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Coalbed Methane Enrichment Characteristics and Exploration Target Selection in the Zhuozishan Coalfield of the Western Ordos Basin, China

The Zhuozishan coalfield at the western margin of the Ordos Basin is one of the main coal-mining areas in China, and recent explorations have revealed the great potential for coalbed methane (CBM) resources in its Carboniferous and Permian strata. In this paper, the controlling factors of CBM enrichment of the major coals are studied in this coalfield and the CBM resources are estimated based on the analysis of the coal petrology and compilation of literature data on the gas content. The result of the coal petrology analysis of 10 samples shows that the vitrinite content of No. 16 coal (71.9–77.3%) is higher than that of No. 9 coal (59.1–65.1%), and the inertinite content of No. 16 coal (18.9–23.5%) is lower than that of No. 9 coal (30.1–34.9%). The Ro,max value of No. 16 coal (1.18–1.35%) is higher than that of No. 9 coal (1.04–1.13%), and both coals are of medium rank. Due to greater thickness, deeper burial depth, and better coal petrology characteristics, the No. 16 coal seam of the Taiyuan Formation is selected as the major coal seam for CBM resource estimation, which has a thickness of 1–6 m and a present-day burial depth of 200–1100 m. The gas content of this coal seam varies mostly between 4 and 10 m3/t. Positive correlation between the coal seam thickness as well as present-day burial depth and the gas content suggests that the thick and deeply buried coal seams are favorable for CBM preservation. The ash yield shows an insignificant negative correlation with the gas content, indicating that ash yield is not an important factor for CBM enrichment. The syncline hinges located below the thrust zones show higher gas content due to greater burial depths. In contrast, the anticline hinges at shallower depths tend to have lower gas contents. Based on the combined information about sedimentary environments, structural patterns, and hydrogeology, two CBM accumulation models are put forward in the study area that include syncline—hydraulic plugging below thrust nappe and fault—confined aquifer plugging. The volumetric method is used to estimate the CBM resources, and results indicate that the CBM resource in the whole coalfield is 428.78 × 108 m3, and the total resource abundance is 0.74 × 108 m3/km2. Two favorable areas for the CBM exploration are optimized based on the resource amount and resource abundance. One of the favorable areas is the Kabuqi area in the northern part of the coalfield, and another is the Baiyunwusu area in the central part of the coalfield. These two areas will be the CBM priority exploration areas at the western margin of the Ordos Basin.


INTRODUCTION
The commercial development of coalbed methane (CBM) resources is well established in a number of countries around the world, including the United States, Australia, China, India, and Canada. 1 The methane in coals occurs in several ways, including adsorbed gas on micropore (<2 nm in diameter) surfaces, trapped gas in pores of the coal matrix, free gas in fractures (cleat system), and dissolved gas in coal seam water. 2,3 CBM exploration and development have many advantages: (1) CBM is a type of clean energy, (2) extraction of CBM could greatly reduce hazards and thus strengthen mine safety, and (3) CBM utilization could decrease the emission of greenhouse gases. 4,5 China is rich in CBM resources, with a geological resource of 30.05 × 10 12 m 3 , 6 and the development and utilization of CBM can optimize China's energy structure and reduce greenhouse gas emissions. 7,8 China has made great progress in the CBM accumulation mechanism and development methods, such as in the Ordos Basin, 9−11 Qinshui Basin, 12,13 and Jungar  40 Basin. 14,15 However, to achieve China's goal of carbon neutrality by 2060, the scale of CBM development in China still needs to be urgently upgraded. Some areas with a low priority for exploration become the current focus of the investigation, such as the Zhuozishan coalfield at the western margin of the Ordos Basin.
Although the CBM exploration and development are successful at the eastern margin of the Ordos Basin, the controlling factors of CBM formation and CBM resources at the western margin of the basin are not well understood. The prospect of CBM exploration in the western Ordos Basin is still not very clear. The Carboniferous−Permian coal seams of the Zhuozishan coalfield at the western margin of the Ordos Basin have a high degree of thermal evolution, a large coal seam thickness, and a moderate burial depth. 16 These conditions should favor the enrichment of the CBM. Therefore, the Zhuozishan coalfield may have high potential for CBM exploration. The study on the accumulation mechanisms of CBM is not only beneficial to the further development of CBM resources in this coalfield but also provides support for the development of the CBM industry in China.
The geological factors controlling the CBM enrichment include characteristics of the coal seams, 17,18 coal reservoir characteristics, 18−22 structure conditions, 23−25 and hydrogeological conditions. 26−29 The Zhuozishan coalfield is located in front of a large and structurally reworked complex foldthrust belt. 30,31 The presence of a large number of faults and folds influences the distribution and quality of CBM resources. The recognition of the control factors of CBM enrichment in the Zhuozishan coalfield allows a selection of favorable target areas for CBM exploration and provides reference for CBM exploration in complex structural areas.
In this paper, the factors such as coal seam thickness, coal burial depth, coal petrology, coal quality, and coal ranks are investigated. The CBM enrichment models are established based on the depositional environments, structural features, and hydrogeological conditions. Furthermore, the exploration potential of CBM is evaluated, and the target areas for CBM exploration are optimized. We hope that the results of this study can provide support for further CBM exploration in the western Ordos Basin.

GEOLOGICAL SETTING
The Zhuozishan coalfield is located in Wuhai City, southwestern Inner Mongolia Autonomous Region, with a south−north length of approximately 100 km and a west−east width of 5− 25 km, and a cover of approximately 1930 km 2 . Geologically, the Zhuozishan coalfield is situated in the northern segment of the western margin of the Ordos Basin ( Figure 1a). The western boundary of the coalfield is the Gander−Xilaifeng thrust belt. The eastern boundary is the Eastern Zhuozishan thrust belt. The north and south sides of the coalfield are the Qianli Mountain and Zhengyiguan strike-slip fault zone, respectively ( Figure 1b). The main coal-bearing strata of the Carboniferous−Permian in the Zhuozishan coalfield include the Taiyuan Formation and overlying Shanxi Formation (Figure 1c), which were formed in the intracratonic paralic setting of the North China Plate, 32 and are now preserved in a series of thrust-related folds (Figure 1d). Both formations were developed with a marine−continental transitional facies. 33,34 The Taiyuan Formation comprises interbedded sandstone, siltstone, mudstone, limestone, and coals, with No. 16 coal being the main mineable seam ( Figure 2). The Shanxi Formation is composed of interbedded thick sandstone, siltstone, mudstone, and coals, with No. 9 coal being the main mineable seam (Figure 2).
After deposition of the coal-bearing strata, the western margin of the Ordos Basin went through the process of tectonic deformation. From the middle Permian to the middle Triassic, the western margin of the Ordos Basin underwent a stable subsidence along with the entire North China Craton, which resulted in the deposition of continental successions: Xiashihezi Formation, Shangshihezi Formation, Sunjiagou Formation, and Liujiagou Formation. 35 In the Late Triassic, due to the Indosinian movement, the sedimentary region of the North China Craton has shrunk largely, and the Ordos Basin was transformed into a separated continental lacustrine basin. 36 Since the Late Jurassic, the western margin of the Ordos Basin was influenced by the Yanshanian movement, resulting in the formation of east−west thrust belts. Subsequently, during the Neogene, this area was affected by the extrusion of the Tibetan Plateau, and the thrust nappe belts continued to develop. 37 At present, the preservation of coals in the Zhuozishan coalfield is generally controlled by two fold-thrust belts. The Eastern Zhuozishan thrust belt was formed in the Late Jurassic−Early Cretaceous, and the Eastern Gander Mountain thrust belt was formed in the Late Cretaceous−Neogene (Figure 1d). Both thrust belts branch out southward into several relatively small faults. 38 The asymmetric anticline belt was developed along the eastern margin of the coalfield, with the Archean or Lower Paleozoic strata being exposed. The syncline belt was developed along the western margin of the coalfield, constituting the footwall of the western neighboring thrust fault (Figure 1d). The Carboniferous−Permian coalbearing series are well preserved in the syncline belt.

Sample Information and Experimental Method.
A total of 10 coal samples were collected from two different coal mines in the Zhuozishan coalfield; 5 samples belong to the No. 9 coal seam of Baiyunwusu mine, and 5 samples were collected from the No. 16 coal seam of the Laoshidan mine.
The experiments performed in this study included the coal macrolithotype identification, maximum vitrinite reflectance (R o , max , %), coal macerals, and coal proximate analysis. The coal macrolithotypes are classified according to the Chinese National Standard GB/T 18023−2000. A Leitz MPV-3 photometer was used to determine R o , max , and coal macerals following the Chinese National Standards GB/T 6948−1998 and GB/T 8899−1998, respectively. The proximate analysis test was conducted following the Chinese National Standard GB/T 30732−2014. Proximate analysis includes determination of ash yield and contents of moisture, volatile matter, and fixed carbon, on an air-dried basis.

Resource Evaluation Method.
The method for the resource evaluation of CBM is different from that of conventional natural gas because the CBM is mainly preserved in the adsorption state in the coal reservoirs. The generation and enrichment of CBM are controlled by more complex factors. Numerous methods have been used to estimate the CBM resources, including the volumetric method, numerical simulation method, GIS-based multilevel fuzzy mathematical method, and analogy forecast method. 41−44 This study uses the volumetric method to evaluate the CBM resources in key areas, and the calculating formula is as follows where n is the total number of subunits subdivided into the ith calculation units.
G i is the geological resource of CBM in the ith calculation unit, in hundred million cubic meters (10 8 m 3 ).
C rj is the coal resource of the jth subunit, in hundred million tonnes (10 8

t).
C j is the average gas content of the coal reservoir in an airdried basis, in cubic meters per tonne (m 3 /t).
For calculation units lacking data on coal resources, the volumetric method can be used with the following formula where G i is the geological resource of CBM in the ith calculation unit, in hundred million cubic meters (10 8

m 3 ).
A i is the gas-bearing area of the coal seam in the ith calculation unit, in square kilometers (km 2 ).
H i is the apparent thickness of the coal seam in the ith calculation unit, in meters (m). D i is the density of coal in the ith calculation unit with air dry basis, in tonnes per cubic meter (t/m 3 ).
C i is the gas content of coal in the ith calculation unit with air-dried basis, in cubic meters per tonne (m 3 /t).
The resource abundance of CBM can reflect the distribution of the CBM resources, and it can be calculated by the total CBM resource divided by the coal distribution area.
The gas contents of the coal seam used in the CBM resource evaluation of this study were collected in the related geological reports and literature by the actual measurement method (Table S1).

Coal Seam Distribution.
The thickness of the Carboniferous−Permian coal-bearing strata ranges from 140 to 380 m in the Zhuozishan coalfield. The total thickness of the coal seams ranges from 10.5 to 20.78 m (Figure 3a). The coal seams in the Taiyuan Formation are thicker than those in the Shanxi Formation ( Figure 2). The regional distribution of the total coal seam thickness is characterized by being thicker in the central parts and thinner in the northeast and southwest parts of the coalfield. The coal accumulation centers are mainly distributed in the Kulihuoshatu and Baiyunwusu mine areas, in which the Kulihuoshatu mine area has the largest coal seam thickness, reaching 24.6 m (Figure 3a). The burial depth of the coal seams varies greatly, ranging from 133.1 to 1232.1 m, with a general deepening trend from the north to the south ( Figure  3b). The maximum burial depth of coal seams, which is based on the bottom surface of the deepest coal seam (No. 17 coal seam), is located in the Hongliushu mine area, followed by the Tiegaisumu mine area, with both areas having a burial depth greater than 600 m. The burial depth of coal seams in the Kulihuoshatu mine area is the shallowest (<300 m) ( Figure  3b,c).

Coal Proximate Analysis and Coal Petrology.
The moisture content of the coals varies from 0.36 to 0.88%, the ash yield ranges from 14.21 to 57.93%, and the volatile matter content ranges between 16.09 and 24.13% ( Table 1). The results of proximate analysis show that the coals in the study area are characterized by extra-low moisture, low to medium volatile matter, and medium to high ash yield, which are classified following the China Coal Industry Standards MT/T 850-2000, MT/T 849-2000, and GB/T 15224.1-2018, respectively.
The coal macrolithotypes are predominantly semi-dull coal, followed by dull coal and semi-bright coal, with moderate hardness. The maceral compositions are dominated by vitrinite, with the content ranging between 59.1 and 77.3%, averaging 68.30%. The content of inertinite varies from 18.9 to 34.9%, averaging 27.36%. The content of the liptinite is generally low, being less than 2%, and its influence may be insignificant and can be neglected (Table 2).  Table 2). Both coal seams belong to medium-rank coal. Previous studies have shown that the gas content increases with the coal rank, 45,46 which can be attributed to the increase of micropore and pore specific surface area. 47 The No. 16 coal seam may have higher gas contents than the No. 9 coal seam due to the higher maturity.

Coal Reservoir Characteristics. 4.4.1. Coal Structure.
Coal structure is generally classified into four types, including primary coal, cataclastic coal, granulitic coal, and mylonitic coal. 48,49 Primary coal is beneficial to the preservation of CBM, but it also has a relatively weak adsorption capacity and permeability. 50 The tectonically deformed coals, represented by cataclastic coal, granulitic coal, and mylonitic coal, 48 have distinct effects on CBM migration. 51 As brittle-ductile and ductile tectonically deformed coals, granulitic coal and mylonitic coal have numerous micropores, which enhance methane adsorption. 52 However, the original fractures and cleat system are completely broken and blocked, which causes a considerable decrease of permeability in coals, making the coals not conducive to the development of coalbed methane. 53 By contrast, cataclastic coal has higher permeability resulting from abundant migration channels provided by fractures, which is most favorable for CBM development. 48,54 In the Zhuozishan coalfield, the coal structure of the No. 9 seam is dominated by primary coal, while that of the No. 16 seam is dominated by primary− cataclastic coal. The No. 16 coal seam should have better advantages than the No. 9 coal seam for the CBM development.

Permeability and Reservoir Pressure.
Permeability is an important parameter in the evaluation of coal reservoirs. The permeability of No. 9 coal in the Zhuozishan coalfield is between 0.010 and 0.039 mD, with an average of 0.018 mD. 55 The permeability of No. 16 coal is between 0.047 and 0.078 mD, with an average of 0.056 mD. 55 (Figure 4a). The largest coal seam thickness is located in the Laoshidan area, with a thickness greater than 6 m, followed by the Kabuqi mine area and the Shen-1 well area of Qipanjing, with a thickness greater than 5 m (Figure 4a). The coal seam thickness in the southern and eastern coalfields is smaller, which is less than 2 m. The burial depth of the No. 16 coal seam in the Zhuozishan coalfield ranges from 200 to 1100 m ( Figure 4b). Overall, the burial depth gradually increases from the east and west flanks toward the central axis of synclines in the coalfield. The maximum burial depth of the No. 16 coal seam is located in the Hongliushu mine area, which is more than 1000 m (Figures 3c and 4b). The roof of the No. 16 coal seam is mainly composed of mudstone, with sandstone and siltstone locally present (Figures 3c and 4c). Except for the local sandstone roof with poor sealing capacity in the Qipanjing mine area, the roof in the rest of the coalfield is mostly composed of mudstones with better sealing capacity, which is conducive to CBM preservation.

Distribution of the CBM content.
In this study, the gas content data in published studies were collected and used in constructing the contours of the gas contents of No. 16 coal in the Zhuozishan coalfield. 16,55,57−61 The contour map shows that the gas content decreases from the northwest to the southeast in the Zhuozishan coalfield. The higher gas content occurs in the Kabuqi mine, the Muergou mine, and the Dilibangwusu mine, with the gas content generally greater than 10 m 3 /t. The gas contents of the coals in the Baiyunwusu and Burgastai mines are generally greater than 8 m 3 /t, while that of the residual area is less than 4 m 3 /t (Figures 3c and 4d). thickness, coal quality, and coal physical properties. 65,66 The coal seam thickness and coal quality will determine the CBM generation, adsorption, and preservation. 17 Coal seam thickness not only controls the CBM-generating potential of coal reservoirs but also plays an important role in CBM preservation, and the thicker the coal seam, the more favorable for CBM preservation. 64 The previous studies of Carboniferous−Permian in the North China Craton have shown that the most favorable coal-forming environments include the interdistributary bay of the delta plain, fluvial plain, and the tidal flat�lagoon. 33 As a part of the North China Craton, the Zhuozishan coalfield was developed with similar coal-forming environments.
In general, the thicker coal seam would be conducive to the enrichment of CBM. 17 Within the individual coal mine areas, such as Kabuqi and Gongwusu, a significant positive correlation exists between the coal seam thickness and the gas content ( Figure 5). Furthermore, Spearman correlation analysis is used to analyze the relationship between the coal thickness and the gas content in the entire coalfield and individual mine area. The Spearman correlation coefficients (SCC) of the entire coalfield and Kabuqi and Gongwusu mine areas are −0.34 (n = 18, p = 0.168 > 0.05), 0.733 (n = 10, p = 0.16 < 0.05), and 0.881 (n = 8, p = 0.004 < 0.01), respectively. The SCC also reveals a significant positive correlation between coal thickness and gas content in Kabuqi and Gongwusu mine areas. However, for the entire Zhuozishan coalfield, the relationship between coal seam thickness and gas content is insignificant and even shows a negative correlation. This indicates that coal seam thickness has a positive effect on the gas content under similar geological conditions, but there are other more critical factors that obscure or diminish this positive effect in the entire coalfield.
The formation of macerals is closely related to the swamp depositional environment. 67 It is generally believed that vitrinite is mainly formed in a reducing environment, which is mostly related to peat swamps with higher water levels. 68 Inertinite is mostly formed in peat swamps with shallow water cover, periodic exposure, and wildfire events. 69,70 Liptinite is a group of macerals derived from nonhumifiable plant matter and relatively hydrogen-rich remains such as sporopollenin, resins, waxes, and fats, which are closely related to the oxygenrich peat swamp environment where the lignocellular tissue of the plant remains can be oxidized and decomposed, and the stable components are relatively enriched. 71 Peat swamps in tidal flats are mostly water-covered low-lying swamps with strong reducibility, and the coal formed in this environment has high vitrinite contents and beneficial reservoir physical properties. 72 In the Taiyuan Formation, the No. 16 coal seam was mainly developed in the tidal flat and has a vitrinite content greater than 70% (Table 2), which is conducive to CBM generation and adsorption. However, the alluvial plain would favor the hydrodynamic and oxidizing conditions. The coal formed in this environment would have a high content of inertinite. 73 The content of inertinite in coal seams is the key parameter controlling the formation of pores and fractures, and the unfilled inertinite also has better adsorption capacity. 74 Where micropores are not developed, inertinite can also be used as the main adsorption medium. 75 In low−medium-rank coals, the higher content of inertinite tends to be associated with a larger proportion of mesopores and macropores, which is conducive to the migration of methane and the development of CBM. 76 In the Shanxi Formation, No. 9 coal seam in the Zhuozishan coalfield was mainly formed in a fluvial swamp. 34 The fluvial swamp could not maintain a long time of stable water cover, which is driven by fluvial flooding. The periodic oxidation environment blocked the formation of vitrinite, while inertinite was relatively enriched, resulting in a high content of inertinite, being higher than 30% (Table 2). In comparison to the No. 9 coal seam, the inertinite content of the No. 16 coal seam is lower but still slightly higher than 20%, which is also conducive to gas migration.
The depositional environment has an obvious influence on the ash yield in coal, thus directly affecting the reservoir's physical properties of the coal. 77 The ash yield is closely related to the terrigenous clastic input during peat accumulation, which is controlled by hydrodynamic conditions and fluctuating water levels in the peat swamp. 78 As for the same rank coals, the higher ash yields tend to be associated with the lower gas adsorption capacity of the coal reservoir. 47,79 During the deposition of the Shanxi Formation, the depositional environments of the coalfield were mainly the fluvial plain and delta plain, 34 where the active hydrodynamic conditions would favor the input of terrigenous siliciclastics. Therefore, the average ash yield in the No. 9 coal seam of the Shanxi Formation is 33.15%. In contrast, the No. 16 coal seam of the Taiyuan Formation, formed in the tidal flat, has a relatively low ash yield of 24.16% ( Table 1).
As for the major coal seam, when the ash yield in coal is lower than 20%, the average value of the gas content is 5.38 m 3 /t, and when the ash yield is higher than 20%, the average value of the gas content is 3.61 m 3 /t ( Figure 6). The SCC between the ash yield and the gas content of the coals for the entire coalfield and Kabuqi, Laoshidan, and Gongwusu mine areas, are −0.483 (n = 18, p = 0.042 < 0.05), 0.036 (n = 7, p = 0.939), −0.5 (n = 3, p = 0.667), and −0.881(n = 8, p = 0.004 < 0.01). It shows that the negative correlation between ash yield and gas content exists in the entire coalfield. However, within the individual coal mines, such as the Kabuqi and Laoshidan mines, the ash yield has insignificant correlations with the gas content, which are shown by the SCC or linear correlation coefficient ( Figure 6). On the contrary, a significant negative linear correlation exists in the Gongwusu, which is consistent with the SCC. Overall, the gas content decreases with the ash yield in the Zhuozishan coalfield, but this trend is not obvious, which manifests that the ash yield is not the key controlling factor for CBM accumulation.  Table S1).  Table S1).

Influence of the Sedimentary Rock Sequence on CBM Preservation
. The caprock of the No. 16 coal seam in the Zhuozishan coalfield is dominated by mudstone, with locally developed sandstone and siltstone (Figure 4c). The areas with high gas contents are mostly developed with mudstone sealing rocks (Figure 4d). The caprock might play more important roles in CBM accumulation than other sedimentary factors. 15 The sedimentary environment of the Taiyuan and Shanxi formations changed gradually from the tidal flat�lagoon and delta facies to fluvial facies. This variation creates distinguishable reservoir−caprock combinations, resulting in different sealing capabilities. Four lithological associations of the reservoirs and caprocks are identified in the Zhuozishan coalfield, which are characterized by different superposing patterns of coal and its surrounding rocks (Figure 7). The first pattern is represented by the upward succession of the medium-grained sandstone−mudstone−thin coal−thick-bedded coarse-grained sandstone, which was deposited in fluvial plain facies (Figure 7a). The second pattern is represented by the succession of the fine-grained sandstone−mudstone−thick coal−medium-grained sandstone, which was deposited in the upper delta plain facies (Figure 7b). These two patterns mainly occur in the Shanxi Formation. The third pattern is represented by the succession of the fine-grained sandstone− thick coal−interbedded fine-grained sandstone−mudstone, which was deposited in lower delta plain facies (Figure 7c).   Table S1).
The fourth pattern is represented by the succession of the mudstone−thick coal−limestone−thick mudstone, which was deposited in the tidal flat�lagoon facies (Figure 7d). The latter two patterns mainly occur in the Taiyuan Formation. Comparing these lithological associations, it can be deduced that the coal seams deposited in the lower delta plain and tidal flat have relatively better sealing capacity resulting from the caprock of thick-bedded mudstone and interbedded thinbedded fine-grained sandstone. On the contrary, the coal seams deposited in the fluvial plain and upper delta facies have a relatively poor sealing capacity due to the thick-bedded coarse-grained sandstone roof. The methane-capped condition of the coal seam can be estimated by identifying the lithological association. The reservoir−caprock combination occurring in the lower delta and tidal flat�lagoon facies is the most favorable for CBM preservation in the Zhuozishan coalfield.  80 In the late Mesozoic, the average paleotemperature gradient in the Ordos Basin reached 4.5°C/ 100 m, 80 which greatly accelerated the maturation of the organic matter in the Upper Carboniferous−Lower Permian. The average R o,max of coal reached 1.26%. The coal-bearing strata began to generate abundant gas in the Middle Jurassic. 81 Subsequently, the peak stage of gas generation occurred in the Early Cretaceous. Then, the gas generation ended and the gas gradually escaped in the Late Cretaceous. 82 The Early Cretaceous was an important stage for the CBM generation in the coals of the Taiyuan and Shanxi formations of the Zhuozishan coalfield.

Present-Day Burial Depth of Coal.
For the entire coalfield, the correlation between the gas contents and the present-day burial depth of coal seams is not significant (Figure 8). For the individual mine areas, including the Kabuqi, Laoshidan, Gongwusu, and Lutian mine areas, it can be revealed that a relatively positive correlation exists between the coal seam burial depth and the gas content ( Figure 8). However, the low coefficients also indicate that the variation in the gas content is not dominated by a single factor. Even though the linear correlation coefficients are low, the higher gas content still occurs in the deeper strata. For example, in the Kabuqi mine, the average gas content in shallow-buried coal (0−300 m) is 6.8 m 3 /t, while that in deep-buried coal (300− 600 m) is 8.8 m 3 /t. To certify the relationship between buried depth and gas content, Spearman correlation analysis is carried out for the entire coalfield and individual coal mines. The SCC of the entire coalfield, Kabuqi, Laoshidan, Gongwusu, and Lutian mine areas are 0.683 (n = 67, p = 0 < 0.01), 0.85 (n = 46, p = 0 < 0.01), 0.576 (n = 9, p = 0.052 > 0.05), 0.857 (n = 8, p = 0.007 < 0.01), and 1 (n = 4, p = 0 < 0.01), respectively. The result demonstrates a significant positive correlation between the entire coalfield and individual coal mines. This means that within similar geological conditions, the gas content tends to increase with the present-day coal burial depth.

Coal Rank.
The R o value of the No. 16 coal seam in the Zhuozishan coalfield is characterized by an overall trend of decreasing from the west to the east. The relationship between the variation of the gas content and R o is not obvious for the entire coalfield. However, on the east side of the coalfield with a relatively low R o value, the gas content is generally less than 2 m 3 /t. The R o is greater than 1.0% in the area with high gas content. The eastern and western margins of the coalfield are located at the syncline margin, but the gas content of the western side is slightly higher than that of the eastern side ( Figure 9). These phenomena indicate that coal rank may still have a positive effect on the CBM accumulation in No. 16 coal seam.

Coalfield Structure. Different geological structures
can lead to differences in the occurrence, coal structure, physical properties, fracture development, and groundwater runoff conditions of coal reservoirs and surrounding rocks, which have an impact on the gas content. 83 The development of geological structures such as faults and folds had a substantial influence on the CBM enrichment in the Zhuozishan coalfield.
The Zhuozishan coalfield is structurally complex, which resulted from the development of numerous thrust faults and folds. The formation of the Kabuqi syncline and Zhuozishan anticline was influenced by thrusting in the northern part of the western margin of the Ordos Basin. The older strata in the study area, which mainly include Ordovician and Cambrian, are thrust over the coal-bearing strata, and some reverse faults are developed on the gentle flank of the syncline. The reverse faults tend to form the structural trap conditions which are favorable for the gas enrichment in coal seams. 45 The Kabuqi mine area is developed with this trap type (Figures 3c and 4d). The gas content increases gradually when it is closer to the axis of the Kabuqi syncline. Generally, synclines have better CBM preservation conditions than anticlines. 17,84 As for a single syncline, due to the compression at the syncline axis area, the gas permeability of surrounding rocks becomes lower, and the CBM can be better sealed in the axis area. As a result, the axes of the syncline have a relatively high gas content in the central parts of the Zhuozishan coalfield (Figure 4d), indicating that the syncline in the study area has a positive impact on the CBM enrichment. The large anticline is not conducive to CBM preservation, and the gas contents in the coals of the Zhuozishan anticline and the Gander anticline are lower.
In terms of controls of the fault types, several major reverse faults were developed in the Zhuozishan coalfield, and these faults have disrupted the continuity of coal seams and their surrounding strata to varying degrees. The reverse faults, formed by tectonic compressive stress, are mostly closed faults. These faults have an obstructive effect on the CBM escaping and thus favor gas preservation. Due to the uplift of the hanging wall of the reverse fault, the burial depth of coals becomes shallow, the pressure of the overlying strata becomes lower, and the adsorption of the coal reservoir is weakened, which leads to a large amount of coalbed methane dissipation. The Laoshidan mine and Gongwusu mine are both located southwest of the Gander−Xilaifeng fault, where the coal in the hanging wall of the reverse fault has a shallow burial depth and a low gas content.

Hydrological Factors.
The coal-bearing strata in the Zhuozishan coalfield can be subdivided into four aquifers and four aquicludes. The water-yield property of all aquifers is relatively low, and the confining capacity of the aquicludes is uneven. 16 The aquiclude at the bottom of the Taiyuan Formation is distributed in the entire area and has the best confining capacity, while the aquiclude at the bottom of the Shanxi Formation is less continuous laterally and its confining capacity is limited to some extent.
Hydrogeological conditions have a great influence on the preservation and transport of CBM, and also on the exploration and extraction of CBM. 29 The gas-controlling role of hydrogeological conditions in the Zhuozishan coalfield is mainly reflected in two aspects, including the hydraulic migration and the hydraulic plugging. The hydraulic plugging effect is conducive to CBM enrichment.
Hydraulic migration and diffusion are common in faultdeveloped areas. 85,86 In the Zhuozishan coalfield, the faults destabilize each aquifer, leading to water migration, and the gas escapes with the flow of water. The Qipanjing mine area has extremely developed fault structures, especially normal faults, and these faults may lead to obvious hydraulic migration and diffusion, which may explain the low gas contents in the coal seams.
Hydraulic plugging commonly exists in asymmetric synclines. 87 For example, in the Kabuqi syncline, the east side is developed with the thrust fault dominated by compression, and the west side is developed with the Gander−Xilaifeng thrust fault, both of which are dominated by thrust faults, which should have plugging effects on the CBM. Each aquifer in the Kabuqi syncline has no hydraulic connection, and the aquifer has less water yield and slow groundwater runoff, which must have reduced CBM escaping. Otherwise, the coal seam would be exposed in some areas along the fold margin. Under this condition, CBM is also enriched in the syncline axis by hydraulic plugging of atmospheric precipitation, which can keep CBM from escaping.

CBM Enrichment Models.
According to the geological conditions and the characteristics of CBM generation and preservation, two types of CBM enrichment models in the Zhuozishan coalfield are proposed, namely, the CBM enrichment in syncline�hydraulic plugging below the thrust nappe and the CBM enrichment by fault�confined aquifer plugging.

CBM Enrichment in Syncline�Hydraulic
Plugging below the Thrust Nappe. The relationship between the main fold styles and the gas contents in the study area has shown that the maximum gas contents are located in the axial area of the Kabuqi syncline ( Figure 10). Combined with the previous research results, the reasons for the higher gas contents in the Kabuqi syncline can be examined in more detail. The depositional environments were dominated by the delta-tidal flat, which favored the deposition of a thick coal reservoir, and a thick regional mudstone sealing cover. The Kabuqi syncline is located at the fold-thrust belt. 31 Due to the compression stress, the axial area of the syncline trapped by thrust faults has better reservoir physical conditions, higher gas adsorption capacity, and favorable gas-bearing conditions. 88−90 The groundwater runoff is slow, which has a better hydraulic plugging effect on the CBM. As a result of these conditions, the Kabuqi mine has relatively better CBM generation and preservation conditions, and thus has higher gas contents, making it a beneficial area for CBM exploration and development. This model can provide an example of CBM exploration and development in the fold-thrust zone, and it is also developed in the northern area of the Zhuozishan coalfield.

CBM Enrichment by Fault�Confined Aquifer
Plugging. In the Zhuozishan coalfield, a number of reverse faults with compression stress are developed. The maximum gas contents are located in the central area between the Xilaifeng reverse fault and the Heilonggui reverse fault ( Figure  11). The adjacent reverse faults compress the coal reservoir,  increasing the burial depth of coals in the central part. The compression stress from faults, together with the pressure of the confined aquifer, could improve the adsorption capacity of the coal seam and seal the upward-escaping CBM in the central part. The CBM near the reverse faults on both sides is blocked from escaping, which protects the central gas preservation. The CBM enrichment in the southern Zhuozishan coalfield, such as Baiyunwusu and Laoshidan mines, is in agreement with this model. The CBM enrichment by fault and confined aquifer plugging will be also conducive to future CBM exploration in complex structural areas with abundant faults.  (Table S1), and in the area without the values of gas contents, the gas contents are obtained by the analogy method.

CBM RESOURCE EVALUATION AND TARGET
6.1.2. Results of CBM Resource Evaluation. After determining the calculation units and calculation parameters, the CBM resource of each calculation unit of the Zhuozishan coalfield is calculated separately, based on formulas (1) and (2) mentioned previously. The resource abundance of the CBM of each calculation unit is calculated by the total CBM resource divided by the coal distribution area. The results show that the total coal-bearing area of the 14 evaluation units in the entire Zhuozishan coalfield is 578.4 km 2 , the total gas-bearing volume is 428.78 × 10 8 m 3 , and the total resource abundance is 0.74 × 10 8 m 3 /km 2 . The resource abundances of CBM in the Hongliushu mine and the Dili Gongwusu mine are the largest, both being greater than 0.7 × 10 8 m 3 /km 2 . The resource abundances in the rest of the area are relatively small (Table  3).  6.2.2. Results of Favorable Area Selection. The contours of thickness, burial depth, gas content, caprock sealing capacity, and fault of the main coal seam in the Zhuozishan coalfield are superimposed, and then the favorable CBM areas of the main coal seam in the study area are preferentially selected ( Figure  12a). According to the above selection principles, two favorable areas for CBM exploration and development were selected in the No. 16 coal seam (Figure 12b), including the Kabuqi favorable area, with a CBM resource of 96.84 × 10 8 m 3 and a resource abundance of 0.61 × 10 8 m 3 /km 2 , and the Baiyunwusu favorable area, with a CBM resource of 17.78 × 10 8 m 3 and a resource abundance of 0.45 × 10 8 m 3 /km 2 ( Table  4).

CONCLUSIONS
(1) The CBM geological conditions in the Zhuozishan coalfield are characterized by a greater total coal seam thickness ( (2) Coal seam thickness in the study area is positively correlated with the gas content, and the ash yield is negatively correlated with the gas content. The coal rank has little effect on the variation of the gas content, and the coal burial depth has a better correlation with the gas content within each individual mine area. Mudstonedominated caprocks are beneficial to the preservation of coalbed methane, and the reservoir−caprock combination developed in the lower delta plain and the tidal flat�lagoon is conducive to CBM accumulation. In terms of geological structures, the gas contents are higher in the syncline below the thrust nappe, while the gas contents are lower in the anticline. Combining the depositional environment, structure features, and hydrogeological conditions, two types of CBM enrichment models are proposed, namely, the syncline�hydraulic plugging below the thrust nappe model and the fault� confined aquifer plugging model.