Multi-Phase Tectonic Movements and Their Controls on Coalbed Methane: A Case Study of No. 9 Coal Seam from Eastern Yunnan, SW China

Multi-phase tectonic movements and complex geological structures limit the exploration and hotspot prediction of coalbed methane (CBM) in structurally complex areas. This scientific problem is still not fully understood, particularly in the Bumu region, Southwest China. The present paper analyses the occurrence characteristics and distribution of CBM based on the comprehensive analysis of CBM data. In combination with the analysis of the regional tectonics setting, geological structure features and tectonic evolution. The control action of multi-phase tectonic movements on CBM occurrence are further discussed. Results show that the Indosinian local deformation, Yanshanian intense deformation, and Himalayan secondary derived deformation formed the current tectonic framework of Enhong synclinorium. The intense tectonic compression and dextral shear action in the Yanshanian and Himalayan movements caused the complex geological structures in Bumu region, composed of the Enhong syncline, associated reverse faults and late derived normal fault. The CBM distribution is complex, which has the central and western NNE-trending high gas content zones along the syncline hinge zone and the reverse faults. The geological structure controls on CBM enrichment are definite and important. Based on geological structure features and responses of gas content, methane concentration, and gas content gradient, the gas controlling patterns of geological structure are determined and can be classified into five types: the reverse fault sealing, syncline sealing, monoclinal enrichment, normal fault dispersion, and buried floor fault dispersion types. The structural compression above the neutral surface plays an important role in the syncline sealing process, which is indicated by an increase in gas content gradient. The EW-trending tectonic intense compression and dextral shear action in the Himalayan movement avoided the negative inversion of NNE-trending Yanshanian compressive structure and its destruction of CBM reservoir. However, the chronic uplift and derived normal fault during Himalayan period caused the constant dissipation of CBM.


Regional Structure
The research area is tectonically located in the southwestern margin of the Yangtze plate ( Figure  1). It occurs at the boundary between the Tethys-Himalaya tectonic domain and the circum Pacific tectonic domain [29,30]. This tectonic unit is bordered by the NE-trending Mi'le-Shizong fault in the south, the NS-trending Xiaojiang fault in the west and the NW-trending Youjiang fault in the northeast [31,32].
The Bumu region is located in the central part of the Enhong synclinorium, which is the main geological structure in this area ( Figure 2). The Enhong synclinorium, with a length of 53 km and width of 6-12 km, is located between the NE-trending Fuyuan-Mi'le fault and the NS-trending Pingguan-Agang fault [33]. The Enhong synclinorium is mainly composed of Enhong syncline and many secondary anticlines and synclines, which extend in the direction of NE, NNE, NS, and NNW from the southwest to the northeast of the Enhong synclinorium. The deformation strength of the fold decreases gradually from west to east and from north to south, with the stratigraphic dip angle of about 10-25°. Meanwhile, the NE-plunging Enhong synclinorium is strongly destroyed by numerous multilevel multidirectional faults ( Figure 2). The secondary derived drag folds, derived faults, and associated faults developed well in the northwestern limb of Enhong synclinorium. The

Regional Structure
The research area is tectonically located in the southwestern margin of the Yangtze plate ( Figure 1). It occurs at the boundary between the Tethys-Himalaya tectonic domain and the circum Pacific tectonic domain [29,30]. This tectonic unit is bordered by the NE-trending Mi'le-Shizong fault in the south, the NS-trending Xiaojiang fault in the west and the NW-trending Youjiang fault in the northeast [31,32].
The Bumu region is located in the central part of the Enhong synclinorium, which is the main geological structure in this area ( Figure 2). The Enhong synclinorium, with a length of 53 km and width of 6-12 km, is located between the NE-trending Fuyuan-Mi'le fault and the NS-trending Pingguan-Agang fault [33]. The Enhong synclinorium is mainly composed of Enhong syncline and many secondary anticlines and synclines, which extend in the direction of NE, NNE, NS, and NNW from the southwest to the northeast of the Enhong synclinorium. The deformation strength of the fold decreases gradually from west to east and from north to south, with the stratigraphic dip angle of about 10-25 • . Meanwhile, the NE-plunging Enhong synclinorium is strongly destroyed by numerous multilevel multidirectional faults ( Figure 2). The secondary derived drag folds, derived faults, and associated faults developed well in the northwestern limb of Enhong synclinorium. The derived brachy drag folds in the south are mainly arranged in echelons to the right row, and the axial traces form an acute angle with the Fuyuan-Mi'le fault, indicating that the deformation mechanism of the folds is derived from the right-lateral slipping of the Fuyuan-Mi'le fault ( Figure 2). The NE-trending secondary associated folds are strongly deformed and extend near the fault zone of Fuyuan-Mi'le fault. The secondary faults in the northwestern limb are mostly the NNE-trending derived faults and the NE-trending associated branch faults of the Fuyuan-Mi'le fault.
Energies 2020, 13, x FOR PEER REVIEW 4 of 18 derived brachy drag folds in the south are mainly arranged in echelons to the right row, and the axial traces form an acute angle with the Fuyuan-Mi'le fault, indicating that the deformation mechanism of the folds is derived from the right-lateral slipping of the Fuyuan-Mi'le fault ( Figure 2). The NEtrending secondary associated folds are strongly deformed and extend near the fault zone of Fuyuan-Mi'le fault. The secondary faults in the northwestern limb are mostly the NNE-trending derived faults and the NE-trending associated branch faults of the Fuyuan-Mi'le fault.   The NS-and NNW-trending folds developed well in the north part of the southeastern limb of the Enhong synclinorium ( Figure 2). The secondary faults in the southeastern limb are commonly the NS-trending associated faults and the NE-, EW-, and NW-trending strike-slip normal derived faults, which are mostly arranged as an echelon pattern and are restricted by the Pingguan-Agang fault at a large angle. Especially the arcuate fault zone, composed by the right-lateral strike-slip normal faults, is formed near the eastern part of the Bumu region. Additionally, the latter small oblique strike-slip faults, mainly the NNW-trending left-lateral strike-slip faults and NEE-trending right-lateral strike-slip faults, cut across the major structure and complicate geological structure.

Coal-Bearing Strata and Coal Seam
The basement of the Yangtze plate in this area is the Mesoproterozoic Kunyang Group. The sedimentary stratum are Siluric, Devonian, Carboniferous, Permian, Triassic, Paleogene, Neogene, and Quaternary [29]. The exposure strata in the research area are mainly the limestone of Yongningzhen Formation (T 1 y) and the siltstone and sandstone of Feixianguan Formation (T 1 f). The buried strata, exposed by drilling, are Kayitou Formation (T 1 k), Changxing Formation (P 3 c), Longtan Formation (P 3 l) and Emeishan basalt Formation (P 3 β) (Figure 3a). The Changxing Formation (P 3 c) and Longtan Formation (P 3 l) are the major coal-bearing strata. They were measured between 214.33 m to 278.55 m in thickness and consist of 28 to 46 coal seams ( Figure 3a). The total thickness of the coal seams ranges from 20.23 m to 35.54 m with an average thickness of 30.22 m. The minable coal seams are the coal seams Nos. 4, 7-9, 12, 13, 15, 16, and 18-24, with an average thickness of 19.57 m, and the R o,max ranges from 1.07% to 1.15% (Table 1). The thickness, coal quality, and lateral stability of coal seams in the upper segment of Longtan Formation (P 3 l 2 ) are favorable for the exploitation of coal and CBM. The No. 9 coal seam, with a thickness of 1.22-5.80 m and an average thickness of 3.28 m, is stable and the unique regional minable coal seam in the research area.

Material and Experimental Methodology
The geological structures and structural features in Bumu region were analyzed and determined according to the field and drilling geological surveys. The structural features, distribution law, structural Energies 2020, 13, 6003 7 of 17 pattern, and mechanical causes of numerous multilevel folds and faults of Enhong synclinorium in the region were systematically studied based on the theory of modern structural geology and geomechanical analysis. In combination with the analysis of regional tectonic setting, the multi-phase tectonic evolution and deformation mechanism in the research area were discussed and determined.
The CBM gas content and gas components of 108 coal samples from different coal seams in Bumu region were measured according to the desorption method and gas chromatography method that following Chinese National Standards GB/T 19559-2008 and GB/T 13610-2003, respectively ( Table 1). The No. 9 coal seam has the most CBM data, obtained from 27 boreholes ( Table 2). The maximum vitrinite reflectance measurement, coal maceral composition analysis, and proximate analysis were performed following the Chinese National Standards GB/T6948-2008, GB/T8899-1998, and GB/T212-2008, respectively (Table 1). These CBM data form the basis for the discussion of the occurrence characteristics and distribution of CBM in Bumu region. Based on the response and variation of CBM gas content, gas component and gas content gradient to the different types and periods geological structures, the control action of different geological structures, fold neutral surface action and fault sealing in multi-phase tectonic movements on CBM occurrence were further discussed.

Geological Structure and Tectonic Evolution
The geological structures in Bumu region are mainly the Enhong syncline, reverse, and normal faults ( Figure 3b). The Enhong syncline is the main geological structure, which controls the overall distribution, orientation, and burial depth of coal seams (Figure 3c  Geological survey has discovered 20 faults, developed well in this area with the directions of NNE, NE, and NW ( Figure 3b). The faults are mainly reverse faults, with a few normal faults. The reverse faults are mostly in NNE direction and the normal faults are mostly in NE and NW directions. The imbricate large reverse faults of F 2-5 , F 2-6 , F 8-10 , and F 5 , with the fault throw of 40-55 m, are located in the NWW limb of Enhong syncline (Figure 3c). The normal faults are limited by the NNE-trending reverse faults and that indicates the normal faults are formed later. Normal faults generally extend unsteadily, especially the F 16 fault, as the southern section of the arcuate strike-slip normal derived faults, extends in NE-NEE direction. There are seven faults, F 2-6 , F 8-10 , F 16 , f 1 , f 2 , f 3 , and f 4 in No. 9 coal seam.
Since the formation of coal measures in the Late Permian, the study area has mainly experienced Indosinian movement, Yanshanian movement, and Himalayan movement. Affected by the NW-trending basement fault on the northeast, the NW-trending folds and reverse faults were formed beside the basement faults under the NS-trending tectonic compression in the Indosinian [34]. In other areas of the Enhong synclinorium, including the Bumu region, the influence of the Indosinian movement was very weak and the Indosinian geological structure was rare. During the late Jurassic and early Cretaceous, the subduction of the Pacific plate and the compression of the South China block led to the intense Yanshanian tectonic movement [35]. The intense tectonic compression in the direction of NW-SE caused formation of the Enhong syncline, Fuyuan-Mi'le fault, Pingguan-Agang fault, and the associated branch faults. In the Bumu region, the NNE-trending large reverse fault of F 2-6 and F 8-10 , as well as small buried reverse fault of f 1 , f 2 , f 3 , and f 4 were formed ( Figure 3). The former Indosinian NW-trending structures ware modified intensely and the Yanshanian NE-trending structures were formed rarely in the northeast of the Enhong synclinorium. During the Eocene, the northward subduction of the Indian plate and the westward subduction of the Pacific plate for the Eurasian plate caused the Himalayan movement, EW-trending tectonic compression and dextral shear action [36][37][38]. It caused a right-lateral shear slip and property transformation of the Fuyuan-Mi'le fault and the Pingguan-Agang fault. Meanwhile, the formation of the NNE-trending secondary derived drag folds and derived compression-shear faults in the NW limb, combined with the NE-and EW-trending strike-slip normal derived faults in the SE limb complicated and formed the current tectonic framework of the Enhong synclinorium. The derived normal fault of F 16 were formed in the southeast of the Bumu region. The Indosinian NW-trending structures in the northeast, the Yanshanian NNE-trending Enhong syncline, large boundary, and associated branch reverse faults, combined with the Himalayan secondary derived faults and drag folds, formed the current tectonic framework of the Enhong synclinorium.

Gas Composition
In the Bumu region, the composition of CBM is complex, mainly composed of CH 4 , N 2 , CO 2 , and heavy hydrocarbon gases. The heavy hydrocarbon gases (C 2+ ) include ethane and propane in the research area. The concentration of CH 4 varies greatly and ranges from 9.17% to 95.32%, with an average value of 65.60%. The N 2 and CO 2 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 13 C 1 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 N 2 concentration decreases with the increase in coal burial depth (Figure 4d).
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).  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. In the Bumu region, the methane concentration of CBM decrease while as the N 2 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.

Gas Content and Its Distribution
In the Bumu region, the gas content of coal seam ranges from 6.94 to 19.27 m 3 /t and is mainly between 8 and 13 m 3 /t. The average value of gas content is 11.19 m 3 /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 different 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 3 /t and with an average value of 11.29 m 3 /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 3 /t 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 differentiation of CBM content distribution, particularly in the west ( Figure 5). With the gas content of 10 m 3 /t 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.
(1) Central NNE-trending high gas content zone, with a length of 6500 m in the NNE direction and a width about 1500 m. The gas content is 10.25-15.30 m 3 /t, with an average of 12.11 m 3 /t. Meanwhile, two secondary high gas content zones have been formed in the southwest and northeast, with the gas content generally higher than 13 m 3 /t and 11 m 3 /t, respectively. The distribution of gas content gradient is similar to that of gas content, and there is an overall trend that the gas content gradient decreases from the southwest to the northeast, in the direction of the Enhong syncline plunging. However, the methane concentration of CBM is high in the southwest and northeast while is low in the central region. (2) In the western NNE-trending high gas content zone, the gas content, gas content gradient, and methane concentration vary greatly and distribute complexly. The gas content is 9.70-19.27 m 3 /t and the highest gas content zone forms in the study area. The gas content gradient is 1.50-3.62 m 3 /t/100 m and the methane concentration is 42.16-83.40%. (3) In the northwest low gas content zone, the gas content, gas content gradient, and methane concentration are all low, with the mean value of 8.77 m 3 /t, 1.21 m 3 /t/100 m and 49.03%, respectively. Such a situation is very unfavourable for the CBM accumulation. (4) In the southeast low gas content zone, the gas content is 7.79-9.81 m 3 /t, with its contour lines extending in NNE direction and decreasing in SEE direction. In general, the contour lines of gas content gradient and methane concentration also extend in NNE direction, while decreasing in NWW direction.

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 effects 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 different 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 different structure parts, the effects of different 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).

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 effects 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 NNEtrending high gas content zone, to its limbs in the northwest and southeast low gas content zones. The development of different 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 different structure parts, the effects of different 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).  (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 3 /t, 70% and 2.6 m 3 /t/100 m, respectively. The mean values of those properties in the test boreholes are 15.38 m 3 /t, 79.26% and 3.13 m 3 /t/100 m, respectively. Those boreholes are located near the large reverse fault of F 2-6 and F [8][9][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 3 /t, 70%, and about 2.0 m 3 /t/100 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 3 /t, 80.07% and 1.88 m 3 /t/100 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 3 /t, 70% and about 1.8 m 3 /t/100 m, respectively ( Table 2). The gas content gradient varies greatly, ranges from 1.06 to 2.94 m 3 /t/100 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 F 1 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 3 /t, 70%, and about 1.8 m 3 /t/100 m, respectively ( Table 2). The right-lateral slipping of the Pingguan-Agang fault derives the F 16 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 f 1 , f 3 , and f 4 reverse faults is characterized by the low gas content, methane concentration, and gas content gradient, which is generally lower than 10 m 3 /t, 60%, and about 1.8 m 3 /t/100 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 different 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 3 /t/100 m in normal and buried floor fault dispersions while is higher than 2.4 m 3 /t/100 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 3 /t in reverse fault type and around 12 m 3 /t in syncline sealing type. Gas content gradient can indicate the influence of geological structure on CBM reservoir from the buried depth, which is lower than 1.8 m 3 /t/100 m in normal and buried floor fault dispersions while is higher than 2.4 m 3 /t/100 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 3 /t in reverse fault type and around 12 m 3 /t in syncline sealing type.
The tectonic movement was dominated by the continuous subsidence movements after the coalbearing 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][46][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.

Conclusions
Based on the largely available geological and CBM data, a comprehensive analysis is performed on the Bumu region, one of the hotspots of CBM development in Southwest China. This study has drawn several conclusions, as follows: (1) The Indosinian local deformation, Yanshanian intense deformation, and Himalayan secondary derived deformation formed the current tectonic framework of Enhong synclinorium. The intense tectonic compression and dextral shear action in the Yanshanian and Himalayan movements caused the complex geological structures in Bumu region, which are composed of the Enhong syncline, associated reverse faults and late derived normal fault. 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][46][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.

Conclusions
Based on the largely available geological and CBM data, a comprehensive analysis is performed on the Bumu region, one of the hotspots of CBM development in Southwest China. This study has drawn several conclusions, as follows: (1) The Indosinian local deformation, Yanshanian intense deformation, and Himalayan secondary derived deformation formed the current tectonic framework of Enhong synclinorium. The intense tectonic compression and dextral shear action in the Yanshanian and Himalayan movements caused the complex geological structures in Bumu region, which are composed of the Enhong syncline, associated reverse faults and late derived normal fault. (2) The methane concentration of CBM decreases while the nitrogen and heavy hydrocarbon gas concentrations increase in Bumu region. The relatively low methane concentration and gas content imply the open to semi-open reservoir space of CBM. The CBM distribution is complex, which has the central and western NNE-trending high gas content zones along the syncline hinge zone and the reverse faults. The geological structure controls on CBM enrichment are definite and important. (3) Based on geological structure features and responses of gas content, methane concentration and gas content gradient, the gas controlling pattern of geological structure are determined and can be classified into five types: the reverse fault sealing, syncline sealing, monoclinal enrichment, normal fault dispersion, and buried floor fault dispersion types. (4) The structural compression above the neutral surface plays an important role in the syncline sealing process, which is indicated by the increase in gas content gradient. The EW-trending tectonic intense compression and dextral shear action in the Himalayan movement avoided the negative inversion of NNE-trending Yanshanian compressive structure and its destruction of CBM reservoir. However, the chronic uplift and derived normal fault during Himalayan period caused the constant dissipation of CBM.