*5.1. Gas Composition*

In the Bumu region, the composition of CBM is complex, mainly composed of CH4, N2, CO2, and heavy hydrocarbon gases. The heavy hydrocarbon gases (C2+) include ethane and propane in the research area. The concentration of CH4 varies greatly and ranges from 9.17% to 95.32%, with an average value of 65.60%. The N2 and CO2 account for 21.76% and 1.61%, respectively. The concentration of heavy hydrocarbon gas is abnormally high, 0.30–25.51%, and the average value is up to 10.47%. The CBM δ<sup>13</sup> C1 ranges from −51.69 ‰ to −43.43 ‰ with an average of −46.57 ‰, indicating that CBM is mainly the pyrolysis product of humic organic matter and possibly the secondary biogenic gas in the shallow coal seam [39]. The local high concentration of heavy hydrocarbon gas is mainly the complete result of the coal maceral, microbial degradation, and coal pore structure [29,33].

There is no apparent linear relationship between the CBM composition and coal burial depth, indicating that the geological influence factors of CBM are complex. The concentrations of methane, heavy hydrocarbon and alkane gas generally tend to increase with coal burial depth (Figure 4a–c), while the N2 concentration decreases with the increase in coal burial depth (Figure 4d).

**Figure 4.** The gas-bearing property parameters vary with the coal burial depth. (**a**) methane concentration, (**b**) heavy hydrocarbon gas concentration, (**c**) alkane concentration, (**d**) N2 concentration, (**e**) gas content, and (**f**) gas content gradient.

In the Bumu region, the methane concentration of CBM decrease while as the N2 and heavy hydrocarbon gas concentration increase. The dissipation of methane, as well as its mutual exchange and mixture with surface atmosphere are mostly the main geological cause. It indicates that the CBM

occurrence space in the study area has relatively low sealing capacity, which may have resulted from destruction of fault in the CBM reservoir. This is consistent with the complicated geological structure in the Bumu region.

#### *5.2. Gas Content and Its Distribution*

In the Bumu region, the gas content of coal seam ranges from 6.94 to 19.27 m<sup>3</sup>/<sup>t</sup> and is mainly between 8 and 13 m<sup>3</sup>/t. The average value of gas content is 11.19 m<sup>3</sup>/t. Generally, the CBM content tends to increase linearly with coal burial depth and the gas content gradient tends to decrease with coal burial depth following the power function [40]. With the increase in the coal burial depth, the gas content gradient exhibits an overall decreasing trend, but there is no significant positive correlation with the gas content in the Bumu region (Figure 4e,f). The overall relatively low methane concentrations and gas contents of coal seam indicate the widely distributed open and semi-open reservoir space of CBM in the study area. The influence of geological structure on the sealing of CBM reservoir is important and will be discussed in greater detail later in this paper. The gas content of di fferent coal seams fluctuate significantly with stratigraphic sequence and these observations were previously interpreted as a result of multiple unattached CBM-bearing systems in the vertical section [41,42]. Therefore, this study selects the No. 9 main coal seam with stable development and largely available gas data as the research object to discuss the gas distribution features (Table 2, Figure 5).

Gas content of No. 9 coal seam in the Bumu region is relatively high, ranging from 7.31 to 19.27 m<sup>3</sup>/<sup>t</sup> and with an average value of 11.29 m<sup>3</sup>/t. The distribution pattern of gas content is complex and has an overall banding distribution in the direction of NNE. The CBM enrichment region, with the gas content higher than 10 m<sup>3</sup>/<sup>t</sup> and about half of the study area, is distributed along the hinge zone of the Enhong syncline. The gas contents of the two limbs of syncline decrease and form two low gas content zones in the northwest and southeast of the study area. At the same time, the development of fault structures complicates the geological structure and the occurrence of CBM in the study area, resulting in the further di fferentiation of CBM content distribution, particularly in the west (Figure 5). With the gas content of 10 m<sup>3</sup>/<sup>t</sup> as the boundary, the central NNE-trending high gas content zone, western NNE-trending high gas content zone, northwest, and southeast low gas content zones can be further divided as follows.


**Figure 5.** Contour maps of No. 9 coal seam floor, structural sketch and gas-bearing property parameters. (**a**) coal seam floor and structural sketch; (**b**) gas content; (**c**) methane concentration; (**d**) gas content gradient.

#### **6. Gas Controlling Pattern of Geological Structure**

In the Bumu region, as a complex geologic structural area with well-developed syncline, normal fault and reverse fault structures, geologic structures play an important role in the occurrence and distribution of CBM, especially the e ffects of multi-phase tectonic movements after the formation of coal seam. A systematic study on the structural characteristics, tectonic evolution, and CBM occurrence shows that the gas distribution pattern is mainly controlled by the development of geological structures. The development of the Enhong syncline leads to the deep burial and structural compression of the coal seam in the syncline hinge zone, which could allow gas enrichment and shows that the central NNE-trending high gas content zone spreads along the Enhong syncline axis and an overall decreasing trend from the hinge zone of the Enhong syncline, in the central NNE-trending high gas content zone, to its limbs in the northwest and southeast low gas content zones. The development of di fferent types of fault structures further complicates the geological structure and CBM content distribution in the study area, especially in the western NNE-trending high gas content zone.

According to the variation of gas content, gas content gradient, and methane concentration in di fferent structure parts, the e ffects of di fferent geological structures on CBM occurrence are determined as the following five types: reverse fault sealing type, syncline sealing type, monoclinal enrichment type, normal fault dispersion type, and buried floor fault dispersion type (Figure 6).

**Figure 6.** Scatter diagram of gas-bearing property parameters of 5 gas controlling pattern. (**a**) plot of methane concentration to gas content gradient; (**b**) plot of methane concentration to gas content; (**c**) plot of gas content to gas content gradient. Gas controlling pattern: I—reverse fault sealing type, II—syncline sealing type, III—monoclinal enrichment type, IV—normal fault dispersion type, V—buried floor fault dispersion type.

(1) Reverse fault sealing type: No. 9 coal seam from boreholes Nos. 0103, 103, 301, 301-1, and 404 is characterized by the high value of gas content, methane concentration, and gas content gradient, which is generally higher than 12 m<sup>3</sup>/t, 70% and 2.6 m<sup>3</sup>/t/<sup>100</sup> m, respectively. The mean values of those properties in the test boreholes are 15.38 m<sup>3</sup>/t, 79.26% and 3.13 m<sup>3</sup>/t/<sup>100</sup> m, respectively. Those boreholes are located near the large reverse fault of F2–6 and F8–10, which are formed during the Yanshanian period. Compared with the Yanshanian reverse faults in the North China plate that are mostly reversed to the normal faults in the later period [7], the Yanshanian large reverse fault, even underwent the strike-slip structural modification during the Himalayan period, still have good seal capacity and are beneficial for the preservation of CBM.

(2) Synclinal sealing type: No. 9 coal seam is characterized by the high gas content, methane concentration and the medium gas content gradient, which is generally higher than 10 m<sup>3</sup>/t, 70%, and about 2.0 m<sup>3</sup>/t/<sup>100</sup> m, respectively (Table 2). The CBM of No. 9 coal seam from boreholes Nos. 0101, 101, 1202, 403, 504, 505, and 602, located in the hinge zone of the Enhong syncline, has a mean value of 11.56 m<sup>3</sup>/t, 80.07% and 1.88 m<sup>3</sup>/t/<sup>100</sup> m, respectively. The development of syncline leads to the increase in coal seam burial depth, which is conducive to the accumulation of CBM and the increase in gas content from limb to hinge zone. In contrast, the gas content gradient tends to decrease with the increase in the burial depth [40]. Therefore, the increase in the gas content gradient of coal seam in the Enhong syncline hinge zone, compared to the limbs, is strong evidence that the structural compression above the neutral surface is conducive to the accumulation of CBM.

(3) Monoclinal enrichment type: No. 9 coal seam located in syncline limbs, such as Nos. 501, 502, 503, 1201, 2301, and EH01 boreholes, is characterized by the medium gas content, methane concentration and the low gas content gradient, which is generally lower than 12 m<sup>3</sup>/t, 70% and about 1.8 m<sup>3</sup>/t/<sup>100</sup> m, respectively (Table 2). The gas content gradient varies greatly, ranges from 1.06 to 2.94 m<sup>3</sup>/t/<sup>100</sup> m, which is caused by its decrease with the increase in burial depth.

(4) Normal fault dispersion type: No. 9 coal seam located in the footwall of F1 normal fault, such as Nos. 0102, 102, 1203, 2302, and 303 boreholes, is characterized by the low gas content, methane concentration, and gas content gradient, which is generally lower than 10 m<sup>3</sup>/t, 70%, and about 1.8 m<sup>3</sup>/t/<sup>100</sup> m, respectively (Table 2). The right-lateral slipping of the Pingguan-Agang fault derives the F16 normal fault during the Himalayan period. The extensional fault destroys the CBM reservoir and provides seepage channels for CBM dissipation as well as its mutual exchange with the surface atmosphere.

(5) Buried floor fault dispersion type: No. 9 coal seam located near the buried f1, f3, and f4 reverse faults is characterized by the low gas content, methane concentration, and gas content gradient, which is generally lower than 10 m<sup>3</sup>/t, 60%, and about 1.8 m<sup>3</sup>/t/<sup>100</sup> m, respectively (Table 2). The scale and displacement of these buried floor faults are small, which fail to form a good sealing property for CBM. In addition, the faults penetrate the Emeishan basalt formation underlying the coal-bearing strata, and it has been reported that natural gas can be migrated and be stored in the Emeishan basalt [43,44]. The well-developed fractures in the basalt also serve as an escaping path and as a reservoir of CBM, respectively [40]. These fractures destroy the sealing of the fault and cause the dispersion of CBM.

For No. 9 coal seam in the Bumu region, methane concentration is sensitive to the sealing of CBM reservoir and is generally larger than 70% in reverse fault and syncline sealing types (Figure 6). Gas content and gas content gradient vary greatly with burial depth resulted from the di fferent geological structure control action (Figure 7).

Gas content gradient can indicate the influence of geological structure on CBM reservoir from the buried depth, which is lower than 1.8 m<sup>3</sup>/t/<sup>100</sup> m in normal and buried floor fault dispersions while is higher than 2.4 m<sup>3</sup>/t/<sup>100</sup> m in reverse fault sealing type (Figures 6 and 7). Gas content is an important parameter for the enrichment of CBM, which is higher than 12 m<sup>3</sup>/<sup>t</sup> in reverse fault type and around 12 m<sup>3</sup>/<sup>t</sup> in syncline sealing type.

**Figure 7.** Gas content (**a**) and gas content gradient (**b**) vary with coal burial depth. Gas controlling pattern: I—reverse fault sealing type, II—syncline sealing type, III—monoclinal enrichment type, IV—normal fault dispersion type, V—buried floor fault dispersion type.

The tectonic movement was dominated by the continuous subsidence movements after the coal-bearing strata deposited in the Late Permian until the Late Jurassic. According to the previous numerical simulation research about the burial and thermal histories of coal seams based on the analysis of vitrinite reflectance and homogenization temperature of fluid inclusion, the largest ancient burial depth of the coal seams can reach 3800 m in the research areas [45–47]. Under the effect of paleotemperature, the coal seams experienced deep metamorphism and reached the fat coal stage, accompanied with the massive CBM generation [45,46]. The influence of the Indosinian movement was very weak and the Indosinian geological structure was rare in the Bumu region. The intense tectonic compression and the formation of compressive structures during the Yanshanian movement, as well as the peak-period of gas generation, are conducive to CBM enrichment. The gas-controlling pattern is manifested as the reverse fault sealing, syncline sealing, and monoclinal enrichment of CBM, which are the main controlling factors to the central and western NNE-trending high gas content zones. The tectonic intense compression and dextral shear action in the Himalayan movement avoided the inversion of compressive structure to extensional structure and the destruction of CBM reservoir, which are common in the North China plate [48]. However, the chronic uplift and derived normal fault during the Himalayan period caused the constant dissipation of CBM. The normal fault dispersion and buried floor fault dispersion are the main reasons for the northwest and southeast low gas content zones in the Bumu region.
