (SQ1–SQ3) are recognized. SQ2, deposited during peak transgression as a braided-river delta plain, provided the optimal environment for peat accumulation. In contrast, SQ3 is dominated by progradational deltas with coarser sediments, where coal accumulation weakened. The results demonstrate that coal accumulation was jointly controlled by tectonic subsidence (providing accommodation space), climate (causing peat oxidation and fine-clastic input), and sedimentation (with interdistributary bays on the delta plain being the most favorable sites). Coal accumulation in the Lower Xishanyao Member resulted from the coupling of tectonic, climatic, and sedimentary processes. This genetic model provides a theoretical basis for regional coal and CBM exploration. Full article

The formation of hard coal seams is the outcome of multi-stage, complex transformations of organic matter that lead to an increase in carbon content, a decrease in volatile components, and a progressive evolution of the rock’s structure and texture. Diagenetic and metamorphic processes, which underpin coal formation, largely determine its petrographic and geochemical characteristics, but they are not the only factors controlling the final properties of coal. An equally important role is played by the tectonic history of the region in which the coal seams occur. In this study, we carried out an integrated analysis of coal rank, based on vitrinite reflectance measurements (R₀), and mechanical properties, using Vickers microhardness tests (Hv). Coal samples were collected from both sides of a fault plane within a single seam. The results show that the presence of the fault is clearly reflected in the measured parameters. Vitrinite reflectance generally increases towards the fault zone, but in the immediate vicinity of the fault, it exhibits a slight decrease. Subtle yet systematic changes are also observed in microhardness, particularly in the Hv values. The results show that vitrinite reflectance (R₀) and microhardness (Hv) vary in a very similar manner—both parameters decrease as the degree of structural degradation of coal increases within the fault zone. This consistent response of R₀ and Hv to local structural damage suggests that they may serve as sensitive indicators of the presence and extent of influence of small-scale tectonic dislocations. Their combined application provides additional information on the potential occurrence of a fault and on the degree of structural disturbance of coal in its vicinity. Full article

Mining above confined aquifers fundamentally depends on understanding the evolution of floor permeability for water hazard control and water conservation mining. A mechanical model was developed to characterize the coordinated deformation of floor aquiclude strata, accounting for non-uniform distributions of stress and water pressure. The competing mechanisms whereby neutral plane strain and flexural deflection dominantly control permeability at different dip angles were elucidated, and the influence of dip angle on the stability of the water-resistant key strata was quantified. On this basis, a quantitative method for assessing the feasibility of in situ water conservation mining above confined aquifers was developed and its effectiveness was verified through field application. The main findings are as follows: The deflection of the floor aquiclude increases with water pressure, advance distance, and panel length. Larger coal seam dip angles correspond to smaller aquiclude deflection, with a strong dependence on the water pressure treatment method. The equivalent permeability of the floor increases with water pressure, panel length, and advance distance, and its variation is most pronounced with water pressure. As the dip angle increases, the equivalent permeability exhibits a trend of first rising and then decreasing; the transition between deflection-dominated and neutral plane strain-dominated control occurs at a dip angle of 35°. Lithological assemblage is found to govern the position of the neutral plane and the bending stiffness matrix, while a soft–hard interbedded floor is effective in suppressing deformation and mitigating the increase in the equivalent permeability. For inclined aquiclude key strata, the ranking of zones most prone to failure and water inrush is as follows: lower end > upper end > coal wall position > behind the goaf. A quadratic multi-parameter response model for the mining-induced equivalent permeability at the Fenyuan Coal Mine is established, yielding the sensitivity ranking under single factor and interaction effects as follows: water pressure > panel length > advance distance > water pressure (quadratic) > water pressure × panel length interaction. The higher the water pressure, the stronger the influence of dip angle on the equivalent permeability. Groundwater ion evolution is dominated by dissolution/leaching, with sulfate (SO₄²⁻) serving as a diagnostic ion for source identification. The stepwise criteria and grouting-reinforcement parameters for in situ protection of confined aquifers are proposed. Using water quality and quantity as evaluation metrics, Working Face 5-103 at the Fenyuan Coal Mine, which is a large-inclination-angle and high-pressure working face, has achieved in situ protection of the floor water. Full article

Ecosystem services (ES) are a key bridge connecting natural ecosystems with human social development. The core significance of ecosystem service value (ESV) is to quantify the contribution of ecosystems to human well-being. The mining of mineral resources causes disturbance to the structure, function, and value of ecosystems. This study focuses on the high groundwater level coal–grain overlapping areas in eastern China, the mining of mineral resources has led to widespread loss of cropland and carbon sinks in the region. Considering the particularity of ecosystem evolution caused by coal mining subsidence, we developed multiple land use demand scenarios under dual objective constraints based on PIM and Markov chain, including Inertial Development (ID), Food Security (FS), Urban Expansion (UE), Ecological Restoration (ER). The PLUS model was used to simulate the spatial changes of land use and the equivalent factor method was used to calculate the changes in ESV, exploring the best path to improve the ecological benefits of the coal–grain overlapping areas. The results indicate that: (1) By 2030, the study area will add 54,249.09 ha of coal mining subsidence, mainly mild and moderate subsidence, and cropland being the most affected by subsidence among all land types. (2) In the multi-scenarios, the total ESV is ranked as follows: ecological governance scenario (CNY 51.21199 billion) > ID scenario (CNY 51.0898 billion) > food security scenario (CNY 48.4767 billion) > UE scenario (CNY 48.27157 billion). Among them, the ER scenario achieves all individual ESV gains and has the highest overall ESV. (3) Spatial analysis shows that in the ER scenario, the ESV of mining townships significantly increases and the ESV gap between other townships has decreased. However, the FS scenario and UE scenario have led to widespread degradation of ESV between various townships in eastern mountainous areas, and severe degradation of ESV in some urban townships. This study validated the accuracy and applicability of the PLUS model in medium scale and plain regions. The study has confirmed our hypothesis that reasonable land use and ecological restoration methods can achieve Pareto improvement in regional ESV, provided a holistic and local dialectical perspective for related research, and a scientific basis for the sustainable development of coal grain overlapping areas. Full article

Water within coal reservoirs exerts dual effects on methane adsorption–desorption by competing for adsorption sites and reducing permeability. The bound water effect, caused by coal wettability, significantly constrains coalbed methane (CBM) production, rendering investigations into coal wettability crucial for efficient CBM development. Compared with other geological formations, coals are characterized by a highly developed microporous structure, making the CO₂ sequestration mechanism in coal seams closely linked to the microscale interactions among gas, water, and coal matrixes. However, the intrinsic mechanisms remain poorly understood. In this study, molecular dynamics simulations are employed to investigate the wettability behaviors of CO₂, CH₄, and water on different coal matrix surfaces under varying temperature and pressure conditions, for coal macromolecules representative of four coal ranks. The study reveals the evolution of water wettability in response to CO₂ and CH₄ injection, identifies wettability differences among coal ranks, and analyzes the microscopic mechanisms governing wettability. The results show the following: (1) The contact angle increases with gas pressure, and the variation in wettability is more pronounced in CO₂ environments than in CH₄. As pressure increases, the number of hydrogen bonds decreases, while the peak gas density of CH₄ and CO₂ increases, leading to larger contact angles. (2) Simulations under different temperatures for the four coal ranks indicate that temperature has minimal influence on low-rank Hegu coal, whereas for higher-rank coals, gas adsorption on the coal surface increases, resulting in reduced wettability. Interfacial tension analysis further suggests that higher temperatures reduce water surface tension, cause dispersion of water molecules, and consequently improve wettability. Understanding the wettability variations among different coal ranks under variable pressure–temperature conditions provides a fundamental model and theoretical basis for investigating deep coal seam gas–water interactions and CO₂ geological sequestration mechanisms. These findings have significant implications for the advancement of CO₂-ECBM technology. Full article

The mechanism of low- to middle-rank coal seam gas accumulation in the Baode block on the eastern edge of the Ordos Basin is well understood. However, exploration efforts in the Shizuishan area on the western edge started later, and the current understanding of enrichment and accumulation rules is unclear. It is important to systematically study enrichment and accumulation, which guide the precise exploration and development of coal seam gas resources in the western wing of the basin. The coal seam collected from the Shizuishan area of Ningxia was taken as the target. Based on drilling, logging, seismic, and CBM (coalbed methane) test data, geological conditions were studied, and factors and reservoir formation modes of CBM enrichment were summarized. The results are as follows. The principal coal-bearing seams in the study area are coal seams No. 2 and No. 3 of the Shanxi Formation and No. 5 and No. 6 of the Taiyuan Formation, with thicknesses exceeding 10 m in the southwest and generally stable thickness across the region, providing favorable conditions for CBM enrichment. Spatial variations in burial depth show stability in the east and south, but notable fluctuations are observed near fault F1 in the west and north. These burial depth patterns are closely linked to coal rank, which increases with depth. Although the southeastern region exhibits a lower coal rank than the northwest, its variation is minimal, reflecting a more uniform thermal evolution. Lithologically, the roof of coal seam No. 6 is mainly composed of dense sandstone in the central and southern areas, indicating a strong sealing capacity conducive to gas preservation. This study employs a system that fuses multi-source geological data for analysis, integrating multi-dimensional data such as drilling, logging, seismic, and CBM testing data. It systematically reveals the gas control mechanism of “tectonic–sedimentary–fluid” trinity coupling in low-gentle slope structural belts, providing a new research paradigm for coalbed methane exploration in complex structural areas. It creatively proposes a three-type CBM accumulation model that includes the following: ① a steep flank tectonic fault escape type (tectonics-dominated); ② an axial tectonic hydrodynamic sealing type (water–tectonics composite); and ③ a gentle flank lithology–hydrodynamic sealing type (lithology–water synergy). This classification system breaks through the traditional binary framework, systematically explaining the spatiotemporal matching relationships of the accumulated elements in different structural positions and establishing quantitative criteria for target area selection. It systematically reveals the key controlling roles of low-gentle slope structural belts and slope belts in coalbed methane enrichment, innovatively proposing a new gentle slope accumulation model defined as “slope control storage, low-structure gas reservoir”. These integrated results highlight the mutual control of structural, thermal, and lithological factors on CBM enrichment and provide critical guidance for future exploration in the Ningxia Autonomous Region. Full article

The eastern Junggar Basin in Xinjiang, China is a key coal-bearing region dominated by the Middle Jurassic Xishanyao Formation. Despite its significance as a major coal resource base, detailed paleoenvironmental reconstructions of its coal seams remain limited. This study investigates the B₁, B₂, B₃, and B₅ coal seams of the Xishanyao Formation using X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) to assess geochemical indicators of the depositional environment during coal formation. The results show that the coal samples are characterized by high inertinite content and low vitrinite reflectance, indicative of low-rank coal. Slight enrichment of strontium (Sr) was observed in the B₁, B₂, and B₅ seams, while cobalt (Co) showed minor enrichment in B₃. Redox-sensitive elemental ratios (Ni/Co, V/Cr, and Mo) suggest that the peat-forming environment ranged from oxidizing to dysoxic conditions, with relatively high oxygen availability and strong hydrodynamic activity. A vertical trend of increasing paleosalinity and a shift from warm–humid to dry–hot paleoclimatic conditions was identified from the lower (B₁) to upper (B₅) coal seams. Additionally, the estimated atmospheric oxygen concentration during the Middle Jurassic was approximately 28.4%, well above the threshold for wildfire combustion. These findings provide new insights into the paleoenvironmental evolution of the Xishanyao Formation and offer a valuable geochemical framework for coal exploration and the assessment of coal-associated mineral resources in the eastern Junggar Basin. Full article

The first coalification jump (FCJ) has a significant effect on changes in the microstructural properties of coal and plays a crucial role in understanding the efficient utilization of low-rank coal. One lignite (QSY-2), two subbituminous (MHJ-10 and YCW-2), and three high-volatile A-grade bituminous coals (YX-12, JF-18, and HY-5) from the Ningdong coalfield were selected for research, avoiding the influence of regional geology. The evolution characteristics of the microstructures before and after the FCJ were investigated via spectroscopic experiments. The complex and unstable molecular structure of low-rank coal gradually decomposes and polymerizes at 350 °C. The aliphatic structure shows a V-shaped change trend as metamorphism increases. The inflection point is around an R_o of 0.6%. Demethylation and polymerization occur simultaneously during the FCJ. The reconnection of benzene substances with the aromatic ring increases the density of aromatic rings in the YCW-2 sample, significantly enhancing its aromaticity. The removal of oxygen-containing functional groups, especially methoxy and carbonyl groups, provides the possibility for the formation of CH₄ and CO₂ during the metamorphosis of lignite to subbituminous coal. Furthermore, high temperatures result in a loss of moisture content during the FCJ, which is the primary factor leading to a reduction in the hydroxyl content in coal. The selected samples are primarily composed of organic matter, with low levels of heteroatoms in the coal. It is preliminarily determined that coalification is not significantly affected. This study provides a theoretical foundation for investigating the molecular structure evolution of low-rank coal during the FCJ. Full article

To address the issues of severe surrounding-rock failure and ground support component failure in advancing working-face driving roadways (AWFDRs) in ultra-close coal seams, this study used the 5202 air-return roadway in Huaye Coal Mine as a case study and for engineering background. Numerical simulation, theoretical analysis, and industrial application methods were adopted to analyze the laws of the dynamic evolution of vertical stress in such roadways. The mine pressure behaviors of AWFDRs in ultra-close coal seams were also clarified, thereby enabling the proposal of a solution; namely, zoned support technology. The results show that the 5202 air-return roadway, as an AWFDR in an ultra-close coal seam, exhibits five different characteristic behaviors of mine pressure zones during excavation. Zone 1 is influenced by the adjacent working-face mining under goaf; Zone 2 is influenced by the adjacent goaf lateral abutment stress under goaf; Zone 3 is influenced by the stress of the overlying solid coal; Zone 4 is influenced by the adjacent goaf lateral abutment stress under the overlying solid coal; and Zone 5 is influenced by stabilized stress under the overlying solid coal. The mine pressure behaviors of these zones were ranked, from most intense to weakest, as follows: Zone 3 > Zone 1 > Zone 4 > Zone 2 > Zone 5. Based on this, a basic support scheme was proposed, which involves using bolt–mesh–beam supports combined with shed supports under the goaf and bolt–mesh–beam supports combined with roof anchor cables under the overlying solid coal. Additionally, in Zones 1 and 3, roof anchor cables or rib anchor cables were supplemented as reinforcing supports, which were combined with the basic support scheme described above to form a zoned support scheme for the AWFDR. The analysis of mine pressure behavior and implementation of a zoned support scheme for AWFDRs in ultra-close coal seams provides technical and engineering references for roadway supports under similar mining conditions. Full article

Permeability variations during coalbed methane (CBM) production remain a critical research focus. However, existing studies have primarily concentrated on nearly horizontal reservoirs, with limited in-depth analyses of the dynamic evolution of permeability in steeply inclined coal reservoirs (SICRs) (>45°). This study examined the dynamic permeability characteristics across different reservoir dip angles, comparing variations in the up-dip (UD) and down-dip (DD) directions of SICRs and investigating their controlling mechanisms. The results indicate an asymmetry in permeability changes between UDs and DDs, with UDs generally exhibiting a greater amplitude of variation. The reservoir dip angle exerts a more pronounced influence on DD permeability changes, primarily through its effects on effective stress (ES) and matrix shrinkage (MS). Specifically, a reduction in the negative impact of ES in the UD enhances the overall reservoir permeability, whereas a reduction in the positive effect of MS in the DD leads to a more significant permeability decline. The comparatively smaller increase in DD permeability contributes to distinct evolutionary trends in the permeability between UDs and DDs. These dynamic permeability changes in SICRs have a more substantial impact on CBM production, particularly in low-strength and low-rank coal reservoirs. The findings of this study provide valuable insights for optimizing CBM production strategies in SICRs. Full article

Many studies have shown that the thermal evolution degree is the main factor affecting the micropore structure of coal reservoirs. However, within the same thick coal seam, the R_o,max of the entire coal seam is not much different, which affects the determination of the main controlling factors of pore structure heterogeneity. Therefore, No. 8 coal collected from Benxi Formation in the eastern margin of Ordos was taken as an example, and 16 samples were selected for low-temperature liquid nitrogen, carbon dioxide adsorption, and industrial component tests. Based on heterogeneity differences of R_o,max, industrial components and pore volume distribution of adsorption pores (pore diameter is less than 100 nm), the main controlling factors affecting the micropore structure of ultra-thick coal seams, were discussed. Then, the surface free energy theory was used to study the influencing factors affecting surface free energy variations during coal adsorption. First of all, R_o,max is not the main controlling factor affecting the micropore-fracture structure, as the effects of industrial components on the micropore structure are obvious, which indicates that industrial components are the main factors affecting vertical differences in the micropore structure within the same thick coal seam. Second of all, R_o,max and industrial components affect the adsorption process. When the adsorption pressure is lower, the adsorption volume and adsorption potential increase rapidly. When the adsorption pressure is higher (pressure is larger than 15 Mpa), the adsorption capacity and potential tend to be stable. Moreover, the maximum surface free energy increases with the increase in coal rank, which indicates that the degree of thermal evolution is the core factor affecting the adsorption free energy, but it is also controlled by the influence of industrial components (ash content). Lastly, micropores affect the adsorption capacity, and mesopores have little effect on the adsorption capacity, since micropores restrict the adsorption capacity and change the adsorption process by affecting surface free energy variations. The refined characterization of pore-fracture structures in deep coal reservoirs plays a crucial role in the occurrence and seepage of coalbed gas. This research can provide a theoretical basis for the efficient development of deep coalbed gas in the target area. This study aims to identify the primary factors controlling micropore structures in No. 8 coal from the Benxi Formation and to analyze the role of industrial components, which has been overlooked in previous research. Full article

The width of a stop-mining coal pillar is of great significance to the stability of the surrounding rock of the main roadway and the safety of production in the mine. This paper focuses on the west panel of Sihe Coal Mine as the engineering background, analyzes the evolution characteristics of front abutment pressure in the mining area under conditions of rapid advancement and large mining height and its sensitivity to influencing factors, explores the coupling mechanism between the width of the stop-mining coal pillar and the surrounding rock of the main roadway, and analyzes the differences in mining pressure characteristics such as internal stress of the coal pillar, vertical stress, deformation, and failure of the main roadway’s surrounding rock under different coal pillar widths with the influence of mining. The comprehensive results indicate that the influence range of front abutment pressure on the working face is 65 m, and the significance ranking of different mining factors acting on it is as follows: mining height > working face length > advancing speed. The rational width of the stop-mining coal pillar is determined to be 80 m while the stress field of the surrounding rock in the main roadway is in a critical state of mining disturbance. Industrial tests have shown that the relative displacements between the roof and floor as well as the ribs of the main roadway are relatively small, at 105 and 260 mm, respectively, which can effectively maintain the stability of the surrounding rock of the main roadway. The research results can provide a scientific basis and engineering reference for the design of stop-mining coal pillars in mines with similar geological conditions. Full article

Understanding the changes in carbon structure during the mid–low-temperature pyrolysis of low-rank coal is important for efficient utilization. Raman spectroscopy is commonly used to analyze the structural order of carbonaceous materials, but traditional methods may overlook the heterogeneity of coal/char. This research explores the heterogeneity of char structure derived from low-rank coal at 700 °C through multi-point micro-Raman analysis. The analysis of parameters such as area (A), intensity (I), full width at half maximum (FWHM/W), and peak position (P) reveals that the carbon structure becomes less ordered as coal transforms into char due to the deposition of small molecules on the surface. The study emphasizes the benefits of multi-point detection for gaining in-depth insights into the structural evolution of carbonaceous materials. The increased standard deviation of Raman parameters indicates diverse structural characteristics resulting from pyrolysis at this temperature, which traditional methods may not capture effectively. The mapping method used in this research visually illustrates the distribution of carbon structures in the region. Full article

The mineralization within the North China Craton (NCC) is intricately linked to Mesozoic large-scale extension in eastern China and is a consequence of a unified geodynamic tectonic background. Despite previous attempts to elucidate the relationship between large-scale mineralization and magmatic activity in the NCC, a lack of systematic research has hindered the identification of connections among deposits with inconsistent metallogenic ages. This study focuses on the coal measures of the Huanghebei Coalfield (HHBC) in western Shandong, presenting a regional magmatic–hydrothermal metallogenic system with a genetic connection. It delves into the intricate interplay between the multi-mineral enrichment mechanism, metallogenic regularity, and the NCC’s destruction. The findings reveal that: (1) Various stages of magmatic intrusion during the Yanshanian period significantly influenced the Late Paleozoic coal measures in the HHBC. The coal measures exhibit distinct ranks, ranging from medium-rank bituminous C to A and high-rank anthracite C, resulting in noticeable differences in gas generation among different coal ranks. The shale between the coal seams C5 and C7 emerges as excellent with a good hydrocarbon-generating capacity during the middle-maturity stage. (2) The “Intrusion along the rock layer type” proves most conducive to shale gas enrichment, while the “laccolith type” is more favorable for shale gas enrichment compared to “dike type” intrusions, which have a limited impact on shale gas enrichment. (3) The mineralization process of CBM, shale gas, and iron ore is influenced by Yanshanian-period magma. The enrichment degree of CBM and shale gas exhibits an inverse correlation with the distance from the magmatic intrusion. Iron deposits demonstrate a close association with the magmatic intrusion, with enhanced enrichment along the rock layer. The results indicate that the destruction of the NCC triggered intense metasomatism in the deep cratonic fluids, serving as the primary driving mechanism for large-scale mineralization during the Yanshanian period. Magmatic intrusions bring hydrothermal fluids conducive to mineralization, and the heat release from these intrusions promotes thermal evolution, hydrocarbon generation, and the enrichment of organic-rich strata. Full article

The Permian Longtan Formation in the Songzao coalfield, Southwest China, has abundant coalbed methane (CBM) stored in high-rank coals. However, few studies have been performed on the mechanism underlying the differences in CBM gas content in high-rank coal. This study focuses on the characterization of coal geochemical, reservoir physical, and gas-bearing properties in the coal seams M₆, M₇, M₈, and M₁₂ based on the CBM wells and coal exploration boreholes, discusses the effects of depositional environment, tectono-thermal evolution, and regional geological structure associated with CBM, and identifies major geological constraints on the gas-bearing properties in high-rank coal. The results show that high-rank coals are characterized by high TOC contents (31.49~51.32 wt%), high T_max and R₀ values (averaging 539 °C and 2.17%), low HI values (averaging 15.21 mg of HC/g TOC), high porosity and low permeability, and high gas-bearing contents, indicating a post-thermal maturity and a good CBM production potential. Changes in the shallow bay–tidal flat–lagoon environment triggered coal formation and provided the material basis for CBM generation. Multistage tectono-thermal evolution caused by the Emeishan mantle plume activity guaranteed the temperature and time for overmaturation and thermal metamorphism and added massive pyrolytic CBM, which improved the gas production potential. Good geological structural conditions, like enclosed fold regions, were shown to directly control CBM accumulation. Full article