A Study on the Heterogeneity Characteristics of Geological Controls on Coalbed Methane Accumulation in Gujiao Coalbed Methane Field, Xishan Coalfield, China

Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024 Shanxi, China Coal Reservoir Laboratory of National Engineering Research Center of CBM Development & Utilization, China University of Geosciences, Beijing 100083, China Shanxi CBM Exploration and Development Branch, PetroChina Huabei Oilfield Company, Shanxi 048000, China Research Institute of Petroleum Exploration and Development-Northwest, Petrochina, Lanzhou 730000, China Exploration Company, Sinopec, Chengdu 610041, China


Introduction
As a clean and unconventional natural gas resource, coalbed methane (CBM) is considered to be the most potential source of energy with broad prospects for commercial development. In recent years, the exploitation of CBM in coal-bearing basins has achieved tremendous success in China, especially in Qinshui Basin and Ordos Basin [1][2][3][4][5][6][7][8][9]. The exploration and exploitation of CBM in Xishan coalfield start relatively late, and the research bases of CBM geology are poor in there. Previous research works show that the geological back-ground of Xishan coalfield is very unique. The coalfield had experienced multistage tectonic deformation accompanied with complex structural morphology [10,11]. The maturity of the Pennsylvanian and Permian coal seams in Xishan coalfield shows strong heterogeneity and ranges from highvolatile bituminous A to anthracite [12]. Gujiao CBM field is one of the most important areas for the exploration and exploitation of CBM in Xishan coalfield. The proved CBM resource in this area is about 82.9 billion m 3 , showing strongly economic developmental potential [13]. Since 2011, more than 700 CBM wells have been drilled in this area. However, the exploitation performance of these CBM wells is not very well. The average of daily gas production is only about 277 m 3 [14]. Bian et al. proposed that the complicated structural character of coal reservoir and the ambiguity of resource evaluation process are the main reasons for this poor gas production phenomenon [15].
In this study, the method proposed in our previous work [45] was used to study this complex dynamic process in Gujiao CBM field. The evolution histories of sedimentaryburial process, thermal field, and hydrocarbon generation in CBM reservoir were simulated by analyzing the geophysical logging data. Then, a dynamic equilibrium model was utilized to explain the evolution of CBM reservoir. Finally, the controlling mechanism of four critical tectonic events on CBM accumulation heterogeneity was investigated.

Geological Settings
The Gujiao CBM field is located in the northwest part of Xishan coalfield, including Dongqu, Tunlan, Malan, and Xingjiashe well fields ( Figure 1). The structural contour map of the No. 8 coal seam is shown in Figure 2. Current tectonic structural morphology is mainly controlled by Malan syncline in this area. A series of large-scale NE-SW normal faults developed in this area such as Gujiao fault, Lijiashe fault, and Yuanxiang fault. Some small folds developed in the western region [38]. For the purpose of assessment, the Gujiao CBM field is subdivided into four structural domains (i.e., blocks B1-B4) according to the structural morphology and fracture development character ( Figure 2). Block B1 locates in the region of the Malan syncline's west limb. This block contains an east-dipping monocline with abundant small-scale folds and normal faults. Block B2 is the nucleus area of the Malan syncline. The structure of this block is relatively simple. Only a few faults and small-scale synclines are developed in the northern region. The region of the Malan syncline's eastern limb is further subdivided into two domains. Block B3 is the north part, which declines from northeast to southwest. A series of parallel large-scale northeast-southwest normal faults are developed in this area. Block B4 is the south part, whose structural characteristic is definitely inconsistent with block B3. The structure is relatively simple in this domain.
The strata preserved in the Gujiao CBM field include rocks of the Archaeozoic, Proterozoic, Cambrian, Ordovician, Pennsylvanian, Permian, Triassic, and Quaternary Systems. The main coal-bearing strata are the Benxi Formation and the Taiyuan Formation of the Pennsylvanian System and the Shanxi Formation and the Lower Shihezi Formation of the Permian System [38]. The principal targets for CBM development are coal seam No. 2 in the Shanxi Formation and Nos. 8 and 9 in the Taiyuan Formation ( Figure 3), with thicknesses of 1-8 m, 1-4 m, and 1-6 m, respectively [42]. No.8 coal seam is the target in this research.

Method and Dataset
3.1. Sampling and Experiments. Four coal blocks were collected in the study area ( Figure 1). All samples were collected from fresh coal mining face in underground coal mines. In order to minimize the influence of tectonic deformation and macroscopic coal type on the experimental results, the primary structural semibright coal samples were selected for experiment. These samples were treated to 60-80 mesh (0.18-0.25 mm). After moisture-equilibrium treatment, isothermal adsorption experiments were carried out at 25, 30, 3 Geofluids 40, 50, 60, and 70°C under an equilibrium pressure of up to 20 MPa. Combining with the simulated results of coal reservoir condition (pressure and temperature) evolution, these isotherms were used to characterize the adsorption capacity evolution character of coal reservoir during an uplifting process.

Simulation
Process. Data of 9 exploration wells were used to simulate the evolution of reservoir formation ( Figure 2). The data of proximate analysis, coal seam burial depth, thickness, vitrinite reflectance, gas content, and others were used for simulation research. Table 1 provides an example of the input database of the XST-019 well. The dynamic equilibrium model proposed by Wei et al. [53,54] was used to simulate the geological evolution history in this study. The tectonic subsidence history, geothermal field evolution history, and methane generation features were simulated by using a commercial computer software program PetroMod 2012. More details can be found in our previous work [45].

Geofluids
tectonic and subsidence evolution, geothermal evolution, and coal maturation history) of coal reservoir in Gujiao CBM field were strongly controlled by the evolution process of Xishan coalfield. Since the Hercynian movement, the tectonic and subsidence evolution process of Xishan coalfield can be divided into six stages, accompanied with two magmatic activities ( Figure 4). In stage I (306.1-250 Ma), affected by the Hercynian movement, the North China plate began to subside, and a series of near north-south direction coal accumulation depressions were formed in Xishan coalfield [25,55]. Then, Pennsylvanian paralic coal-bearing rock series, Permian continental coal-bearing rock series, and continental clastic rock series were deposited continuously (Figure 4(a)).
In stage III (215-180 Ma), the whole coalfield experienced tectonic uplift and sedimentary rocks suffered denudation. As a result of the Early Yanshanian orogeny, the coalfield had shown heterogeneous evolution characteristics. Affected by the continuous development of the east-west direction tectonic zone of Yangqu-Meng county in the north of Xishan coalfield, the northern part of Xishan coalfield was uplifted (Figure 4(c)) [55]. The Permo-Pennsylvanian coal seams and Ordovician limestone were exposed to the surface in this area, and the Xishan coalfield was separated with the northern Ningwu coalfield. For the southeast part, the Qixian concealed rock was developed in this stage which caused an uplift and denudation in this area (Figure 4(c)) [36].
In stage IV (180-165 Ma), the coalfield experienced tectonic subsidence. Jurassic strata were deposited in depression areas in this stage [20]. Influenced by the east-west compression force, a series of north-south direction syncline struc-tures were formed. Malan syncline and Shuiyuguan syncline were formed initially (it is estimated that they were connected at this time). The Jiaocheng fault was also formed initially, and it was thrust nature at this time ( Figure 4(d)) [58].
In stage V (165-65 Ma), during the Middle-Late Yanshanian orogeny period, influenced by the composite effects of regional east-west compressive stress, slowly uplifting of Lvliang mountain (120-65 Ma), and the development of Huyan mountain intrusion (141-125 Ma) [22], the whole coalfield experienced tectonic uplift, especially the western part of the coalfield. The direction of Malan syncline's axis changed, and the Malan syncline separated from Shuiyuguan syncline in this period (Figure 4(e)) [20].
In stage VI (65 Ma-Now), affected by the accelerated uplift of Lvliang mountain in the Himalayan orogeny period [59,60], Xishan coalfield sustained uplift and suffered denudation. Influenced by the northwest-southeast tension stress field, a series of normal faults were formed in the coalfield [59,61]. At this time, the Jiaocheng fault changed from thrust to normal fault [25]. Significant subsidence occurred in the area on the upper wall of the fault (Figure 4(f)). Finally, the current structural character was formed.

Tectonic and Subsidence Evolution in Gujiao CBM Field.
As a part of Xishan coalfield, the evolution process of coalbearing strata in Gujiao CBM field could also be divided into six stages, which are deposition, burial, uplift, reburial, rapid uplift, and step erosion ( Figure 5).  V L and P L are Langmuir volume (in air-dry base) and Langmuir pressure. a The data are obtained from wellbore, geophysical logging, and experiments (isothermal adsorption tests, maceral measurements, vitrinite reflectivity measurements, proximate analysis). b The data are obtained from the subsidence, geothermal, and organic maturation history simulations.

Geofluids
Taiyuan period, the paralic sedimentary coal-bearing strata had been deposited with a deposition rate of 12. The whole coalfield descended affected by regression.
The northern part was uplifted affected by theYangqu-Mengxian tectonic subzone.

Stage III (215-180 Ma)
A series of meridional structures were formed under the action of east-west compression stress.

Geothermal Evolution and Coal
Maturation. The geothermal evolution and coal maturation characters are illustrated in Figure 6. In stage I (306.5-250 Ma), the main deposition period of the coal-bearing strata, the geothermal gradient was about 2.8°C/100 m and the coal reservoir was buried shallowly. The low temperature of coal seam led to a limited organic matter maturity (Table 2). In stage II (250-215 Ma), the study area subsided rapidly. The temperature of coal seam was increasing gradually during the long-term deposition process. At the end of Middle Triassic period, the temperature of coal seam was about 105°C when the burial depth of coal seam reached its maximum ( Table 2). The vitrinite reflectance (R o , %) of the coal seam was approximately 0.7% at that time ( Figure 6). Then, in the uplifting stage (stage III), affected by the development of Qixian concealed rock (215-180 Ma) in the southeast of study area (Figure 4(c)), an abnormal geothermal field was formed ( Figure 6). Due to the superposition of magma heat, the geothermal gradient in the study area increased significantly, reaching 11.84°C/100 m in some areas [22]. The regional magma thermal metamorphism led to the significant increase of coal reservoir organic maturity. The vitrinite reflectance (R o , %) of coal seam was approximately 1.3-2.06% (Table 2), showing a decrease trend from southeast to northwest. In stage IV (180-165 Ma), the study area experienced a short-term subsidence and deposited the Jurassic strata. The geothermal field returned to normal level, and the geotemperature decreased substantially ( Figure 6). Coal organic matter maturing stagnated in this stage ( Table 2).  Figure 5: Typical tectonic movement and burial history curve for the coal-bearing strata in the study area. P N P 1 P 2 T 1-2 T 3 J 1 J 2 J 3 Q Biogenetic gas Thermogenic gas

Geofluids
Then, in the next stage (stage V from 165 to 65 Ma), coalbearing strata suffered denudation due to a long-term structural uplift. For the early period in stage V (165-141 Ma), structural uplifting led to further decreasing of coal seam temperature, and coal organic matter maturing continued to stagnate (Table 2). For the middle period in stage V (141-125 Ma), the formation of the Huyan mountain rock mass resulted in a significant increase of geothermal gradient in the western part of the Xishan coalfield (Figures 4(e) and 6(i)). The thermal contact metamorphism caused further maturation of coal organic matter ( Table 2). The coal rank was distributed around the Huyan mountain in a circular belt. Within a short distance, the coal rank changed from super anthracite to high-volatile bituminous C [22]. Limited by the scale of the intrusive rock mass, only a small range of the study area was affected by this magma thermal event, which corresponds to the vicinity of the XSM-169 well in the southwest study area. For the last period in stage V (125-65 Ma) and stage VI (65 Ma-Now), the geothermal gradient reverted to a lower level gradually, the coal reservoir temperature decreased substantially, and the maturation of coal organic matter stopped (Table 2).

CBM Accumulation Process.
In each evolution stage, the in situ gas content of the coal seam was mainly affected by the coal adsorption capacity and reservoir conditions (pressure and temperature) during the evolution process. An experimental simulation method modified from Bustin and Bustin [62] was used to investigate the evolutionary process of CBM accumulation in our previous work [45]. In this work, a similar experimental simulation work was also conducted. As the pressure and temperature of coal reservoir decreased, the coal adsorption capacity increased firstly and then decreased (i.e., the uplifting process) (Figure 7). Table 3 shows the adsorption capacity evolution characteristics in the domains of B1 and B3.
Combining with the results from simulation using the dynamic equilibrium model, the experimental results shown in Figure 7 and Table 3 were used to simulate the CBM accumulation processes and the formation history in the study

Geofluids
area. The output of the simulation includes burial depth; cumulative gas production; cumulative gas diffusion; R o ; in situ gas content at each geological time; and gas production, diffusion, and retention features during each geological evolutionary stage. Table 4 provides two examples of the simulation results for simulation nodes that contain well XSM-169 and XST-019, which represent that the coal reservoir experienced three-stage and two-stage thermal metamorphism processes, respectively. The evolution of the CBM reservoir of the No. 8 coal is shown in Figure 8. Consisting with the tectonic evolution history ( Figure 4) and thermal evolution history (Figure 6), the entire CBM reservoir formation evolution can also be divided into six stages ( Figure 8 and Table 4).
In the first stage (306.5-250 Ma), the shallowly buriedimmature stage, biogenerating gas was the main products in this low-temperature reservoir condition. However, the biogenic gas was almost dissipated due to the shallow depth and absence of an effective local reservoir cap. In the second stage (250-215 Ma), the deeply buried stage, the coal seam had undergone deep metamorphism process, accompanied with the generation of primary plutonic metamorphism gases. A few amounts of this primary CBM were stored in coal seam, which was the most dissipated. In the third stage, during the Late Triassic-Early Jurassic (215-180 Ma), affected by the concealed rock mass in Qixian County, an abnormal geothermal field was formed which made the coal organic matter maturated rapidly in most parts of the study area ( Figure 8 and Table 4). A large amount of secondary coalbed gas generated in this stage, accompanied with strong gas dif-fusion. The fourth stage was the short-term deposition stage occurring between 180 and 165 Ma. As the organic matter maturation and hydrocarbon generation stagnated, only gas diffusion existed in this period. The fifth stage (165-130 Ma) was subdivided into three substages. The substage V 1 was a dissipation stage, from 165 to 141 Ma. The organic matter maturation and hydrocarbon generation continue to stagnate; only gas diffusion existed in this substage (Table 4). Then, in the substage V 2 , the thermal contact metamorphism caused further maturation of coal organic matter in the southwest part of research area (e.g., well XSM-169), whereas the gas further diffused in the other areas (e.g., well XST-019) as shown in Figure 8 and Table 4. In the southwestern part, the generation of third coalbed gas led to strong gas diffusion (Table 4). For the substage V 3 (125-65 Ma) and stage VI (65 Ma-Now), affected by the Late Yanshanian orogeny and Himalayan orogeny, the overlying strata were uplifted and eroded continuously, and more gas escaped from coal reservoir.

Hydrocarbon System Evolution and Geological Control
Effects. The Gujiao CBM field contains one hydrocarbon system, as shown in Figure 9. During the histories of hydrocarbon generation, expulsion, and migration processes, there are three major hydrocarbon generation and accumulation periods, i.e., the Indosinian, the Early Yanshanian, and the Middle Yanshanian, respectively. The Permo-Pennsylvanian coal seams are the source rocks of gas in this hydrocarbon system. The Late Permian strata are seal rocks T MAX is the maximum temperature of coal reservoir in each geothermal evolution stage. 10 Geofluids that prevent gas diffusion. Hydrogeological conditions and tectonic structures had different effects on hydrocarbon migration and preservation. Four critical tectonic events have had a significant impact on gas production, accumulation, and diffusion ( Figure 9). Coal-bearing strata were deposited in the Late Pennsylvania and Early Permian periods (stage I) during which the primary biogenic methane was produced and almost dissipated (Figure 8). After this shallowly buried period, the generation, expulsion, and accumulation processes of CBM were affected by the following episodic tectonic movements.
During stage II, the Early-Middle Triassic Indosinian orogeny was the first critical tectonic event that affected the generation of primary CBM (Figure 9). The maturity of coal increased with the increase of depth from the edge of the basin to the center (i.e., from north to south). Gas content at the end of this stage and gas diffusion during this stage were 6.2-7.4 m 3 /t and 11.2-13 m 3 /t, respectively ( Figure 10). The distribution of gas content was consistent with the distri-bution characteristic of coal organic vitrinite reflectance ( Table 2). The deeper buried coal seams had a higher paleotemperature, accompanied with a higher organic maturity. The higher organic maturity led to a higher gas production capacity and gas diffusion ( Figure 10). The retention and dissipation of primary gases were controlled by the amount of gas produced and the sealing conditions of the coal reservoir.
The second critical tectonic event was the Early Yanshanian orogeny (Stage III in Figures 5 and 9). In this stage, the development of the Late Triassic-Early Jurassic Qixian concealed rock mass led to an abnormal geothermal field, which accelerated the maturation process of coal organic matter. The reservoir temperature decreased gradually from southeast to northwest (Table 2). Hydrocarbon generation and dissipation in most parts of the study area reached their maximum values over the entire geological evolution history. The maturity of coal organic matter increased significantly, reaching a maximum of 2.06% (well GJ-06) in the southeast part of study area. The gas production was about 28.12-  Q S and P are the gas content and reservoir pressure obtained from the simulations of CBM reservoir accumulation history. Q C is the sorption capacity obtained from the adsorption capacity evolution simulation. S is the gas saturation of the sampling sites. Well XSM-169 and XST-019 represent that the coal reservoir had experienced three-stage and two-stage thermal metamorphism processes, respectively. 12 Geofluids content, and gas diffusion were similar to the distribution of paleotemperature ( Figure 11 and Table 2). The second critical tectonic event controlled the paleotemperature field variation which resulted in the high gas content in B4 and the south area of B2. During the following period of 185-145 Ma (stages IV and V 1 in Table 4), the study area was deposited briefly and then uplifted. Due to the low temperature of coal reservoir, hydrocarbon generation stagnated, and only gas diffusion existed in this period. Gas content at the end of this stage and gas diffusion during this stage were 10.7-35.6 m 3 /t and 6.5-23.8 m 3 /t, respectively ( Figure 12). The amount of gas diffusion was high in the southeast (B4 and the south area of B2 domain), which inherited the gas content distribution characteristic at the previous stage (Figures 11(a) and 12(b)). This result implies that the methane concentration was a much more important factor than others for gas diffusion in this stage.
After this gas diffusion period, the third critical tectonic event occurred in the Middle Yanshanian orogeny (145-130 Ma: stage V 2 in Table 4) (i.e., the formation of the Huyan mountain rock mass). Due to the intrusion of Huyan mountain rock mass, an abnormal geothermal field was formed in the southwest part of study area and the coal seams in there experienced their third hydrocarbon-generation process. The reservoir temperature was distributed in a ring band and decreased from the southwest corner to the northeast in southwest research area (south area of B1 and southwest of B2). Hydrocarbon generation and gas diffusion showed the same trend that decreased from the southwest to the northeast (Figure 12(b)). The coal rank reached 2.5% in the area of well XSM-169, and the volume of gas production was 101.4 m 3 /t in this area during this stage. As for the other areas, due to the further decrease of temperature, the hydrocarbon generation stopped and the gas in coal seams further escaped. In these low-paleotemperature field areas, the gas diffusion was dominated by the in situ hydrocarbon  13 Geofluids concentration. The gas content at the end of the previous evolution period (Figure 12(a)) controlled the variation of gas accumulation and dissipation in these low temperature areas. Gas content at the end of this stage and gas diffusion during this stage were 9.7-38.8 m 3 /t and 1-90.6 m 3 /t, respectively ( Figure 13). The gas content and gas diffusion reached their maximum value in the southwest region and decreased radially to the northeast, whereas in the areas unaffected by Huyan mountain rock mass, the hydrocarbon variation characteristics of the previous period were inherited.
In substage V 3 , influenced by the uplifting of Lvliang mountain in this period and the intrusion of Huyan mountain rock mass in the former period, the west to southwest part of the study area uplifted more than elsewhere Uplifting stage (for the whole area); the third gas generation stage (locally in the southwest part) Coalbed methane dissipation stage Figure 9: Extended essential element diagram showing the hydrocarbon system present in the study area with relation to tectonic activity and general tectonic behavior.  accompanied with a higher gas escape degree. The amount of gas diffusion was 1.6-16.2 m 3 /t, showing a decrease trend from southwest to northeast (Figure 14(b)). At the end of the stage, the gas content was 8.1-22.7 m 3 /t, as shown in Figure 14(a).
Since the Cenozoic Era, the study area has experienced three stages of uplift [51]. During the first and second uplifting stages (substage VI 1 and VI 2 ), under the influence of the Early and Middle Himalayan tectonic movement, the study area was uplifted in a stepped manner. The gas diffusion  16 Geofluids was dominated by the hydrocarbon concentration at the end of the previous stage (Figures 14(a) and 15(b)). After this evolution period, the gas content was about 6.5-17.4 m 3 /t ( Figure 15(a)), and its distribution characteristics were roughly consistent with the distribution characteristics at the previous stage (Figure 14(a)). The gas saturation was relatively low in the west part of study area (B1 domain and north of B2 domain) due to sustaining uplifting of Lvliang mountain in the west ( Table 3). The fourth critical tectonic event occurred in the Late Himalayan period (substage VI 3 ). During this stage, the study area experienced a sharp uplift. In this period, the development of the Fenhe graben caused the structural inversion in the northeast to eastern regions of the study area [59]. The stress field transformed from the squeezed state to the stretched state. Affected by the northwest-southeast tension stress, a series of northeast-southwest normal faults were formed, mainly in the northeastern part of the study area. The development of these extensional structures resulted in strong gas diffusion ( Figure 16). The gas saturation and gas content decreased rapidly in the northeast part of the study area (east part of B3 domain) ( Table 3 and Figure 16). Moreover, during this stage, the limestone areas in the northwest and north part of the coalfield were the main recharge runoff area of the study area. The groundwater flowed from these recharge areas to the low-lying zone of the study area. The gas was carried to the axis area of syncline for accumulation [16,26].
In general, the combining effects of basin uplifting, tectonic inversion (the formation of extensional faults), and hydrogeological characteristics controlled the gas content and dissipation characteristics of reservoirs in this evolution period.
The formation, preservation, and dissipation of coalbed methane were mainly affected by four critical tectonic events. The first critical tectonic event during the Indosinian period of the Early-Middle Triassic controlled the first hydrocarbon generation process in the study area. The second critical tectonic event in the Early Yanshanian period, Qixian concealed rock mass magma-heat event, affected the distribution of secondary gas generation distribution characteristic and resulted in higher gas generation in southeast part of research area. The third critical tectonic event in the Middle Yanshanian period, Huyan mountain intrusive rock mass magmaheat event, dominated local third gas generation distribution characteristic and resulted in higher gas generation in the southwest research area (south area of B1 and southwest of B2). The fourth critical event, i.e., the Late Himalayan tectonic inversion, caused the further diffusion of gas and reduced the gas content and saturation in coal reservoir, especially in the northeast part of study area (east part of B3 domain). The combination of these four critical tectonic events led to the high gas heterogeneity distribution of coalbed methane reservoirs in Gujiao coalbed methane field, Xishan coalfield.

Data Availability
All data are derived from my experimental results, which can be provided in the appendix of the paper if necessary.

Conflicts of Interest
The authors declare that they have no conflicts of interest.