Next Article in Journal
From CO2 to Mg Carbonates in Ultramafic Rocks: Isotopic and Kinetic Constraints from Fluid-Limited Serpentinization
Next Article in Special Issue
Development Characteristics and Reservoir Significance of Laminae in the Cambrian Qiongzhusi Formation Shale in the Southern Sichuan Basin
Previous Article in Journal
Carbothermic Processing of Low-Grade Lithium-Bearing Aluminosilicate Ores with the Production of a Lithium-Containing Slag
Previous Article in Special Issue
Brittleness Evaluation Method and Brittle–Plastic Transition Law of Deep Shale Based on Energy Evolution
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Minerals
Volume 16
Issue 5
10.3390/min16050529
minerals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

The Origin of Organic Matter Pore Destruction in Post-Mature Shales of the Qiongzhusi Formation, Southwestern Upper Yangtze, China: Evidence from Scanning Electron Microscopy

by
Huajun Min
*,
Jinhui Xu
,
Shuangqing Liang
,
Chunyan Liu
and
Limin Zhao
*
School of Energy and Electric Power Engineering, Xinjiang Vocational University of Technology, Kashi 844004, China
*
Authors to whom correspondence should be addressed.
Minerals 2026, 16(5), 529; https://doi.org/10.3390/min16050529
Submission received: 9 April 2026 / Revised: 10 May 2026 / Accepted: 12 May 2026 / Published: 15 May 2026
(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)
Browse Figures
Versions Notes

Abstract

Considerable debate remains regarding the mechanisms responsible for the reduction in organic matter (OM) pores in post-mature shales. To address this issue, complementary techniques including scanning electron microscopy (SEM), total organic carbon (TOC) analysis, and helium porosity measurement were employed to characterize the microstructure and porosity of post-mature shales from the Qiongzhusi Formation in the southwestern Upper Yangtze region, China. The results show that OM pores in these shales are poorly developed and exhibit highly irregular morphologies. Notably, the degree of OM pore development is negatively correlated with TOC. Interestingly, in samples with TOC < 2.5 wt.%, well-preserved spongy migrated OM is still observable under SEM. The average porosity of Qiongzhusi mudstones is 1.8%; siltstone samples with TOC < 2 wt.% yield an average porosity of 3.5%, whereas samples with TOC > 4 wt.% have an average porosity of only 1.9%. These findings do not support the hypothesis that graphitization causes the significant destruction of OM pores in post-mature shales. Instead, we propose that compaction has been the dominant factor controlling OM pore destruction. Accordingly, we introduce a “depth window” for the development of high-quality shale gas reservoirs: Beyond a certain maximum paleoburial depth, compaction leads to extensive OM pore destruction and a marked decline in reservoir quality. This study advances our understanding of pore evolution in post-mature shales and provides practical guidance for shale gas exploration.

1. Introduction

Shale refers to the rocks primarily composed of detrital particles with a grain size of less than 62.5 μm, including siltstone (grain sizes mainly ranging from 3.9 to 62.5 μm) and mudstone (grain sizes mainly less than 3.9 μm). Shale gas is the gas trapped in shales following hydrocarbon generation and expulsion. In recent years, shale gas exploration and development have garnered significant scientific and industrial interest. Reservoirs are one of the key factors for the enrichment of oil and gas, so the formation mechanism of the reservoirs has been widely studied [1,2,3,4,5].
OM pores, which are pores that develop within organic matter, are commonly the dominant pores in shale-gas reservoirs and have a significant impact on reservoir quality [6,7,8,9,10,11,12,13]. The formation and evolutionary mechanisms of OM pores have thus drawn great attention [1,5,8,14,15,16,17,18]. These pores are predominantly bubble-like pores generated during the thermal pyrolysis and hydrocarbon generation of organic matter (bitumen). The development of the OM pores is affected by multiple factors, including organic matter maturity [1,6,19], organic matter type [20,21,22,23], chemical composition, compaction, and hydrocarbon expulsion [20,24,25,26]. Shales with organic matter maturity—quantified by vitrinite reflectance (Ro)—in the range of 1.2% to 3.5% typically display high total porosity and well-developed OM pore networks, rendering them high-quality shale-gas reservoirs [13,18,19,27,28]. In contrast, post-mature shales (Ro > 3.5%)—in which organic matter has undergone varying degrees of graphitization—typically exhibit significantly reduced OM pore development, markedly degraded reservoir quality, and are even considered non-reservoirs [18,29]. Studies have established pore evolution patterns of shale-gas reservoirs as a function of thermal evolution [17,18,30]. These studies suggested that, during and after the wet gas stage, the porosity of shale-gas reservoirs exhibits a unimodal trend—initially increasing and subsequently decreasing—with the inflection point at an Ro of approximately 3.5%.
The origin of the significant decrease in OM pores in post-mature shales remains controversial. During the post-mature stage, diagenesis mediated by water is significantly inhibited, and diagenesis is instead dominated by thermal evolution of organic matter and compaction. Consequently, several viewpoints have been proposed regarding the origin of the decrease in OM pores in post-mature shales, including graphitization, compaction, and a combination of both. In some cases, OM pores in post-mature shales are relatively poorly developed [29,31]. Combined with the increase in the ordered degree of aromatic nuclei of the organic matter after graphitization, the increase in the density of the organic matter, and the reduction in the OM pores, it is generally believed that the graphitization of organic matter would lead to a significant reduction in OM pores [29,31,32,33,34]. However, there are studies suggested that OM pores in graphitized shales can be well developed [8,35], challenging the above viewpoint. Furthermore, Wang et al. (2013) reported that, in Qiongzhusi post-mature shales, porosity reduction occurred not only in OM pores but also in mineral-associated pores [31]. This concurrent decline implies that factors beyond thermal maturity may collectively influence OM pore evolution. Song et al. (2022) conducted simulation experiments and found that the volume of the OM pores decreased after graphitization, and the OM pores gradually evolved into organic microcracks [30]. However, no organic microcracks were observed in the Qiongzhusi post-mature shales. Therefore, they proposed that the combined effect of graphitization and compaction had led to the significant reduction in OM pores in post-mature shales. Most previous studies have generally assumed, based on common sense, that compaction has an impact on the evolution of pores in post-mature shales [36]. However, a quantitative and mechanistic understanding of how compaction influences the evolution of the pores remains lacking.
The Lower Cambrian Qionzhusi shales—characterized by post-mature thermal maturity—are widely distributed in the Upper Yangtze in China. The Qionzhusi shales exhibit considerable heterogeneity in lithology and TOC, rendering them an ideal object for investigating reservoir properties of the post-mature shales. However, the mechanisms responsible for the reduction in OM pores during the post-mature stage remain incompletely understood and are actively debated. Taking the Qiongzhusi shales as the study object, we conducted a comprehensive petrographic analysis of OM pores to provide petrological evidence for OM pore reduction in post-overmature shales. Our findings provide insights into the evolution of post-mature shale reservoirs and offer practical guidance for the exploration of shale gas in post-mature shales.

2. Geological Background

The study area is located in the western margin of the Upper Yangtze in China (Figure 1a,b). The Qiongzhusi Formation consists of a sequence of clastic sedimentary rocks deposited on a passive continental margin during the Early Cambrian. Prior to the deposition of the Qiongzhusi Formation, a regional tectonic uplift led to the development of an unconformity at the base of the formation. This was followed by a major transgressive event across the Yangtze area, which promoted widespread deposition of the Qiongzhusi Formation. During the deposition of the Qiongzhusi Formation, the ancient landform descended from the west to the east, the sediment originated from the ancient land in the west, and facies evolved progressively from shoreland and shallow-water shelf environments in the west to deep-water shelf settings in the east (Figure 1a) [37,38,39]. Organic-rich shales within the Qiongzhusi Formation are predominantly developed in the lower interval deposition in both deep- and shallow-water shelf settings [38,40]. In the Majinzi section and regions to the west, the Qiongzhusi shales mainly consist of siltstones interbedded with mudstones and limestones [40]. To the east of the Majinzi section, the turbid sediments increase, with mudstone constituting the main part of Qiongzhusi shales [40]. During burial, the Qiongzhusi Formation underwent multiple episodes of tectonic subsidence and uplift. By the Late Cretaceous, the Qiongzhusi Formation reached its maximum burial depth and subsequently gradually exhumed to its present-day burial depth [40,41]. In the study area, the Qiongzhusi Formation attained a maximum paleo-burial depth of 8000–9500 m, corresponding to peak formation temperatures in excess of 260 °C (Figure 1c) [40]. The Qiongzhusi shales are high in the degree of thermal evolution of organic matter, with Ro reaching 4.0%–5.5% [40]. The high geothermal temperature also led to the graphitization of organic matter [29,31,42,43,44].

3. Materials and Methods

The samples in this study were collected from outcrop sections and drilling cores in the study area, all of which are from the lower part of the Qiongzhusi Formation (Figure 1, Table 1). To allow for comprehensive observations and comparative analyses, samples were collected from shallow and deep shelf facies, with considerable variations in lithology, mineral composition, and TOC (Table 1), thereby ensuring that the results of this study are highly representative.
Thin Section Preparation and Observation: To prepare a thin section, fresh rock samples were first cut into rock blocks approximately 2 cm in length and width. One surface of each block was ground flat and mounted onto a glass slide using epoxy resin adhesive. The mounted specimen was then progressively ground—through coarse, medium, fine, and precision stages—to a final thickness of 30 μm, with a tolerance of ±5 μm. The resulting thin sections were examined under an optical microscope to characterize rock texture and microfabric.
High-Resolution Observation of the Microstructure of Shales: For sample preparation, fresh samples were first cut into rock blocks approximately 10 mm × 10 mm in size. Subsequent mechanical polishing was performed stepwise using silicon carbide abrasive papers graded from 400# to 2000#. Finally, the polished samples were subjected to Ar ion beam milling using a GATAN PECS II 685 instrument (Gatan, Pleasanton, CA, USA) and then coated with platinum to enhance conductivity during imaging. The samples were observed using an FEI Quanta 650 field emission scanning electron microscope (Thermo Fisher Scientific, Hillsboro, OR, USA). This instrument provides high spatial resolutions (approximately 1 nm at 30 kV) and secondary electron and backscattered electron (BSE) imaging modes, and it is capable of energy-dispersive X-ray spectroscopy, enabling semi-quantitative elemental analysis and mineral phase identification.
Porosity Measurement: Cylindrical rock core samples (25 mm in diameter × 30 mm in length) were extracted from drilling cores parallel to the bedding plane. The porosity was determined using the gas porosity measurement method, with reference to the Chinese national standard (GB/T 34533-2017) [45], and the schematic diagram of the porosity measurement apparatus is shown in Figure 2. Prior to measurement, the core plugs were dried at 105 °C. Following drying, the samples were placed in the sample chamber, and the initial pressure of the chamber was recorded. Helium gas was then introduced into the reference chamber at a pressure of 200 psi. After approximately 30 s of equilibration, the pressure in the reference chamber was recorded. Subsequently, the valve connecting the reference and sample chambers was opened to permit helium expansion from the reference chamber into the sample chamber. After approximately 30 h, the system reached thermodynamic equilibrium, and the final equilibrium pressure was recorded. Upon attainment of the pressure equilibrium, the skeletal volume of the shale sample was calculated using Boyle’s law. The pore volume was then derived by subtracting the skeletal volume from the total geometric volume of the sample, which was measured independently via the Archimedes immersion method. The porosity of the shale sample was the ratio of the pore volume to the total volume. Then, 7 silty shale and 4 mudstone shale samples were tested in this study (Table 1).
TOC Determination: TOC was determined using the combustion method. Prior to analysis, the samples were crushed to a size between 0.075 mm and 0.18 mm and then dissolved completely in dilute hydrochloric acid at a temperature of 60 °C to remove inorganic carbon compounds. The dissolved residue was then repeatedly rinsed with distilled water and dried. Next, the sample was heated in an oxygen-enriched pyrolysis furnace at ~1500 °C to convert all organic carbon to CO2. The resulting CO2 was detected and quantified using an infrared detector. Calibration against certified carbon standards enabled quantification of TOC based on the integrated absorption peak area of the analyte. The TOC determination was carried out using a Leco CS230 carbon analyzer (Leco, Saint Joseph, MI, USA), and a total of 21 samples were tested (Table 1).

4. Results

4.1. Petrology

The Qiongzhusi shales are predominantly composed of mudstone and siltstone. In the Majinzi section and Well Z1, Qiongzhusi shales consist mainly of argillaceous siltstone, in which silt grains range predominantly from 20 to 40 μm in size and exhibit poor roundness—typically angular to subangular in shape (Figure 3a–c). Integrated analysis of grain morphology, BSE imaging grayscale contrast, and energy-dispersive X-ray spectroscopy reveals that the silt grains are primarily composed of quartz and albite (Figure 3c); minor constituents include potassium feldspar, muscovite, calcite, and apatite. The matrix and pore-filling phases consist chiefly of clay minerals, microcrystalline quartz, pyrite, and organic matter, with subordinate amounts of mud-sized terrigenous detrital quartz and authigenic albite. In contrast, the Qiongzhusi shales at the Songlin section are dominated by mudstone (Figure 3g–i), with a small amount of argillaceous siltstone at the bottom. The argillaceous siltstones at Songlin are characterized by finer particles, typically 10–20 μm in size (Figure 3d–f). Both the mudstones and siltstones at Songlin contain abundant microcrystalline quartz, occurring either as discrete grains or as aggregates (Figure 3f).
Siltstones with relatively low TOC (TOC < 2 wt.%) exhibit predominantly linear or concavo-convex grain contacts—indicative of intense compaction (Figure 3a–c). By contrast, siltstones with higher TOC (TOC > 3.5 wt.%) display an increased proportion of point contacts between silt grains, attributable to pervasive pore-filling by organic matter and microcrystalline quartz (Figure 3d–f). Consequently, particle packing in the high-TOC siltstones is less compact than that of siltstones with low TOC.

4.2. OM Pores

OM pores constitute a critical pore type in shale-gas reservoirs and are frequently the most abundant pore class [6,7,8,9,10,11,12,13]. Based on their morphological characteristics and spatial relationships, OM pores in the Qiongzhusi shales are classified into six types (Table 2).

4.2.1. OM Pores Within Sedimentary Organic Matter

Sedimentary organic matter (SOM) refers to organic material derived from the remains, secretions, and excretions of organisms that enter sediments directly or indirectly—either through deposition, biodegradation, burial, or subsequent condensation reactions leading to the formation of new organic compounds and their derivatives [46]. The pores in sedimentary organic matter are generally less developed [11,20,21,23]. In the Qiongzhusi shales, only a few pores, which are isolated and regular in shape, measuring generally <20 nm in diameter, developed along SOM margins or within SOM (Figure 4).

4.2.2. OM Pores Within Migrated Organic Matter

Migrated organic matter originates from the thermal evolution of kerogen and subsequently migrates into adjacent pores upon generation and is then thermally degraded into solid bitumen or pyrobitumen [21]. In shale-gas reservoirs, migrated organic matter constitutes the dominant fraction of total organic matter [26]. During the thermal degradation of bitumen, natural gas is generated concurrently with the formation of numerous pores within the bitumen [6]. In commercially productive shale reservoirs, such OM pores are typically well developed [6,7,8,10,13]. The characteristics of OM pores developed within migrated organic matter in the Qiongzhusi shales are as follows:
  • OM pores are poorly developed and exhibit highly irregular morphologies. Compared with commercially productive shales, the migrated organic matter in the Qiongzhusi shales has far fewer OM pores [6,8,9,10,13,47,48]. Most of the OM pores are isolated and of varying sizes, with pore sizes ranging from a few nanometers to several hundred nanometers. The plane porosity of organic matter generally does not exceed 5% (Figure 5). Unlike the OM pores in commercially developed shales—which are of predominantly nearly circular or elliptical in shape—the OM pores in the Qiongzhusi shales are highly irregular in shape (Figure 5h,i).
  • OM pores are relatively less developed in the shales with TOC > ~4 wt.%. For samples with TOC > ~4 wt.%, most of the organic matter is dense, with no pores that can be observed under SEM (Figure 5a–f), except for locally developed, coalesced, sheet-like pores (Table 2). The organic matter within the framboidal pyrite is also dense in shales with TOC > ~4 wt.% (Figure 6a,b).
  • OM pores are relatively well developed in samples with TOC < ~2.5 wt.%. Samples with TOC < 2.5 wt.% exhibit less developed OM pores; however, the OM pores are nonetheless common and—locally—relatively well developed (Figure 7). These pores are predominantly isolated and irregular in shape (Table 2, Figure 5g–i). In some compaction shadows, the OM pores are well-developed and exhibit nearly circular or elliptical morphologies (Figure 7d–f), resembling those observed in commercially exploited shales of relatively low thermal maturity. Some organic matter appears to have been subjected to unidirectional compaction, with internal pores arranged in a direction (Figure 7c). In these shales, the organic matter within the framboidal pyrite has well-developed OM pores—either homogeneous or complex sponge-like in morphology (with a maximum plane porosity of 15%) (Table 2, Figure 6c–f).

4.3. Other Pores in the Shales

The other pores in the Qiongzhusi shales include grain-edge intergranular pores (Figure 8a), residual intergranular pores (Figure 8b), pyrite intergranular pores (Figure 8c), interlayer pores within clay minerals (Figure 8d) and mica minerals (Figure 8e), and intragranular dissolution pores (Figure 8f). Among these, grain-edge intergranular pores are relatively more common—particularly in shales with relatively low TOC (TOC < 2.0 wt.%). Residual intergranular pores are less developed due to intense compaction. Interlayer pores associated with mica minerals are commonly observed in the siltstones. Intragranular dissolution pores occur predominantly within feldspar grains—especially potassium feldspar—and are generally less developed.

4.4. Porosity of the Shales

The porosity of the Qiongzhusi shales is low. The porosity values of seven siltstone samples range from 1.6% to 4.5%, with an average of 2.8% (Table 1). In contrast, the porosity of mudstone is lower than that of siltstone, with porosity values ranging from 1.3% to 2.3% across four samples and averaging 1.8% (Table 1). In siltstones, samples with TOC less than 2 wt.% are higher in porosity than those with TOC greater than 4 wt.%. The former exhibits porosity values of 2.6%–4.5% (mean: 3.5%) (Table 1), whereas the latter shows porosity of 1.6%–2.4% (mean: 1.9%) (Table 1). The porosity of the Qiongzhusi shales is negatively correlated with TOC—this relationship is more pronounced in siltstones than in mudstones (Figure 9).

5. Discussion

5.1. Reservoir Quality of the Qiongzhusi Shales

Compared with commercially productive shale-gas reservoirs, the reservoir quality of the Qiongzhusi shales is notably inferior. In terms of rock porosity, commercially productive shales from China and North America typically exhibit porosity exceeding 3%, with maximum values reaching up to 15% [1,27,42]. In contrast, the Qiongzhusi shales display significantly lower porosity (Table 1). Qiongzhusi shales with TOC higher than 4 wt.% have porosities of approximately 2% or lower, rendering them inadequate as shale-gas reservoirs. Although siltstones with relatively low TOC (TOC < 2 wt.%) have relatively high porosity (average 3.4%) (Table 1), they are generally not regarded as high-quality shale-gas reservoirs owing to their insufficient TOC levels [49]. In terms of OM pore development, the migrated organic matter in commercially productive shales typically exhibits a sponge-like morphology characterized by well-developed pores and high plane porosity (up to 30%); these OM pores are generally regular in shape and well interconnected, and they constitute most pore networks in shale reservoirs [6,9,13,42,47]. In these shales, TOC is generally positively correlated with porosity [9,12,18,19,48]. In the Qiongzhusi shales, however, sponge-like organic matter is significantly less abundant; OM pores are markedly underdeveloped compared with those in commercially exploited shales and are predominantly isolated and irregular in morphology. The porosity exhibits a negative correlation with TOC in the Qiongzhusi shales (Figure 9), indicating that OM pores contribute minimally to the overall pore system and that organic matter functions primarily as pore fillings. According to the calculation method of shale matrix porosity [43], the contribution of organic matter (OM) pores to the total porosity of the Qiongzhusi Formation shale was quantitatively estimated. In the calculation, the densities of shale and organic matter were taken as 2.58 g/cm3 and 1.35 g/cm3, respectively [50,51], and the TOC and plane porosity of organic matter were determined in this study. For shales with TOC < 2 wt.%, OM pores account for ≤15% of the total shale porosity; in contrast, for shales with TOC > 4 wt.%, the contribution of OM pores to total porosity is <10%. The results indicate that OM pores in the Qiongzhusi shales contribute minimally to the total porosity. In conclusion, the reservoir quality of the Qiongzhusi shales is poor. Our findings are consistent with those of previous studies, in which the average porosity of the Qiongzhusi shales ranges from 1.48% to 1.95%, and the plane porosity of organic matter is less than 5% [42,43].
The Qiongzhusi shales in surrounding areas—characterized by lower thermal maturity—generally exhibit higher porosity and more extensively developed OM pores compared to those in the study area [34,52,53,54,55]. Notably, several of these shales have produced industrial gas flow [56,57,58], suggesting that the poor reservoir quality of the Qiongzhusi shales in the study area is not innate but the result of destruction of the pore network of the shales (including OM pores).

5.2. Origin of the Destruction of OM Pores in the Qiongzhusi Shales

The Qiongzhusi shales in the study area experienced ultra-deep burial, reaching peak paleotemperatures exceeding 260 °C; consequently, the organic matter within these shales has undergone graphitization [13,29,42]. The origin of widespread OM pore destruction in the post-mature Qiongzhusi shales remains a subject of ongoing debate. Most existing studies attribute this phenomenon predominantly to organic matter graphitization [29,31,32,33,34].

5.2.1. Graphitization and OM Pore Destruction

When the Ro exceeds 3.5%, organic matter progressively transforms into graphite, resulting in a marked decrease in shale resistivity. Consequently, electrical resistivity is widely used as a proxy for estimating the degree of organic matter graphitization in black shales [32,35,59,60,61]. In graphitized shales, resistivity exhibits a positive correlation with TOC; notably, the logging resistivity of graphitized, organic-rich shales (TOC > 2 wt.%) typically falls below 10 Ω·m [59]. Laser Raman spectroscopy of graphitized organic matter typically exhibits a prominent D-band peak—often even more intense than the G-band peak—and may additionally resolve a distinct G′-band [33,34,43]. Considering the densification of organic matter following graphitization, several studies have attributed the underdeveloped OM pores in the Qiongzhusi post-mature shales to this process [29,32,33,34].
In this study, sponge-like organic matter exhibiting exceptionally well-developed pores was observed in the Qiongzhusi post-mature shales (Figure 6c–f and Figure 7d–f). Its morphological and textural characteristics closely resemble those of migrated organic matter commonly found in high-quality, relatively low-maturity shale reservoirs [6,9,13]. Although less abundant, it still challenges the prevailing view that graphitization leads to a substantial reduction in OM pores. Curtis et al. (2012) and Piane et al. (2018) similarly reported that OM pores remain well developed in the graphitized Woodford and Marcellus shales [8,35]; notably, even the highly graphitized Marcellus shales (Ro > 4%) continue to produce commercial quantities of shale gas [35]. The organic-rich interval of the Longmaxi Shale in Well Y12 in the study area exhibits an Ro exceeding 4.0%, coupled with exceptionally low formation resistivity (<3 Ω·m) (Figure 10) (data sourced from PetroChina Zhejiang Oilfield Company), indicating that the organic matter has been graphitized [35,59,61]. The Longmaxi shales in this well contain well-developed OM pores, which are predominantly elliptical in morphology (Figure 11). These shales possess favorable physical properties, with porosity ranging from 3.8% to 4.9%, indicating that graphitization has not induced a substantial reduction in OM pores. The above evidence indicates that graphitization does not—at least at the SEM resolution—significantly reduce OM pores in post-mature shales.
Following graphitization, the density of organic matter increases—potentially accompanied by volumetric shrinkage. In the absence of mechanical compaction, both total porosity and organic-matter-hosted porosity would theoretically increase rather than decrease. Physical simulation experiments on organic matter graphitization conducted by Song et al. (2022) revealed a reduction in pore volume measured via gas adsorption techniques, primarily attributable to the loss of mesopores and micropores within the organic matter matrix—consistent with the hypothesis of organic matter shrinkage [30]. However, these studies also observed that pre-existing elliptical and sponge-like OM pores progressively coalesced into microcracks within the organic matter [30], suggesting that relatively larger pores may undergo structural reorganization rather than net reduction. Further experimental and theoretical investigation is required to validate this interpretation. Nevertheless, porosity and the degree of organic matter pore development in Qiongzhusi shales are markedly lower than those observed in commercially productive shale formations (Table 1 [31,42]), a discrepancy that cannot be fully accounted for by organic matter graphitization alone.
In fact, studies suggesting that graphitization leads to a substantial reduction in OM pores were predominantly based on observations of samples of Qiongzhusi shales [29,32,33,34]. These samples not only underwent organic matter transformation toward graphitization but also experienced significant compaction; thus, the influence of compaction on pore evolution cannot be ruled out. Following graphitization, the physical and chemical properties of organic matter undergo significant changes, accompanied by corresponding variations in pore volume and the specific surface area of the organic matter [30,34]. We tentatively propose that, during the graphitization of organic matter, the structural reorganization of aromatic rings—from a disordered to a more ordered arrangement—enhances OM compactness and induces concomitant adjustments in pore architecture. Nevertheless, mesopores and macropores of relatively large dimensions—resolvable at standard SEM resolution—are largely preserved, although localized modifications to pore morphology cannot be excluded. Thus, temperature is not the sole controlling factor governing OM pore evolution during the post-mature stage.

5.2.2. Compaction and OM Pore Destruction

The wet-gas window for the Qiongzhusi shales in the study area began at an approximate burial depth of 5000 m [40], where OM pores were theoretically expected to begin extensive development [1,6,30]. The maximum burial depth experienced by these shales reached 8000–9500 m [40], implying that after the OM pores formed, the shale underwent deep to ultra-deep (burial depth greater than 6000 m) compaction. While deep to ultra-deep compaction is generally regarded as having only a limited influence on pore evolution in conventional reservoirs [62], its impact may be significantly more pronounced in organic-rich shale-gas reservoirs—particularly those with low matrix porosity (~5%)—where mechanical resistance to compaction is inherently reduced [63]. Consequently, the effect of compaction on shale-gas reservoirs warrants careful evaluation. Although prior studies have documented the influence of compaction on OM pore evolution [9,24,63], the specific role of deep to ultra-deep compaction remains underexplored.
Multiple lines of evidence indicate that the Qiongzhusi shales in the study area likely experienced intense compaction during deep to ultra-deep burial:
  • Formation of OM pores provides essential space for compaction. Prior to thermal maturity, kerogen in source rocks typically generates substantial quantities of bitumen, which infiltrates and fills pores—thereby significantly reducing shale porosity [4]. In shales with high TOC, pore space may be nearly exhausted following bitumen saturation [2], severely constraining compaction due to the absence of pore space. However, during and after the wet-gas window (Ro ≤ 3.0%), abundant OM pores develop [21,64], creating additional pore volume that accommodates further compaction as burial depth increases. This mechanism is theoretically applicable to the Qiongzhusi shales in the study area.
  • A substantial increase in shale burial depth provided the driving force for compaction. Between the onset of the wet-gas window and the maximum paleo-burial depth, the burial depth of the Qiongzhusi shales in the study area increased by 3000–4500 m [40]. This substantial increase in overburden pressure provided the requisite energy to sustain ongoing compaction.
  • Absence of Overpressure Favors Compaction: Carbonate karst reservoirs developed at the top of the underlying Dengying Formation likely enhanced the efficiency of hydrocarbon expulsion—particularly from the basal intervals of the Qiongzhusi shales—during hydrocarbon generation [42,65]. As a result, the development of overpressure was inhibited, thereby establishing favorable geomechanical conditions for compaction.
  • The relatively low mechanical strength of organic-rich shales rendered them particularly susceptible to compaction. Elevated organic matter and clay mineral content can collectively reduce shale strength and diminish its resistance to compaction [63,66]. Shales rich in organic matter or clay minerals—owing to their relatively low mechanical strength—are particularly susceptible to compaction, which may have significantly impacted pore evolution even during the early stage of extensive organic matter pore development [9,67]. Milliken et al. (2013) observed that in the Marcellus shale (Ro ranging from 1.0% to 2.1%), porosity exhibited a strong positive correlation with TOC only up to TOC ≈ 5.5 wt.%; beyond this threshold, porosity plateaued or even declined—a trend attributed by the authors to compaction [9]. Similarly, Lis et al. (2025) noted that although OM pores are theoretically expected to develop during the wet-gas to dry-gas stage, they remain poorly developed in Silurian shales of the Baltic Basin [67]. This anomaly was ascribed to an exceptionally high clay mineral content (~50 vol.%), which compromises the development of a rigid mineral framework and eliminates pressure-shadow protection essential for preserving OM pores. In the basal Qiongzhusi Formation shales—characterized by exceptionally high TOC (up to 13.7 wt.%)—such pronounced organic enrichment would inevitably lead to substantial reductions in rock strength, thereby strongly compacting.
The microstructure of the shales indicates that organic matter in the Qiongzhusi shales has undergone compaction. The siltstones in the Qiongzhusi shales with TOC > ~4 wt.% display a looser grain arrangement compared to those with relatively low TOC (TOC < 2 wt.%) (Figure 3b,c,e,f), attributable to the abundance of organic matter and microcrystalline quartz within intergranular pores in the former (Figure 3f and Figure 5c,e). In high-TOC mudstones, intergranular organic matter is densely packed, lacks internal pores, and intimately adheres to adjacent mineral grains—exhibiting load-bearing structural characteristics (Figure 5a–f). This structural configuration indicates that organic matter and associated pore fillings support the overburden static pressure, demonstrating that compaction has significantly influenced both the organic matter itself and the evolution of OM pores.
The irregular morphology of the OM pores in the Qiongzhusi shales is likely attributable to OM deformation induced by compaction. As shown in Figure 5h,i, most of the OM pores exhibit irregular shapes—markedly contrasting with the near-circular or ellipsoidal morphologies typically observed at the time of initial pore formation [21]. Although experimental studies have indicated that graphitization can modify OM pore morphology [30], the persistence of nearly circular OM pores in some Qiongzhusi samples (Figure 7d–f) suggests that graphitization under natural diagenetic conditions may not substantially alter the pore shape as resolved by SEM. At the onset of OM pore formation, the pores are near-circular [21]. With progressive burial and increasing overburden stress, organic matter undergoes gradual compaction, accompanied by viscoelastic creep and consequent internal pore distortion. So, the irregular pore geometries observed in the Qiongzhusi shales likely reflect the strain-driven pore deformation during compaction. Continued compaction leads to progressive densification of the organic matrix; under intense compaction, internal pores may be entirely obliterated—as evidenced in Qiongzhusi shales with TOC exceeding ~4 wt.% (Figure 5a–f). At the same time, in some compaction shadow positions, OM pores may be perfectly preserved (Figure 7d–f).
Differences in OM pore development within migrated organic matter hosted in framboidal pyrite among Qiongzhusi shales with varying TOC may be attributed to OM creep induced by compaction. In the study area, Qiongzhusi shales with TOC < ~2.5 wt.% exhibit well-developed OM pores within migrated OM enclosed by framboidal pyrite (Figure 6c–f), whereas those with TOC > ~4 wt.% show poorly developed OM pores (Figure 6a,b). In contrast, for Qiongzhusi shales with relatively low maturity (Ro < 3.5%) from regions adjacent to the study area, OM pores within migrated OM hosted in framboidal pyrite are consistently well developed [15,52,68], suggesting that analogous OM pores may have once developed in the Qiongzhusi shales in the study area. The high Young’s modulus of pyrite (~300 GPa)—exceeding that of most silicate minerals [69]—combined with the near-spherical geometry of framboidal pyrite, suggests that these framboids can serve as rigid mineral frameworks and generate pressure shadows internally during burial. Direct compaction of OM alone cannot account for the observed degradation of OM pores within framboidal pyrite in high-TOC Qiongzhusi shales (TOC > ~4 wt.%) in the study area. Given that migrated OM in shales is interconnected over a finite spatial scale [21,70], OM adjacent to framboidal pyrite is driven inward during compaction, thereby compressing the internally hosted OM and ultimately collapsing its pore network. If compaction persists, progressively greater quantities of organic matter will be incorporated into framboidal pyrite, leading to increasingly severe damage to OM pores within the framboids—potentially resulting in their complete obliteration. In comparison with shales characterized by higher TOC, those with lower TOC exhibit greater resistance to compaction [63] and possess less organic matter available to undergo creep deformation under compaction. Consequently, under identical geological conditions, the degradation of OM pores within framboidal pyrite proceeds more slowly in low-TOC shales than in high-TOC shales. As a result, OM pore development within framboidal pyrite is better preserved in low-TOC samples (Figure 6).
In summary, under identical geological conditions, shales with higher TOC or greater clay mineral abundance would undergo more intense compaction, leading to more severe pore degradation. In the study area, both bulk porosity and OM pore development in the Qiongzhusi shales exhibit a statistically significant negative correlation with TOC (Figure 5, Figure 7 and Figure 9), corroborating the preceding analysis and further underscoring compaction as the dominant control on pore deterioration in these shales. Specifically, the widespread destruction of OM pores observed in the Qiongzhusi shales is predominantly attributable to compaction, whereas the contribution of graphitization to OM pore reduction appears markedly less significant than previously hypothesized. Consequently, compaction must be recognized as a critical factor governing pore evolution in shale reservoirs during the post-mature stage; its underlying mechanisms, therefore, warrant systematic and quantitative investigation.

5.3. Implications for Hydrocarbon Exploration

There may be a depth window for the development of high-quality shale-gas reservoirs. In the Qiongzhusi shales, siltstones with TOC exceeding 4.0 wt.% and mudstones exhibiting porosities of approximately 2% or lower (Table 1) are classified as poor or non-reservoirs [42]. Similarly, reports documenting degraded reservoir quality in post-mature shales from other basins [71] suggest that late-stage deterioration of shale-gas reservoirs may be a widespread phenomenon. Based on previous findings and the results here, it can be found that the evolution of shale-gas reservoirs would go through a process from the formation of high-quality reservoirs to non-reservoirs. Specifically, during the wet-gas generation stage, OM pores begin to develop extensively [4,19], leading to progressive porosity enhancement that culminates in a maximum value [2,42]. Subsequently, porosity gradually decreases with increasing compaction, ultimately rendering the rock non-reservoir. Therefore, there may be a depth window for the development of high-quality shale-gas reservoirs. This depth window range is directly affected by the thermal evolution and compaction of organic matter, and relevant influencing factors include organic matter content and type, geothermal gradient, formation pressure coefficient, etc. Notably, abnormally high formation pressure can significantly mitigate the effects of compaction, thereby enhancing pore preservation over an expanded depth range [72,73]. Based on the burial and thermal evolution history of the Qiongzhusi Formation in the study area [40], the depth window of the formation for the development of high-quality shale-gas reservoirs is estimated to range from approximately 5000 m to no deeper than 8000 m. Consequently, except for persistently uplifted ancient structural highs—such as the Chuanzhong paleo-uplift—the maximum paleo-burial depth of the Qiongzhusi Formation across the Sichuan Basin and its periphery exceeds this depth range [74]. Recently, high-yield shale gas production from the Qiongzhusi Formation has been achieved in paleo-uplift areas of the Sichuan Basin [56,57,58], thereby validating the exploration potential of Qiongzhusi Formation shale gas in such paleo-uplift settings. It should be emphasized that the “depth window” for the development of high-quality shale-gas reservoirs is derived from an integrated analysis of the Qiongzhusi shales within the study area. As it is put forward under a certain geological background, its wide applicability still needs to be verified through more case studies in different basins.
Siltstones exhibit significant potential as effective reservoirs under ultra-deep burial conditions. In the study area, Qiongzhusi siltstones—characterized by TOC below 2 wt.% and maximum burial depths exceeding 8000 m—display porosities ranging from 2.6% to 4.5%, confirming their viability as productive reservoirs [42]. Although siltstones have traditionally been excluded from consideration as high-quality shale-gas reservoirs due to their relatively low TOC values [49], recent high-yield shale gas production from the siltstone intervals in the Sichuan Basin highlights their substantial reservoir potential [75,76]. Although the siltstones are relatively low in hydrocarbon generation potential, their inorganic pores are well-developed. Under favorable preservation conditions, they can accumulate natural gas sourced either from within the same unit or from adjacent source rocks, thereby forming shale gas reservoirs [76]. Nevertheless, reservoir characteristics of the siltstones—and, critically, the genetic mechanisms controlling their pore development and preservation—remain incompletely understood and thus warrant further investigation.

6. Conclusions

In this study, the Qiongzhusi shales were taken as an example to investigate the development characteristics of OM pores in post-mature shales. The following conclusions are drawn:
(1)
The Qiongzhusi shales exhibit low porosity and poorly developed OM pores. However, this poor development of the OM pores is the result of post-formation destruction rather than an original feature. Despite the generally poor OM pore development, highly porous spongy-like migrated OM is still observable under SEM, indicating that the widespread destruction of OM pores was not caused by graphitization. This finding negates the traditional view that graphitization leads to the massive destruction of OM pores in post-mature shales and improves our understanding of pore evolution in post-mature shales.
(2)
Compaction likely played a major role in the destruction of OM pores. Consequently, anti-compaction mechanisms (e.g., overpressure) are critical for pore preservation in post-mature shales. In the presence of overpressure, post-mature shales may still serve as high-quality shale-gas reservoirs (e.g., the Marcellus Shale).
(3)
The continuous compaction-induced destruction of OM pores after their formation results in a “depth window” for the development of high-quality shale gas reservoirs: Beyond a certain depth, OM pores are largely destroyed, leading to a significant decline in reservoir quality, potentially to the extent of becoming non-reservoir. Therefore, in shale-gas exploration, priority should be given to areas where the maximum paleoburial depth falls within this window.
(4)
It should be noted that the mechanisms, controlling factors, and extent of compaction-driven OM pore destruction require further investigation. Moreover, the conclusions of this study are primarily based on the Qiongzhusi shales and need to be validated by studies of shales from more basins.

Author Contributions

Conceptualization, H.M.; methodology, H.M.; validation, H.M.; formal analysis, H.M.; investigation, H.M., and J.X.; data curation, J.X.; writing—original draft preparation, H.M.; writing—review and editing, H.M., J.X., and C.L.; supervision, S.L.; project administration, L.Z.; funding acquisition, H.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Major Project, grant number 2017ZX05063002-009.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of this manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
BSEBackscattered electron;
GPaGiga Pascal;
OMOrganic matter;
ROVitrinite reflectance;
SOMSedimentary organic matter;
SEMScanning electron microscopy;
TOCTotal organic carbon.

References

  1. Jarvie, D.M.; Hill, R.J.; Ruble, T.E.; Pollastro, R.M. Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 2007, 91, 475–499. [Google Scholar] [CrossRef]
  2. Mastalerz, M.; Schimmelmann, A.; Drobniak, A.; Chen, Y. Porosity of Devonian and Mississippian New Albany Shale across a maturation gradient: Insights from organic petrology, gas adsorption, and mercury intrusion. AAPG Bull. 2013, 97, 1621–1643. [Google Scholar] [CrossRef]
  3. Khan, M.; Nawaz, S.; Gao, X.; Khan, N.M. Reservoir Characterization of Eocene Carbonates and Cambrian Sands in the Eastern Potwar Region, Pakistan. Reserv. Sci. 2026, 2, 111–125. [Google Scholar] [CrossRef]
  4. Pommer, M.; Milliken, K. Pore types and pore-size distributions across thermal maturity, Eagle Ford Formation, southern Texas. AAPG Bull. 2015, 99, 1713–1744. [Google Scholar] [CrossRef]
  5. Wang, T.; Wang, X.; Zhang, D.; Wang, Q.T.; Luo, H.Y.; Wang, J.; Ma, Z.L.; Chen, Z.X.; Liu, W.H. Pore formation and evolution mechanisms during hydrocarbon generation in organic-rich marl. Pet. Sci. 2025, 22, 557–573. [Google Scholar] [CrossRef]
  6. Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Jarvie, D.M. Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale. J. Sediment. Res. 2009, 79, 848–861. [Google Scholar] [CrossRef]
  7. Ambrose, R.J.; Hartman, R.C.; Campos, D.M.; Akkutlu, I.Y. New pore-scale considerations for shale gas in place calculations. In Proceedings of the Society of Petroleum Engineers Unconventional Gas Conference, Pittsburgh, PA, USA, 23–25 February 2010. [Google Scholar]
  8. Curtis, M.E.; Cardott, B.J.; Sondergeld, C.H.; Rai, C.S. Development of organic porosity in the Woodford Shale with increasing thermal maturity. Int. J. Coal Geol. 2012, 10, 6–31. [Google Scholar] [CrossRef]
  9. Milliken, K.L.; Rudniki, M.; Awwiller, D.N.; Zhang, T. OM–hosted pore system, Marcellus Formation (Devonian), Pennsylvania. AAPG Bull. 2013, 97, 177–200. [Google Scholar] [CrossRef]
  10. Chalmers, G.R.; Bustin, R.M. A multidisciplinary approach in determining the maceral (kerogen type) and mineralogical composition of Upper Cretaceous Eagle Ford Formation: Impact on pore development and pore size distribution. Int. J. Coal Geol. 2017, 171, 93–110. [Google Scholar] [CrossRef]
  11. İnan, S.; Badairy, A.H.; İnan, T.; Al Zahrani, A. Formation and occurrence of organic matter-hosted porosity in shales. Int. J. Coal Geol. 2018, 199, 39–51. [Google Scholar] [CrossRef]
  12. Song, L.; Martin, K.; Carr, T.R.; Ghahfarokhi, P.K. Porosity and storage capacity of Middle Devonian shale: A function of thermal maturity, total organic carbon, and clay content. Fuel 2019, 241, 1036–1044. [Google Scholar] [CrossRef]
  13. Zhang, K.; Jiang, S.; Zhao, R.; Wang, P.; Jia, C.; Song, Y. Connectivity of organic matter pores in the Lower Silurian Longmaxi Formation shale, Sichuan Basin, Southern China: Analyses from helium ion microscope and focused ion beam scanning electron microscope. Geol. J. 2022, 57, 1912–1924. [Google Scholar] [CrossRef]
  14. Li, X.; Jiang, Z.; Song, Y.; Zhai, G.; Bao, S.; Li, Z.; Tang, X.; Wang, P.; Li, T.; Wang, G.; et al. Porosity evolution mechanisms of marine shales at over-maturity stage: Insight from comparable analysis between Lower Cambrian and Lower Silurian inside and at the margin of the Sichuan Basin, South China. Interpretation 2018, 6, T739–T757. [Google Scholar] [CrossRef]
  15. Li, X.; Jiang, Z.X.; Wang, P.F.; Song, Y.; Li, Z.; Tang, X.L.; Li, T.W.; Zhai, G.Y.; Bao, S.J.; Xu, C.L. Porosity-preserving mechanisms of marine shale in Lower Cambrian of Sichuan Basin, South China. J. Nat. Gas Sci. Eng. 2018, 55, 191–205. [Google Scholar] [CrossRef]
  16. Liu, B. Organic Matter in Shales: Types, Thermal Evolution, and Organic Pores. Earth Sci. 2023, 48, 4641–4657. [Google Scholar] [CrossRef]
  17. Wang, X.; Zhang, K.; Jiang, Z.; Lin, Y.; Song, Y.; Jia, C.; Jiang, S.; Jiang, L.; Niu, H.; Li, J.; et al. Hydrocarbon generation and pore evolution in marine organic-rich shales across maturation stages. Fuel 2025, 402, 136050. [Google Scholar] [CrossRef]
  18. Xie, W.D.; Fu, X.F.; Wang, H.X.; Sun, Y.; Vandeginste, V.; Li, X.P. Thermal maturity effect on the microscopic pore structure and gas adsorption capacity of shale gas reservoirs: A review. Pet. Sci. 2025, 22, 3967–3990. [Google Scholar] [CrossRef]
  19. Song, L.; Carr, T.R. The pore structural evolution of the Marcellus and Mahantango shales, Appalachian Basin. Mar. Pet. Geol. 2020, 114, 104226. [Google Scholar] [CrossRef]
  20. Bernard, S.; Wirth, R.; Schreiber, A.; Schulz, H.M.; Horsfield, B. Formation of nanoporous pyrobitumen residues during maturation of the Barnett Shale (Fort Worth Basin). Int. J. Coal Geol. 2012, 103, 3–11. [Google Scholar] [CrossRef]
  21. Loucks, R.G.; Reed, R.M. Scanning-Electron-Microscope petrographic evidence for distinguishing organic matter pores associated with depositional organic matter versus migrated organic matter in mudrocks. GCAGS 2014, 10, 51–60. [Google Scholar]
  22. Ko, L.T.; Ruppel, S.C.; Loucks, R.G. Pore-types and pore-network evolution in Upper Devonian-Lower Mississippian Woodford and Mississippian Barnett mudstones: Insights from laboratory thermal maturation and organic petrology. Int. J. Coal Geol. 2018, 190, 3–28. [Google Scholar] [CrossRef]
  23. Piane, C.D.; Ansari, H.; Li, Z.; Mata, J.; Rickard, W.; Pini, R.; Dewhurst, D.N.; Sherwood, N. Influence of organic matter type on porosity development in the Wufeng-Longmaxi Shale: A combined microscopy, neutron scattering and physisorption approach. Int. J. Coal Geol. 2022, 249, 103880. [Google Scholar] [CrossRef]
  24. Wang, G. Deformation of organic matter and its effect on pores in mud rocks. AAPG Bull. 2020, 104, 21–36. [Google Scholar] [CrossRef]
  25. Wang, Y.; Yang, J. Origin of chemical structure and pore heterogeneity of over-matured organic matter in the Paleozoic marine shales, China. Int. J. Coal Geol. 2025, 303, 104742. [Google Scholar] [CrossRef]
  26. Ma, Y.; Ardakani, O.H.; Li, D.; Ma, J.; Zhong, N.; Huang, H. OM-hosted pore systems in Ordovician Wufeng–Silurian Longmaxi shales in Sichuan Basin SW China: Origin, gas storage, and flow behavior. Int. J. Coal Geol. 2025, 312, 104897. [Google Scholar] [CrossRef]
  27. Zagorski, W.A.; Wrightsone, G.R.; Bowman, D.C. The Appalachian Basin Marcellus gas play: Its history of development, geologic controls on production, and future potential as a world-class reservoir. AAPG Mem. 2012, 97, 172–200. [Google Scholar] [CrossRef]
  28. Chalmers, G.R.; Bustin, R.M.; Power, I.M. Characterization of gas shale pore systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units. AAPG Bull. 2012, 96, 1099–1119. [Google Scholar] [CrossRef]
  29. Wang, S.; Zhang, Z.; Dong, D.; Wang, Y.; Li, X.; Hu, J.; Huang, J.; Guang, Q. Microscopic pore structure and reasons making reservoir property weaker of Lower Cambrian Qiongzhusi shale, Sichuan Basin, China. Nat. Gas Geosci. 2016, 27, 1619–1628. [Google Scholar] [CrossRef]
  30. Song, D.; Wu, C.; Tuo, J. Relationships between Organic Structure Carbonization and Organic Pore Destruction at the Overmatured Stage: Implications for the Fate of Organic Pores in Marine Shales. Energy Fuels 2022, 36, 6906–6921. [Google Scholar] [CrossRef]
  31. Wang, D.; Wang, Y.; Dong, D.; Wang, S.; Huang, J.; Huang, Y.; Wang, S.; Li, X. Quantitative characterization of reservoir space in the Lower Cambrian Qiongzhusi Shale, Southern Sichuan Basin. Nat. Gas Ind. 2013, 33, 1–10. [Google Scholar] [CrossRef]
  32. Li, A.; Su, S.; Li, X.; Zhang, J.; Jing, C.; Zhao, Z. Effects of Organic Matter Graphitization on Shale Gas Accumulation in the Lower Paleozoic Longmaxi Formation from the Changning Area, Southern Sichuan Basin. Lithosphere 2022, 12, 7102509. [Google Scholar] [CrossRef]
  33. Hou, Y.; Yu, R.; Li, J.; Chen, Z.; Wang, C.; Chen, X.; Yang, R.; He, S. Molecular structure characterization of kerogen in contact metamorphic shales: Insights into the effect of graphitization on organic matter pores. AAPG Bull. 2024, 108, 633–662. [Google Scholar] [CrossRef]
  34. Liang, F.; Zhao, Q.; Zhao, S.; Shi, Z.; Zhang, Q.; Zhou, T.; Wang, Y.; Cui, H.; Chen, H.; Wang, Y. Genesis Mechanism of Low Resistivity in the Lower Cambrian Qiongzhusi Formation Shale and Its Response Characteristics to Pore Structure—Take the Z201 Well as an Example. ACS Omega 2025, 10, 43995–44011. [Google Scholar] [CrossRef]
  35. Piane, C.D.; Bourdet, J.; Josh, M.; Clennell, M.B.; Raven, M.D. Organic matter network in post-mature Marcellus Shale: Effects on petrophysical properties. AAPG Bull. 2018, 102, 2305–2332. [Google Scholar] [CrossRef]
  36. Kun, J.; Ye, Y.H.; Liu, S.G.; Ran, B.; Deng, B.; Li, Z.W.; Li, J.X.; Yong, Z.Q.; Sun, W. Characterization and Evolution of Nanoporosity in Superdeeply Buried Shales: A Case Study of the Longmaxi and Qiongzhusi Shales from MS Well #1, North Sichuan Basin, China. Energy Fuels 2018, 32, 191–203. [Google Scholar] [CrossRef]
  37. Zhang, A.Y.; Wu, D.M.; Guo, L.N.; Wang, Y.L. Geochemistry of Marine Black Shale Formation and Its Metallogenic Implication; Science Press: Beijing, China, 1987; pp. 13–30. [Google Scholar]
  38. Yan, J.; Li, Q.; Zhu, X. Main factors controlling shale gas accumulation and exploration targets in the Lower Cambrian, Sichuan Basin and its periphery. Pet. Geol. Exp. 2016, 38, 445–452. [Google Scholar] [CrossRef]
  39. Xu, H.; Shen, J.; Yang, F.; Hu, Q.; Yu, Y.; Xiong, L.; Zhang, T.; Deng, H.; He, J. Provenance, sedimentary paleoenvironment and organic matter accumulation mechanisms in shales from the Lower Cambrian Qiongzhusi Formation, SW Yangtze Block, China. Mar. Pet. Geol. 2025, 181, 107520. [Google Scholar] [CrossRef]
  40. Min, H.J. Characteristics and Formation Mechanism of Highly Over-Mature Shale Gas Reservoirs in Lower Cambrian Qiongzhusi Formation in the Southwestern Yangtze Plate. Doctoral Thesis, Chengdu University of Technology, Chengdu, China, 2020. [Google Scholar]
  41. Zhang, S.; Wang, H.; Su, J.; Wang, X.; He, K.; Liu, Y. Control of Earth system evolution on the formation and enrichment of marine ultra-deep petroleum in China. Pet. Explor. Dev. 2024, 51, 759–773. [Google Scholar] [CrossRef]
  42. Zhao, W.Z.; Li, J.Z.; Yang, T.; Wang, S.F.; Huang, J.L. Geological difference and its significance of marine shale gases in South China. Pet. Explor. Dev. 2016, 43, 547–559. [Google Scholar] [CrossRef]
  43. Wang, Y.M.; Li, X.J.; Chen, B.; Wu, W.; Dong, D.Z.; Zhang, J.; Han, J.; Ma, J.; Dai, B.; Wang, H.; et al. Lower limit of thermal maturity for the carbonization of organic matter in marine shale and its exploration risk. Pet. Explor. Dev. 2018, 45, 385–395. [Google Scholar] [CrossRef]
  44. Zhang, Q.; Zhao, Q.; Luo, C.; Liang, F.; Zhou, S.W.; Wang, Y.M.; Lu, B.; Qiu, Z.; Liu, H.L.; Liu, W. Effect of graphitization of organic matter on shale gas reservoirs: Take the marine shales in the southern Sichuan Basin as examples. Nat. Gas Ind. 2022, 42, 25–36. [Google Scholar] [CrossRef]
  45. GB/T 34533-2017; General Administration of Quality Supervision, Inspection and Quarantine and Standardization Administration of the People’s Republic of China. Measurement of Helium Porosity and Pulse Decay Permeability of Shale. China Standards Press: Beijing, China, 2017; pp. 1–12.
  46. Dai, H.M.; Wang, S.Y.; Chen, Y.C. Geochemistry of Petroleum Exploration, 2nd ed.; Petroleum Industry Press: Beijing, China, 2011. [Google Scholar]
  47. Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Hammes, U. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores. AAPG Bull. 2012, 96, 1071–1098. [Google Scholar] [CrossRef]
  48. Guo, X.S.; Li, Y.P.; Liu, R.B.; Wang, Q.B. Characteristics and controlling factors of micro pore structures of Longmaxi Shale Play in the Jiaoshiba area, Sichuan Basin. Nat. Gas Ind. 2014, 34, 9–16. [Google Scholar] [CrossRef]
  49. Dong, D.Z.; Liang, F.; Guan, Q.Z.; Jiang, Y.Q.; Zhou, S.W.; Yu, R.Z.; Gu, Y.F.; Zhang, S.R.; Qi, L.; Liu, Y. Development model and identification evaluation technology of Wufeng-Longmaxi Formation quality shale gas reservoirs in the Sichuan Basin. Nat. Gas Ind. 2022, 42, 96–111. [Google Scholar] [CrossRef]
  50. Okiongbo, K.S.; Aplin, A.C.; Larter, S.R. Changes in type II kerogen density as a function of maturity: Evidence from the Kimmeridge clay formation. Energy Fuel 2005, 19, 2495–2499. [Google Scholar] [CrossRef]
  51. Ardakani, O.H.; Sanei, H.; Ghanizadeh, A.; Lavoie, D.; Chen, Z.H.; Clarkson, C.R. Do all fractions of organic matter contribute equally in shale porosity? A case study from Upper Ordovician Utica Shale, southern Quebec, Canada. Mar. Pet. Geol. 2018, 92, 794–808. [Google Scholar] [CrossRef]
  52. Luo, S.Y.; Chen, X.H.; Li, H.; Liu, A.; Wang, C.S. Shale Gas Accumulation Conditions and Target Optimization of Lower Cambrian Shuijingtuo Formation in Yichang Area, West Hubei. Earth Sci. 2019, 44, 3598–3615. [Google Scholar] [CrossRef]
  53. Ma, Z.J.; Tang, X.; Zhang, J.C.; Zhai, G.Y.; Wang, Y.F.; Liang, G.D.; Luo, H. Organic matter-hosted pores in the Cambrian Niutitang shales of the Upper Yangtze region: Pore development characteristics and main controlling factors. Earth Sci. 2023, 30, 124–137. [Google Scholar] [CrossRef]
  54. Gao, H.T.; Cheng, P.; Wu, W.; Luo, C.; Xu, L.; Li, T.F.; Gai, H.F.; Tian, H. The geochemical, pore development and water-bearing characteristics of deep and ultra-deep marine shales and their effects on gas content: New implications from the shales of the first Lower Cambrian high yield gas well (Z201) in China. Pet. Sci. 2025, 22, 1839–1853. [Google Scholar] [CrossRef]
  55. Lei, H.; Sun, W.J.; Zuo, Y.J.; Wu, Z.H.; Dai, Q.C.; Lv, W.; Li, Y.Q.; Xie, X.G. Impact of micromechanical properties of organic matter on the micro-mesopore structures of the over-mature shale in the Niutitang Formation. Sci. Rep. 2025, 15, 4542. [Google Scholar] [CrossRef]
  56. He, X.; Liang, F.; Li, H.; Zheng, M.J.; Zhao, Q.; Liu, Y.; Liu, W.P. Breakthrough and enrichment mode of marine shale gas in the Lower Cambrian Qiongzhusi Formation in high-yield wells in Sichuan Basin. China Pet. Explor. 2024, 29, 142–155. [Google Scholar] [CrossRef]
  57. Yong, R.; Shi, X.W.; Luo, C.; Zhong, K.S.; Wu, W.; Zheng, M.J.; Yang, Y.R.; Li, Y.Y.; Xu, L.; Zhu, Y.Q.; et al. Aulacogen-uplift enrichment pattern and exploration prospect of Cambrian Qiongzhusi Formation shale gas in Sichuan Basin, SW China. Pet. Explor. Dev. 2024, 51, 1211–1226. [Google Scholar] [CrossRef]
  58. Zou, C.N.; Zhao, Z.F.; Pan, S.Q.; Yin, J.; Lu, G.W.; Fu, F.L.; Yuan, M.; Liu, H.L.; Zhang, G.S.; Luo, C.; et al. Unveiling the Oldest Industrial Shale Gas Reservoir: Insights for the Enrichment Pattern and Exploration Direction of Lower Cambrian Shale Gas in the Sichuan Basin. Engineering 2024, 42, 278–294. [Google Scholar] [CrossRef]
  59. Wang, Y.M.; Dong, D.Z.; Cheng, X.Z.; Huang, J.L.; Wang, S.F.; Wang, S.Q. Electric property evidences of the carbonification of organic matters in marine shales and its geologic significance: A case of the Lower Cambrian Qiongzhusi Shale in southern Sichuan Basin. Nat. Gas Ind. 2014, 34, 1–7. [Google Scholar] [CrossRef]
  60. Yang, A.; Firdaus, G.; Heidari, Z. Electrical resistivity and chemical properties of kerogen isolated from organicrich mudrocks. Geophysics 2016, 81, D643–D655. [Google Scholar] [CrossRef]
  61. Wang, Z.; Li, X.Q.; Qi, S.; Yao, S.P.; Wang, F.Y. Characteristics of Microscopic Pore Structures and Its Influencing Factors of the Qiongzhusi Formation Shales in the Southern Sichuan Basin. Acta Metall. Sin. 2018, 24, 273–284. [Google Scholar] [CrossRef]
  62. Cao, Y.C.; Yuan, G.H.; Yang, H.J.; Wang, Y.Z.; Liu, K.Y.; Zan, N.M.; Xi, K.L.; Wang, J. Current situation of oil and gas exploration and research progress of the origin of high-quality reservoirs in deep-ultra-deep clastic reservoirs of petroliferous basins. Acta Pet. Sin. 2022, 43, 112–140. [Google Scholar] [CrossRef]
  63. Kumar, V.; Sondergeld, C.; Rai, C.S. Effect of mineralogy and organic matter on mechanical properties of shale. Interpretation 2015, 3, SV9–SV15. [Google Scholar] [CrossRef]
  64. Chen, J.P.; Zhao, W.Z.; Wang, Z.M.; Zhang, S.C.; Deng, C.P.; Sun, Y.G.; Xiao, Z.Y. A discussion on the upper limit of maturity for gas generation by marine kerogens and the utmost of gas generative potential: Taking the study on the Tarim Basin as an example. Chin. Sci. Bull. 2007, S1, 125–132. [Google Scholar] [CrossRef]
  65. Zhou, W.; Xu, H.; Yu, Q.; Xie, R.C.; Deng, K. Shale gas-bearing property differences and their genesis between Wufeng-Longmaxi Formation and Qiongzhusi Formation in Sichuan Basin and surrounding areas. Lithol. Reserv. 2016, 28, 18–25. [Google Scholar] [CrossRef]
  66. Xie, G.Y.; Sun, M.D.; Ma, Y.Q.; Mohammadian, E.; Ostadhassan, M.; Pan, Z.J.; Duan, X.G. Impact of laminae characteristics on pore-fracture connectivity in the Wufeng-Longmaxi shale. Mar. Pet. Geol. 2025, 182, 107562. [Google Scholar] [CrossRef]
  67. Lis, G.P.; Topór, T.; Mastalerz, M. Organic matter content and its role in shale porosity development with maturity: Insights from Baltic Basin Silurian shales. Int. J. Coal Geol. 2025, 301, 104713. [Google Scholar] [CrossRef]
  68. Li, X.; Jiang, Z.X.; Jiang, S.; Wang, S.; Miao, Y.N.; Wu, F.; Cao, X.N. Synergetic effects of matrix components and diagenetic processes on pore properties in the Lower Cambrian shale in Sichuan Basin, South China. J. Nat. Gas Sci. Eng. 2021, 94, 104072. [Google Scholar] [CrossRef]
  69. Eliyahu, M.; Emmanuel, S.; Day-Stirrat, R.J. Mechanical properties of organic matter in shales mapped at the nanometer scale. Mar. Pet. Geol. 2015, 59, 294–304. [Google Scholar] [CrossRef]
  70. Lewan, M.D. Primary oil migration and expulsion as determined by hydrous pyrolysis. In Proceeding of the 13th World Petroleum Congress; OnePetro: Richardson, TX, USA, 1991; Volume 2, pp. 215–223. [Google Scholar]
  71. Kock, M.O.D.; Beukes, N.J.; Adeniyi, E.O.; Cole, D.; Götz, A.E.; Geel, C.; Ossa, F.G. Deflating the shale gas potential of South Africa’s Main Karoo basin. S. Afr. J. Sci. 2017, 113, 87–98. [Google Scholar] [CrossRef]
  72. Xie, G.L.; Jiao, K.; Deng, B.; Hao, W.D.; Liu, S.G. Pore characteristics and preservation mechanism of over-6000-m ultra-deep shale reservoir in the Sichuan Basin. Front. Earth Sci. 2023, 11, 1059869. [Google Scholar] [CrossRef]
  73. Yin, J.; Wei, L.; Sun, S.S.; Shi, Z.S.; Dong, D.Z.; Gao, Z.Y. Overpressure Generation and Evolution in Deep Longmaxi Formation Shale Reservoir in Southern Sichuan Basin: Influences on Pore Development. Energies 2023, 16, 2533. [Google Scholar] [CrossRef]
  74. Liu, S.G.; Deng, B.; Zhong, Y.; Ran, B.; Yong, Z.Q.; Sun, W.; Yang, D.; Jinag, L.; Ye, H. Unique geologic features of burial and superimposition of the Lower Paleozoic shale gas across the Sichuan basin and its periphery. Front. Earth Sci. 2016, 23, 11–28. [Google Scholar] [CrossRef]
  75. Guo, T.L.; Xiong, L.; Ye, S.J.; Dong, X.X.; Wei, L.M.; Yang, Y.T. Theory and practice of unconventional gas exploration in carrier beds: Insight from the breakthrough of new type of shale gas and tight gas in Sichuan Basin, SW China. Pet. Explor. Dev. 2023, 50, 27–42. [Google Scholar] [CrossRef]
  76. Guo, T.L.; Deng, H.C.; Zhao, S.; Wei, L.M.; He, J.H. Formation mechanisms and exploration breakthroughs of new type of shale gas in Cambrian Qiongzhusi Formation, Sichuan Basin, SW China. Pet. Explor. Dev. 2025, 52, 64–78. [Google Scholar] [CrossRef]
Minerals 16 00529 g001
Figure 1. (a) Sedimentary facies distribution of the Lower Cambrian Qiongzhusi Formation in the Upper Yangtze region (modified from [38]); (b) The regional tectonic position of the study area; (c) Burial history diagram of the Qiongzhusi Formation in the study area (modified from [40]); (d) Stratigraphic column of the Qiongzhusi Formation in the study area.
Figure 1. (a) Sedimentary facies distribution of the Lower Cambrian Qiongzhusi Formation in the Upper Yangtze region (modified from [38]); (b) The regional tectonic position of the study area; (c) Burial history diagram of the Qiongzhusi Formation in the study area (modified from [40]); (d) Stratigraphic column of the Qiongzhusi Formation in the study area.
Minerals 16 00529 g001
Minerals 16 00529 g002
Figure 2. Schematic diagram of the porosity measurement apparatus.
Figure 2. Schematic diagram of the porosity measurement apparatus.
Minerals 16 00529 g002
Minerals 16 00529 g003
Figure 3. Thin section and BSE images of the Qiongzhusi black shale samples: (a–c) Z1-6; (a) thin section image. Silt particles are angular–subangular in shape; (b,c) SEM images, and (c) is an enlarged view of the yellow box in (b). Less organic matter and microcrystalline quartz are in the intergranular pores, and silt particles are interconnected primarily by linear and concave-convex contacts; (d–f) ZNC-2; (d) thin section image. Silt particles are angular–subangular in shape; (e,f) SEM images, and (f) is an enlarged view of the yellow box in (e). Much more organic matter and microcrystalline quartz (yellow arrows) are in the intergranular pores, and silt particles are interconnected primarily by point and line contacts; (g–i) ZNC-5; (g) thin section image; (h,i) SEM images, and (i) is an enlarged view of the yellow box in (h). Much more organic matter is in the matrix; organic matter occurs in sheet-like forms with irregular shapes and closely adheres to the particles (red arrows). OM, Organic matter; SOM, sedimentary organic matter; Q, quartz; Ab, albite; Kf, potassium feldspar.
Figure 3. Thin section and BSE images of the Qiongzhusi black shale samples: (a–c) Z1-6; (a) thin section image. Silt particles are angular–subangular in shape; (b,c) SEM images, and (c) is an enlarged view of the yellow box in (b). Less organic matter and microcrystalline quartz are in the intergranular pores, and silt particles are interconnected primarily by linear and concave-convex contacts; (d–f) ZNC-2; (d) thin section image. Silt particles are angular–subangular in shape; (e,f) SEM images, and (f) is an enlarged view of the yellow box in (e). Much more organic matter and microcrystalline quartz (yellow arrows) are in the intergranular pores, and silt particles are interconnected primarily by point and line contacts; (g–i) ZNC-5; (g) thin section image; (h,i) SEM images, and (i) is an enlarged view of the yellow box in (h). Much more organic matter is in the matrix; organic matter occurs in sheet-like forms with irregular shapes and closely adheres to the particles (red arrows). OM, Organic matter; SOM, sedimentary organic matter; Q, quartz; Ab, albite; Kf, potassium feldspar.
Minerals 16 00529 g003
Minerals 16 00529 g004
Figure 4. BSE images of SOM in the Qiongzhusi shales: (a,b) no pores were observed in the SOM, ZNC-4; (b) an enlargement of the yellow box in (a); (c,d) the pores in the SOM are poorly developed, Zhao-1; (d) an enlargement of the yellow box in (c); (e) an enlargement of the position indicated by the green arrow in Figure 3h, and the pores in the SOM are poorly developed; SOM, sedimentary organic matter.
Figure 4. BSE images of SOM in the Qiongzhusi shales: (a,b) no pores were observed in the SOM, ZNC-4; (b) an enlargement of the yellow box in (a); (c,d) the pores in the SOM are poorly developed, Zhao-1; (d) an enlargement of the yellow box in (c); (e) an enlargement of the position indicated by the green arrow in Figure 3h, and the pores in the SOM are poorly developed; SOM, sedimentary organic matter.
Minerals 16 00529 g004
Minerals 16 00529 g005
Figure 5. BSE images of the migrated organic matter in Qiongzhusi shales: (a) organic matter is distributed parallel to bedding and tightly adheres to surrounding inorganic particles, ZNC-4, TOC = 13.7 wt.%; (b) an enlarged view of the yellow box in (a) showing a tight fit between organic matter and surrounding inorganic particles. No OM pores can be observed under SEM; (c,d): (d) is an enlarged view of the yellow box in (c). No OM pores can be observed under SEM, ZNC-1, TOC = 6.95 wt.%; (e) organic matter closely adheres to surrounding inorganic particles, Z1-8, TOC = 4.0 wt.%; (f) an enlarged view of the yellow box in (e). No OM pores can be observed under SEM; (g,h): (h) an enlarged view of the yellow box in (g). A few pores are developed within the organic matter with a plane porosity of 4.1 wt.%. Z1-9, TOC = 2.1 wt.%; (i) a few pores are developed within the organic matter with a plane porosity of 3.1 wt.%. Z1-4. OM, Organic matter; Op, OM pores; MQ, microcrystalline quartz.
Figure 5. BSE images of the migrated organic matter in Qiongzhusi shales: (a) organic matter is distributed parallel to bedding and tightly adheres to surrounding inorganic particles, ZNC-4, TOC = 13.7 wt.%; (b) an enlarged view of the yellow box in (a) showing a tight fit between organic matter and surrounding inorganic particles. No OM pores can be observed under SEM; (c,d): (d) is an enlarged view of the yellow box in (c). No OM pores can be observed under SEM, ZNC-1, TOC = 6.95 wt.%; (e) organic matter closely adheres to surrounding inorganic particles, Z1-8, TOC = 4.0 wt.%; (f) an enlarged view of the yellow box in (e). No OM pores can be observed under SEM; (g,h): (h) an enlarged view of the yellow box in (g). A few pores are developed within the organic matter with a plane porosity of 4.1 wt.%. Z1-9, TOC = 2.1 wt.%; (i) a few pores are developed within the organic matter with a plane porosity of 3.1 wt.%. Z1-4. OM, Organic matter; Op, OM pores; MQ, microcrystalline quartz.
Minerals 16 00529 g005
Minerals 16 00529 g006
Figure 6. BSE images of migrated organic matter in the framboidal pyrite in Qiongzhusi shales: (a,b): (b) is an enlarged view of the yellow box in (a). No OM pores can be found under SEM. Z1-8, TOC = 4.0 wt.%; (c,d): (d) is an enlarged view of the yellow box in (c). The pores within the organic matter (OM pores) are well developed and appear as homogeneous sponge-like pores, with a plane porosity of 15.1%. Z1-9, TOC = 2.1 wt.%. (e) The pyrite has been dissolved, leaving behind moldic pores, ZNC-6, TOC = 1.6 wt.%. (f) An enlarged view of the yellow box in (e). The pores within organic matter (OM pores) are well developed and appear as complex sponge-like pores, with a plane porosity of 15.2%. Sp, Framboidal pyrite; OM, organic matter; Mp, pyrite moldic pore.
Figure 6. BSE images of migrated organic matter in the framboidal pyrite in Qiongzhusi shales: (a,b): (b) is an enlarged view of the yellow box in (a). No OM pores can be found under SEM. Z1-8, TOC = 4.0 wt.%; (c,d): (d) is an enlarged view of the yellow box in (c). The pores within the organic matter (OM pores) are well developed and appear as homogeneous sponge-like pores, with a plane porosity of 15.1%. Z1-9, TOC = 2.1 wt.%. (e) The pyrite has been dissolved, leaving behind moldic pores, ZNC-6, TOC = 1.6 wt.%. (f) An enlarged view of the yellow box in (e). The pores within organic matter (OM pores) are well developed and appear as complex sponge-like pores, with a plane porosity of 15.2%. Sp, Framboidal pyrite; OM, organic matter; Mp, pyrite moldic pore.
Minerals 16 00529 g006
Minerals 16 00529 g007
Figure 7. BSE images of the well-developed OM pores in the Qiongzhusi shales with relatively low TOC: (a,b): (b) is an enlarged view of the yellow box in (a). The plane porosity of the organic matter in (b) is 6.9%, ZNC-7, TOC = 1.7 wt.%. (c) The plane porosity of the organic matter is 8.3%, Z1-4, TOC = 1.8 wt.%. (d) Organic matter between framboidal pyrite, exhibiting well-developed pores, JZ-1, TOC = 1.2 wt.%. (e,f) OM pores are nearly elliptical in shape, and the plane porosity of organic matter is 8%, YS-2, TOC = 0.6 wt.%. Op, OM pores; Qz, quartz.
Figure 7. BSE images of the well-developed OM pores in the Qiongzhusi shales with relatively low TOC: (a,b): (b) is an enlarged view of the yellow box in (a). The plane porosity of the organic matter in (b) is 6.9%, ZNC-7, TOC = 1.7 wt.%. (c) The plane porosity of the organic matter is 8.3%, Z1-4, TOC = 1.8 wt.%. (d) Organic matter between framboidal pyrite, exhibiting well-developed pores, JZ-1, TOC = 1.2 wt.%. (e,f) OM pores are nearly elliptical in shape, and the plane porosity of organic matter is 8%, YS-2, TOC = 0.6 wt.%. Op, OM pores; Qz, quartz.
Minerals 16 00529 g007
Minerals 16 00529 g008
Figure 8. The other pores in the Qiongzhusi shales: (a) grain-edge intergranular pores (yellow arrow), M-1; (b) residual intergranular pores (yellow arrow), Z1-4; (c) pyrite intergranular pores (yellow arrow), Z1-6; (d) interlayer pores within clay minerals (yellow arrow), ZNC-6; (e) interlayer pores within mica minerals (yellow arrow), YS-1; (f) dissolution pores within potassium feldspar (yellow arrow), ZNC-3; Ab, albite; Q, quartz; Sp, framboidal pyrite; Cm, clay minerals; Mi, mica minerals; Kf, potassium feldspar; Dp, dissolved pores.
Figure 8. The other pores in the Qiongzhusi shales: (a) grain-edge intergranular pores (yellow arrow), M-1; (b) residual intergranular pores (yellow arrow), Z1-4; (c) pyrite intergranular pores (yellow arrow), Z1-6; (d) interlayer pores within clay minerals (yellow arrow), ZNC-6; (e) interlayer pores within mica minerals (yellow arrow), YS-1; (f) dissolution pores within potassium feldspar (yellow arrow), ZNC-3; Ab, albite; Q, quartz; Sp, framboidal pyrite; Cm, clay minerals; Mi, mica minerals; Kf, potassium feldspar; Dp, dissolved pores.
Minerals 16 00529 g008
Minerals 16 00529 g009
Figure 9. Scatter plot of porosity versus TOC of the Qiongzhusi shales.
Figure 9. Scatter plot of porosity versus TOC of the Qiongzhusi shales.
Minerals 16 00529 g009
Minerals 16 00529 g010
Figure 10. Stratigraphic column of the Longmaxi Formation of Well Y12 in the study area.
Figure 10. Stratigraphic column of the Longmaxi Formation of Well Y12 in the study area.
Minerals 16 00529 g010
Minerals 16 00529 g011
Figure 11. OM pores in the Longmaxi post-mature shales in Well Y12 in the study area: (a,b) 2285.8 m, TOC = 3.54 wt.%; (c,d) 2289.2 m, TOC = 3.92 wt.%.
Figure 11. OM pores in the Longmaxi post-mature shales in Well Y12 in the study area: (a,b) 2285.8 m, TOC = 3.54 wt.%; (c,d) 2289.2 m, TOC = 3.92 wt.%.
Minerals 16 00529 g011
Table 1. Information on shale samples from the Qiongzhusi Formation.
Table 1. Information on shale samples from the Qiongzhusi Formation.
Sample NumberSection/WellSampleLithologyTOC/wt.%Porosity/%
1MajinziM-1argillaceous siltstone3.6/
2Z1Z1-1argillaceous siltstone4.772.4
3Z1Z1-2argillaceous siltstone4.241.8
4Z1Z1-3argillaceous siltstone1.843.4
5Z1Z1-4argillaceous siltstone1.8/
6Z1Z1-5argillaceous siltstone1.772.6
7Z1Z1-5argillaceous siltstone1.683.3
8Z1Z1-7argillaceous siltstone1.684.5
9Z1Z1-8argillaceous siltstone4.0/
10Z1Z1-9silty mudstone2.1/
11SonglinZNC-1argillaceous siltstone6.951.6
12SonglinZNC-2argillaceous siltstone6.96/
13SonglinZNC-3argillaceous siltstone6.61/
14SonglinZNC-4mudstone13.71.3
15SonglinZNC-5mudstone10.92.1
16SonglinZNC-6mudstone1.6/
17SonglinZNC-7mudstone1.7/
18JingkouJK-1marl1.332.3
19JingkouJK-2marl1.771.6
20Zhao 1Zhao-1mudstone//
21YS6YS-1argillaceous siltstone//
22YS6YS-2mudstone0.6/
23Jinzhu1JZ-1mudstone1.15/
Table 2. Types of OM pores in the Qiongzhusi shales.
Table 2. Types of OM pores in the Qiongzhusi shales.
Pore TypesPore PatternTypical ImageDescription
Isolated irregular poresMinerals 16 00529 i001Minerals 16 00529 i002The pores formed in isolation, exhibiting heterogeneous sizes, irregular morphologies, and poor interconnectivity.
Isolated regular poresMinerals 16 00529 i003Minerals 16 00529 i004The pores formed in isolation, exhibiting relatively uniform morphologies and poor interconnectivity.
Homogeneous sponge-like poresMinerals 16 00529 i005Minerals 16 00529 i006The pores are extensively and uniformly developed and exhibit excellent interconnectivity. Morphology of the pores is nearly ellipsoidal.
Complex sponge-like poresMinerals 16 00529 i007Minerals 16 00529 i008The pore network is highly developed, exhibiting significant heterogeneity in pore size and excellent interconnectivity. Morphology of the pores is nearly ellipsoidal.
Coalesced sheet-like poresMinerals 16 00529 i009Minerals 16 00529 i010The sheet-like pores are concentrated and aligned along a single direction.
Dense organic matterMinerals 16 00529 i011Minerals 16 00529 i012Organic matter has no pore development inside and closely adheres to minerals.
Note: In the pore pattern images, the black part represents pores, the gray part represents organic matter, and the blue part represents minerals.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Min, H.; Xu, J.; Liang, S.; Liu, C.; Zhao, L. The Origin of Organic Matter Pore Destruction in Post-Mature Shales of the Qiongzhusi Formation, Southwestern Upper Yangtze, China: Evidence from Scanning Electron Microscopy. Minerals 2026, 16, 529. https://doi.org/10.3390/min16050529

AMA Style

Min H, Xu J, Liang S, Liu C, Zhao L. The Origin of Organic Matter Pore Destruction in Post-Mature Shales of the Qiongzhusi Formation, Southwestern Upper Yangtze, China: Evidence from Scanning Electron Microscopy. Minerals. 2026; 16(5):529. https://doi.org/10.3390/min16050529

Chicago/Turabian Style

Min, Huajun, Jinhui Xu, Shuangqing Liang, Chunyan Liu, and Limin Zhao. 2026. "The Origin of Organic Matter Pore Destruction in Post-Mature Shales of the Qiongzhusi Formation, Southwestern Upper Yangtze, China: Evidence from Scanning Electron Microscopy" Minerals 16, no. 5: 529. https://doi.org/10.3390/min16050529

APA Style

Min, H., Xu, J., Liang, S., Liu, C., & Zhao, L. (2026). The Origin of Organic Matter Pore Destruction in Post-Mature Shales of the Qiongzhusi Formation, Southwestern Upper Yangtze, China: Evidence from Scanning Electron Microscopy. Minerals, 16(5), 529. https://doi.org/10.3390/min16050529

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop