**1. Introduction**

Coalbed methane (CBM) resources are abundant in China, where early-stage large-scale CBM development is undertaken actively [1]. However, the complex geological settings of CBM need further

investigation in order to achieve large-scale commercial production e fficiently [1,2]. The coal-bearing basins in China experienced multi-phase tectonic movements in geologic history and developed complex geological structures [3]. Geological structure plays an important role in the process of CBM generation, occurrence, migration, and enrichment in coal seams [4–11]. Especially, the influence on CBM occurrence from the geological structure modification of the coal reservoir physical properties, such as porosity, adsorption, and permeability [12–20]. Research has been focused on the variation of gas content with geological structure; generally, fold and fault. CBM resources within depth less than 2000 m in China are estimated to be 36.81 trillion m3, of which more than 84% occur in nine large-scale basins, such as the Junggar, Tianshan, Tuha, Santanghu, Ordos, Qinshui, Erenhot, Hailar, and western Guizhou–eastern Yunnan basins [1]. Most of these basins exhibit the syncline or synclinorium structure. Syncline structure is a favorable area in terms of CBM reservoir formation, mainly due to the increase in coal seam buried depth and gas content [7,21]. On the other hand, the compression above the neutral surface and the extension below the neutral surface of the syncline are also important for the CBM occurrence [22–24]. However, there is still a lack of e ffective methods for determining the neutral surface and its e ffects on CBM. The influence of fault structure on CBM is generally summarized as gas contents are usually higher near the reverse faults and lower adjacent to the normal faults [1,25]. The normal faults can also increase the gas content in closed and occluded types [9]. The early faults are mainly modified and reversed during the late tectonic movement, especially the early compression and late extension. The evolution of fault and the e ffects of geological structure on other gas-bearing characteristics, such as the gas composition, gas pressure, and gas content gradient, are still not well understood.

The Enhong synclinorium is located in the eastern Yunnan region, Southwest China, with abundant coal resources (Figure 1) [26]. The coal-bearing stratum is the Upper Permian Changxing Formation (P3c) and Longtan Formation (P3l). The estimated coal-bearing area is about 485 km2. The geological coal reserves with burial depth less than 2000 m is 5.25 × 10<sup>9</sup> tons, which makes this synclinorium one of the largest coking coal producing areas in China [27]. The amount of CBM resources with burial depth less than 2000 m is 6.13 × 10<sup>10</sup> m3, and 82% of the volume is located at a depth less than 1000 m, showing a good prospect for CBM exploitation [28]. 12 CBM wells have been drilled in the research area. However, the Enhong synclinorium is characterized by the complex geological structure, which is composed of various scales and orientations of dense folds and faults. The complex structure restricts the exploration and commercial exploitation of CBM. Considering the complexity of the geological structure, as well as the insu fficient and uneven CBM data for the whole region, this work takes the Bumu region as the research area because of the rich geological and CBM data available, which is located in the central part of the Enhong synclinorium.

This article presents the occurrence characteristics and distribution of CBM based on the comprehensive analysis of those CBM data. Multi-phase tectonic movements in structurally complex CBM fields are determined according to the analysis of the regional tectonics setting, geological structure features and structural evolution. The gas controlling patterns of geological structure are jointly proposed by the multiple CBM parameters analysis. The multi-phase tectonic movements controls on CBM occurrence are further discussed. The results of this study provide new insights and improved understanding of the influence of fold neutral surface and fault sealing on CBM, which may have significant implications for CBM exploitation in structurally complex areas.

**Figure 1.** Tectonic location of the study area.1-Weining-Ziyun-Nandan fault, 2-Anninghe-Lvzhijiang fault, 3-Luoci-Yimen fault, 4-Puduhe-Dianchi fault, 5-Xiaojiang fault, 6-E'shan-Tonghai fault, 7-Huanian fault, 8-Red river fault, 9-Mi'le-Shizong fault, 10-Nanpanjiang fault, 11-Youjiang fault.

## **2. Geological Setting**

## *2.1. 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.

**Figure 2.** Geological sketch map and structural cross section of Enhong synclinorium.

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.

#### *2.2. 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 (T1y) and the siltstone and sandstone of Feixianguan Formation (T1f). The buried strata, exposed by drilling, are Kayitou Formation (T1k), Changxing Formation (P3c), Longtan Formation (P3l) and Emeishan basalt Formation (P3β) (Figure 3a). The Changxing Formation (P3c) and Longtan Formation (P3l) 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 segmen<sup>t</sup> of Longtan Formation (P3l2) 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.


**Table 1.** Characteristic and gas content of main coal seam in the Bumu region.

Note: *R*o,max—maximum vitrinite reflectance, *V*daf—volatile yield (dry ash-free base), *V*—vitrinite, *I*—inertinite, *L*—liptinite.

**Figure 3.** Stratigraphic column of coal-bearing strata (**a**), structural outline map (**b**) and structural cross sections (**<sup>c</sup>**,**d**) of Bumu region.

#### **3. 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 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 di fferent 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 di fferent types and periods geological structures, the control action of di fferent geological structures, fold neutral surface action and fault sealing in multi-phase tectonic movements on CBM occurrence were further discussed.


**Table 2.** List of gas-bearing property parameters and structure position of No. 9 coal seam.

\* 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.

#### **4. 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,d). The Enhong syncline is an open NNE-plunging symmetrical syncline, with the hinge line plunges in the direction of NE17◦ and the plunge angle of 15–30◦. The coal seams are generally distributed in NE and NW direction, with a dip angle of about 5–20◦ in the south and about 15–30◦ in the north.

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 F2–5, F2–6, F8–10, and F5, 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 F16 fault, as the southern section of the arcuate strike-slip normal derived faults, extends in NE–NEE direction. There are seven faults, F2–6, F8–10, F16, f1, f2, f3, and f4 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. A ffected 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 F2–6 and F8–10, as well as small buried reverse fault of f1, f2, f3, and f4 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 [36–38]. 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 F16 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.

#### **5. Gas Occurrence Features**
