Geological Conditions and Reservoir Formation Models of Low- to Middle-Rank Coalbed Methane in the Northern Part of the Ningxia Autonomous Region

Abstract

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.

1. Introduction

Deep CBM exploration is regarded as an important supplement to China’s natural gas production, which is of great significance for ensuring China’s energy security and achieving the double carbon strategic goal [1,2,3,4,5,6,7,8,9,10]. China has extremely abundant low- to middle-rank CBM resources, with exclusively low-rank CBM resources accounting for 14.70 × 10¹² m³, representing about 40% of the total [11,12,13]. A key area for the exploration and development of low- to middle-rank CBM is the Baode block on the eastern edge of the Ordos Basin, with a cumulative CBM production of over 50 × 10⁸ m³. The enormous potential for the development of low- to middle-rank CBM resources has been demonstrated. The study of geological conditions for CBM reservoir formation is considered a key factor in evaluating resource potential. The accumulation of low- to middle-rank CBM reservoirs is controlled by structures, sedimentary microfacies, and hydrodynamic conditions [14,15,16,17,18,19,20,21]. However, the main controlling factors and models for reservoir formation vary in different regions.

The United States had the earliest commercial development and utilization of CBM in the world, with 22.19 × 10¹² m³ of resources. The San Juan Basin develops high- to middle-rank coal, while the Powder River Basin develops low-rank coal [22]. Ramos A B used three-dimensional AVO (amplitude changes with offset) analysis and anisotropic modeling to study CBM in the San Juan Basin. Faults generated by multi-stage tectonic activities form dominant seepage channels for the migration of CBM, and a three-dimensional framework of AVO, anisotropy parameters, and the fracture network was proposed [23]. The San Juan model is applicable to high-rank coal with strong fractures, while the Shizuishan model provides a new paradigm for fracture lithology involving the dynamic balance of medium-rank coal in areas with medium levels of tectonic activity (such as the western edge of Ordos). Zhang et al. revealed a synergistic mechanism of methane adsorption energy parameters under temperature and pressure changes by analyzing the pore structure of coal seams, adsorption models, and molecular dynamics simulations [24,25,26,27]. Shi et al. constructed a deep CBM content prediction model based on pore compression characteristics, taking into account the influence of gas state factors and effective porosity [28]. Yang et al. used a variable temperature pressure pore permeability test and a variable temperature pressure equilibrium water isothermal adsorption test to clarify the occurrence and distribution of methane in different phase states [29]. Zhang et al. used carbon isotope fractionation tracing to analyze the differences in the changes of ¹³C₁ in different analytical time ranges and clarified the characteristics of the ratio of free gas/adsorbed gas and its influencing factors [30]. However, the exploration of deep CBM in the Shizuishan area on the western margin of the Ordos Basin started late, and the rules for enrichment and accumulation are unclear. It is urgent to clarify these mechanisms to guide the precise exploration and development of CBM resources in the western wing of the basin.

The Yinchuan Basin, with abundant coal and CBM resources, is regarded as an important energy base in the Ningxia Hui Autonomous Region. Since 2020, Ningxia CBM Technology Development Co., Ltd., located in Shizuishan City, Huining District, Hongguozǐ Town, Hongli Street North Side, 51, has achieved a breakthrough of 20,000 cubic meters per day in single-well screen-completion horizontal wells and a cumulative production of 4.5 million cubic meters in the Shizuishan mining area. However, the exploration and development of CBM in regions with complex geological settings, such as basin-marginal structural belts, low-to-medium rank coal, and small-scale yet resource-abundant reservoirs, face a series of theoretical and technological challenges. It is imperative to conduct targeted research to address these bottlenecks, thereby advancing the efficient exploitation of CBM resources in Ningxia. Such advancements are crucial to supporting energy transition, ecological conservation, and high-quality economic development within the Yellow River Basin.

The Carboniferous-Permian coal seams in the Shizuishan area, Ningxia, were selected as research subjects for this study. Based on extensive datasets, including drilling logs, well logging, seismic profiles, and CBM test data, the geological conditions of Carboniferous-Permian CBM reservoirs were systematically analyzed. Key controlling factors governing CBM enrichment were identified, and genetic accumulation models were established using integrated analysis. Through the integrated application of seismic data, drilling logs, well logging, and coal rock test data, a comprehensive geological analysis of CBM was conducted, enrichment patterns of CBM were systematically investigated, and a genetic accumulation model was proposed. These findings provide a robust theoretical basis and actionable technical support for the exploration and development of low-to-medium rank CBM reservoirs in geologically analogous regions.

2. Geological Setting and Experimental Principle

2.1. Geological Setting

The study area is located in the Helan Mountain tectonic belt and Ordos Basin, exhibiting a rhombic shape with a general NNW-SSE orientation. The basin extends over 180 km in a north-south direction and spans approximately 60 km from east to west. The basin’s structural framework is primarily controlled by four major fault zones from west to east—the Helan Mountain eastern piedmont fault (F1), Luhuatai Fault (F2), Yinchuan-Pingluo Fault (F3), and Yellow River Fault (F4) [31,32,33] (Figure 1a). As the western boundary-controlling fault of Yinchuan Basin, the eastern piedmont fault of Helan Mountain (F1) has dominated the basin’s formation and subsidence since the Cenozoic era [34,35]. The study area is located at the northwestern margin of Yinchuan Basin, immediately adjacent to the Helan Mountain fold-thrust belt. It is characterized by a thrust-fold structural style. The structural configuration is mainly governed by NE-trending reverse faults, with the hanging wall developing a flexural structure exhibiting higher elevation in the northeast and lower elevation in the southwest. The footwall is predominantly characterized by a northwest-dipping monoclinal structure with formation dip angles ranging between 15° and 25°. The study area is subdivided into three secondary structural units from west to east—the western slope zone, central fault development zone, and eastern slope zone (Figure 1b).

The coal-bearing strata consist of Carboniferous Yanghugou Formation, Carboniferous-Permian Taiyuan Formation, among others. Taiyuan Formation comprises barrier island-lagoon tidal flat deposits, containing six depositional cycles and coal seams 5 to 9. The developed Shanxi Formation is characterized by fluvial–deltaic–lacustrine sedimentation and well-developed peat swamps. This stratigraphic unit comprises three depositional cycles. Moreover, coal seams 2 and 3 of the Shanxi Formation and coal seams 5 and 6 of the Taiyuan Formation are the principal coal seams. Samples were taken from the northern part of the Ningxia Hui Autonomous Region (Figure 1c).

2.2. SEM Experimental Method

Coal samples (3 mm × 3 mm) were prepared using liquid nitrogen cryofracturing. A 5 nm platinum film was sputtered onto the samples to eliminate charging effects. High-resolution imaging was conducted at an accelerating voltage of 15 kV and a working distance of 20 mm.

2.3. Low-Temperature Liquid Nitrogen Adsorption Experimental Method

Coal samples were vacuum degassed at 105 °C before being tested using a QASAP 2460 automatic physical adsorption instrument to measure the nitrogen adsorption–desorption isotherms at 77 K. The micropore–mesopore pore size distribution was calculated based on the BET theory and DFT model.

2.4. High-Pressure Mercury Intrusion Porosimetry Method

Samples were pretreated and tested using an AutoPore IV 9510 by McMurdic Corporation in the United States automatic mercury intrusion porosimeter to determine the pore size distribution. The fractal dimension was introduced to quantify the heterogeneity of fractured coal pores, and a multi-scale pore-permeability model was constructed.

3. Results and Discussion

3.1. Thickness and Buried Depth

The thickness of coal seams 2 and 3 in the footwall of fault F1 is 4.7–11.7 m, with an average of 5.8 m. The thickness in the north is larger than that in the west, and the distribution of the coal seams is stable. The thickness of coal seams 5 and 6 is 5.9–13.3 m, with an average of 6.5 m. The thickness in the south is larger than that in the north. The thickness in the southwest of the coal seam exceeds 10 m (Figure 2). The overall thickness distribution is more stable, which provides a basis for CBM enrichment.

The burial depth of the main coal seam in the footwall of fault F1 is affected by the structure. Along the trend of the Helanshan fault, it is characterized by a southeastward shallow northwestward deep variation. The depth of coal seams 2 and 3 collected from the Shanxi Formation is 250–1150 m, with an average of 750 m. While the Taiyuan Formation seams 5 and 6 are between 300 and 1300 m deep, with an average of 810 m. The variation of burial depth in the east and south is more stable, and the burial depth of the coal seams near fault F1 in the west and north varies (Figure 2).

3.2. Coal Petrographic Characteristics and Gas-Bearing Properties

3.2.1. Coal Quality

The coal body structure of coal seams 2 and 3 is characterized by massive rock, with a minor presence of a powdery structure; these seams are classified as clastic coal. Coal seams 5 and 6 belong to cataclastic coal, with some primary structure coal. In terms of macrocoal composition, bright coal predominates, followed by dark coal, intermixed with vitrinite strips. Regarding coal seams 2 and 3, the organic matter content ranges from 81.65% to 85.64%, averaging at 83.55%. The inorganic matter content falls between 14.55% and 16.09%, with an average of 15.32%. Within the organic components, the vitrinite content accounts for 42.75–45.69%, averaging 44.20%; the inertinite content is 35.85–36.27%, averaging 36.06%; and the exinite content is 3.09–3.50%, averaging 3.30%. As for coal seams 5 and 6, the organic matter content varies from 88.72% to 93.15%, with an average of 90.94%, and the inorganic matter content is 6.78~11.10%, with an average of 8.94%. Among organic components, vitrinite content accounted for 40.66–56.48%, with an average of 48.75%; inertinite content accounted for 36.09~47.24%, with an average of 41.67%; and the exinite content accounted for 0.58~0.82%, with an average of 0.70%.

The degree of metamorphism increases with burial depth. However, the degree of metamorphism in the southeast is lower than that in the northwest, but the degree variation of metamorphism is smaller. The R_o,max of is 0.98~1.07%, and belongs to middle-rank coal.

The coal body structure has an impact on CBM reservoir through modulating four key petrophysical parameters: the gas storage capacity, adsorption–desorption variation, permeability, and pulverized coal production variation of coal reservoirs [36,37]. Based on logging data, coal seams 2 and 3 are dominated by primary texture coal, and coal seams 5 and 6 are dominated by primary cataclastic texture coal, which is distributed in proximity to the fault (Figure 3).

Based on the analysis of mud logging and well logging data, the two main coal seams are characterized as complex structure coal seams (complex structure coal refers to coal seams characterized by intricate structural features resulting from multi-phase tectonic deformation and diagenetic–metamorphic processes), and 1~2 sets of dirt bands have developed in the middle of the coal seams 2 and 3, with a thickness of 0.5~1.0 m. Meanwhile, only 2~4 sets of dirt bands have developed in the lower part of coal seams 5 and 6, the thickness of which varies from 0.6 m to 1.5 m.

3.2.2. Coal Seam Gas-Bearing

The sampling test results for the CBM wells reveal that the gas content in coal seams 2 and 3 falls within the range of 5.90–12.61 m³/t, averaging 8.50 m³/t. This indicates that the gas content in the southeast region is higher than that in the northwest, and the gas content in the high gas area in the southeast is above 10.0 m³/t. The gas content of coal seams 5 and 6 ranges from 4.39 to 13.88 m³/t, with an average of 8.11 m³/t. Meanwhile, the gas content in the high gas area in the south is above 12.0 m³/t in the eastern region of the coal seam, which is higher than in the western region (Figure 4).

Gas saturation is an important parameter for studying CBM reservoir enrichment, and the longitudinal enrichment could be well reflected by the relationship between gas saturation and burial depth, and the relationship between gas saturation and burial depth is positively correlated with CBM production [38]. The measured gas content and isothermal adsorption show that the gas saturation of two-layer coal seams is higher, and gas saturation is 70% on average. In the transverse direction, gas saturation of higher gas content in the east and southeast is above 70%. In the longitudinal direction, the gas saturation of coal seams 5 and 6 is higher than that of coal seams 2 and 3, and gas saturation shows an increasing trend with increasing depth.

3.2.3. Characteristics of Coal Seam Roof and Floor

The gas content of coal seams is affected by the lithologic assemblage and air permeability of the rock surrounding the coal reservoir. The greater the permeability of the coal seam and its surrounding rock, the easier it is for the CBM to be lost, and the smaller the coal seam gas content. On the contrary, when permeability is lower, CBM is more effectively preserved, leading to elevated gas content levels. The three-dimensional seismic and logging data show that the roof and floor of the main coal seam are mainly mudstone, carbonaceous mudstone, silty mudstone, argillaceous siltstone, and siltstone, and the roof of coal seams 2 and 3 is siltstone in the northwest. The rocks in the central and southern areas of the roof of coal seam 6 are mainly sandstone (Figure 5). Overall, the surrounding rock formations of the coal reservoir exhibit tight lithology with excellent sealing capacity, which refers to a geological stratum’s ability to act as an impermeable barrier, effectively preventing the upward migration of fluids from underlying reservoir units.

3.2.4. Physical Characteristics of Coal Reservoirs

The physical properties of coal reservoirs are evaluated from different dimensions using testing techniques such as scanning electron microscopy, low-temperature liquid nitrogen, and a high-pressure mercury intrusion test. In the collected samples, the macroscopic coal rock types are mainly bright coal, semi-bright coal, and semi-dull coal. As shown in Figure 6a–c, bright coal is mainly composed of vitrinite, with inertinite, low mineral content, and locally common structural vitrinite. Due to high-temperature and high-pressure conditions, the developed organic matter pores and cell volumes are mostly compacted. Semi-bright coal is mainly composed of vitrinite, and its submicroscopic components are mainly homogeneous vitrinite. Organic matter pores develop in a honeycomb-like pattern, occasionally filled with carbonate minerals (Figure 6d–f). Due to the influence of microstructure and ash content, the content of vitrinite in dull coal is relatively low, and there are clay mineral kaolinite layer-like aggregates on the surface of vitrinite (Figure 6g–i).

Pore volumes of three coal samples were studied based on high-pressure mercury intrusion testing. The results in Figure 7 show that the mercury injection mercury removal curve is parallel, indicating that the pores of this type of sample are mostly semi-open and the pore structure is relatively simple. The selected sample has an average porosity of 5.4% and a specific surface area of 3.79 m²·g⁻¹.

Micropore and mesopore diameter distribution has been obtained using N₂ and CO₂ adsorption. The results shows that the mesopore of all the selected samples is 0.001~0.006 cm³·g⁻¹, with an average of 0.004 cm³·g⁻¹; the pore diameter is 6.2~12.3 nm, with an average of 7.3 nm. Based on the mesopore distribution, the main pore peak is 60 nm; this type of pore provides the majority of the mesopore volume. The results show that the micropore of all the selected samples is 0.011~0.014 cm³·g⁻¹, with an average of 0.012 cm³·g⁻¹; the pore diameter is 0.3~0.6 nm, with an average of 0.5 nm. Based on the micropore distribution, the main pore peak is 0.5~0.6 nm and 0.8 nm, this type of pore provides the majority of the micropore volume.

Compared to mesopores, micropores have a more uniform distribution.

3.2.5. In-Situ Stress Conditions

Injection and pressure drop well test data obtained from coal seam 9 collected from 7 wells reveal that the rupture pressure of the coal seam buried at depths of 547.9~893.51 m (average 763.12) is 7.63~15.78 MPa (average 12.5), and the closing pressure is 6.52–14.16 MPa (average 11.14). It is evident that the rupture pressure shows a linear increase as the burial depth increases. Addtionally, there is a positive correlation between the rupture pressure and the closing pressure. (Figure 8a).

The relative magnitudes of three principal stresses is an important basis for assessing the stress field in situ. Anderson categorized the in-situ stress field as a reverse fault stress field (SH > Sh > Sv), a strike–slip fault stress field (SH > Sv > Sh), and a normal fault stress field (Sv > SH > Sh) based on the relationships among three principal stresses. The in-situ stress test shows that the normal fault stress field is mainly developed in the vertical direction of the coal seam (Figure 9).

3.3. Geological Conditions for CBM Enrichment

3.3.1. Construction Conditions

The influence of structural conditions on CBM enrichment is primarily manifested in three respects—tectonic activity, burial depth, and structural types. The coupling of structural evolution and morphology affects the coal seam, the distribution of roof and floor, and the development of the fracture network, and reduces the accumulation of coalbed gas. The study area is located on the northwestern margin of the Yinchuan Basin, adjacent to the Helan Mountain fold belt, and is characterized by a thrust-fold belt dominated by the F1 thrust fault. Since the Late Carboniferous, the study area has sequentially developed thrust-nappe tectonic episodes in the Taiyuan–Shanxi–Shihezi Formations, transitioning to regional subsidence during the Sunjiagou Formation depositional period. The Cenozoic era witnessed differential subsidence extremes in the northwestern region. Tectonic-sedimentary primarily controls coal seam distribution and structural styles (Figure 9). On the hanging wall of thrust fault F1, fractures are well developed, with steep dips of approximately 25°. The deformation is intense, resulting in pronounced folding and coal fragmentation, which facilitates the dissipation of CBM (Figure 10). On the footwall of the F1 thrust fault, the strata have gentle dips ranging from approximately 1° to 10°, with relatively undeveloped faulting. The coal seams are composed of original and cataclastic structures, which provide a sealing effect conducive to CBM enrichment. In the completed MK05 and MK06 wells, gas content tests of coal seams 5 and 6 show a sharp contrast—0.27 m³/t on the hanging wall and 7.44 m³/t on the footwall. The geological structure elucidates the binary differentiation effect of the upper plate dissipation–lower plate preservation resulting from the structural reversal of F1 thrust fault zone: upper plate exhibits high-angle shear fracture networks (dip angles greater than 25°) that form dominant gas escape pathways, resulting in a sharp decline in gas content; the lower plate, due to the release of tectonic stress, forms a gently dipping zone with low dip angles (1–10°), coupled with a self-sealing effect caused by coal fragmentation, establishing a composite preservation mechanism of tectonic stress shielding and physical trapping (Figure 11).

3.3.2. Sedimentary Environment

The sedimentary environment plays a decisive role in determining the lithology and spatial distribution of coal-bearing rocks, which controls the thickness of the coal seams and the petrology of the roof and floor plates, which in turn controls the enrichment of CBM. During the period of Taiyuan Formation, the study area developed a barrier island tidal flat lagoon sedimentary environment under the relatively stable transgressive background. The peat accumulation rate and the increase rate of accommodation space maintained a long-term balance, giving rise to the wide distribution and thick coal seams 5 and 6. The lagoon environment during the Taiyuan Formation resulted in the lithology of the roof and floor of coal seams 5 and 6 being mainly mudstone, with some of them being barrier island tidal flat sandstone, which was favorable for the preservation of CBM. During the period of the Shanxi Formation, the sea level declined, and the sedimentary environment transformed into a continental one, including rivers, deltas, and lakes. Affected by terrigenous clasts, the delta plain environment was conducive to the formation of thicker coal seams 2 and 3. During the Shanxi Formation period, the lithology of the roof and floor of coal seams 2 and 3 in the distributary bay of the delta plain is mainly mudstone, and part is distributary channel sandstone from the delta plain, which is conducive to the enrichment and preservation of CBM. The coal accumulation of the Taiyuan Formation is stronger than that of the Shanxi Formation, which provides more material basis for the formation of CBM.

3.3.3. Hydrodynamic Effects

The migration and dispersion of CBM are often influenced by strong hydrodynamic effects, which are unfavorable for enrichment. In contrast, groundwater pressure in the retention zone can impede the migration of CBM, facilitating enrichment and preservation. Groundwater originates from the fractured water of Helan Mountain and Yellow River water replenishing along ancient river channels. It migrates from shallow to deep parts through coal seams, subjecting the CBM within the coal body to hydrodynamic influences, leading to continuous release, desorption, and diffusion. The gases escaping from the coal seams are trapped by the coal and water bodies, forming CBM reservoirs, which are conducive to the preservation of CBM.

The micro-differential mapping method and resistivity identification method were used to identify the aquifers. Differing from the Shanxi Formation, the main aquifers are sandstone aquifers, and the aquifers of the Taiyuan Formation have better water-bearing properties (Figure 12). The aquifer water abundance, quantified using the differential interpretation method, shows higher values in the northern and southern sectors, while the central zone demonstrates lower productivity. The calculated water level ranges from 623 to 894 m, with high water levels (above 800 m) located in the central and southern parts of the block, and low water levels (below 760 m) located in the northern and southeastern parts of the block. This reflects that the overall groundwater flow direction is from the central area to both limbs.

3.4. CBM Accumulation Patterns

3.4.1. Steep Limb of Syncline-Structural-Fault Escape Type

The accumulation of CBM in the hanging wall area of F1 in the western part is significantly influenced by the fault, forming the subcline steep limb–structural–fault escape type CBM accumulation pattern. The roofs of coal seams 6 and 3 are composed of shale, which is conducive to the preservation of CBM. However, given the influence of the F1 reverse fault, the coal seams 3 and 6 have a shallow burial depth, leading to a large amount of CBM escaping along the fault, which is unfavorable for the accumulation of CBM. These factors collectively result in lower gas content in the coal seams.

3.4.2. Syncline Axial Zone-Structural-Hydrodynamic Sealing Type

Enrichment of CBM in the central part is affected by structural and hydrodynamic conditions, forming a syncline axial zone–structural–hydrodynamic sealing type CBM accumulation model. This region is located in the western part of the lower panel of fault F1, where the syncline axial zone has a larger burial depth and a thicker overlying strata above the coal seam, which is conducive to the preservation of CBM. The syncline axial zone has weaker hydrodynamic conditions, resulting in favorable preservation conditions for CBM. The roof of the coal seam, dominated by shale, also contributes to the enrichment of CBM. These factors lead to higher gas content observed within the coal seam.

3.4.3. Gentle Limb of Syncline-Lithology-Hydraulic Sealing Type

The enrichment of CBM in the eastern area is affected by lithology and hydraulic conditions, forming a gentle limb of syncline–lithology–hydraulic sealing type CBM accumulation pattern. This area is located on the eastern side of the lower panel of F1, within the gentle limb of the syncline, far from the F1 reverse fault, which has a minor impact on the preservation of CBM. The roof of the coal seam is mainly composed of shale, providing a good cap rock for the enrichment of CBM. The weaker hydraulic environment with stagnant water is also conducive to the enrichment of CBM. These factors collectively contribute to higher gas content in the coal seams.

4. Conclusions

(1)

The Permian–Triassic coal-bearing strata, including coal seam 6 of the Taiyuan Formation and coal seam 3 of the Shanxi Formation, exhibit significant thickness and are dominated by thermogenic coalbed methane (CBM) (the thickness of coal seams 2 and 3 in the footwall of the fault F1 is 4.7–11.7 m, with an average of 5.8 m. The thickness of coal seams 5 and 6 is 5.9–13.3 m, with an average of 6.5 m). These strata are buried at suitable depths with mudstone as the primary roof rock, providing favorable gas-bearing properties and good reservoir conditions.

(2)

The primary factors influencing CBM enrichment include structural conditions, depositional environments, and hydrodynamic regimes. The broad, gentle syncline and a series of reverse faults formed by compressional forces provide effective sealing for CBM accumulation. The barrier island–shoreface–lagoon depositional environment of the Taiyuan Formation and the fluvial–delta–lacustrine environment of the Shanxi Formation created favorable conditions for the formation of thick coal seams in seams 6 and 3, while also affecting the distribution of roof lithology. The groundwater within the coal-bearing strata shows a high degree of mineralization and a relatively connate water environment, which is conducive to CBM preservation and enrichment.

(3)

Based on the analysis of CBM enrichment conditions, three typical CBM accumulation models are identified: the syncline steep limb–structural–fault escape type, the syncline axis–structural–hydrodynamic sealing type, and the syncline gentle limb–lithologic–hydrodynamic sealing type.