1 Introduction
With the decrease of new proven available reserves in shallow buried areas, the exploration and development technologies of deep buried coalbed methane (CBM) have become a hot topic in recent years (
Qin and Shen, 2016). The areas that conduct commercial development of deep buried CBM resources in China mainly distributed in the eastern margin of Ordos Basin (e.g., Yanchuannan and Daning-Jixian Blocks) and Qinshui Basin (e.g., Zhengzhuang and Shizhuang Blocks) (
Li et al., 2016). Among these blocks, Daning-Jixian Block is one of the most important areas for the co-exploration and co-exploitation of coal measure gas (CMG). The proved resources of deep buried CBM and tight sandstone gas in this area is about 148.3 billion m
3 and 70.9 billion m
3, respectively, which shows great economic developmental potential (
Li et al., 2011). Many research works have been carried out in Daning-Jixian Block for deep buried CBM reservoir exploration, including distribution characteristics of CBM (
Sun et al., 2004), sedimentary environment analysis (
Sun and Wang, 2006),
in situ stress field characteristics (
Jiang et al., 2016), analysis on origin types of CBM (
Chao and Wang, 2016;
Mu et al., 2016 ), production characteristics of deep buried CBM (
Huang et al., 2018;
Nie et al., 2018;
Nie et al., 2022), favorable area evaluation (
Yan et al., 2021a;
Li et al., 2022), and qualitative research on CBM accumulation (
Li et al., 2022). However, to the best of the authors’ knowledge, effective research on the coal reservoir evolution and CBM accumulation process has not been conducted in this area.
The evolution of CBM reservoir is a complex dynamic process that includes coal organic matter maturation and gas generation processes (
Scott et al., 1994;
Boreham et al., 1998;
Johnson and Flores, 1998;
Wu et al., 2000;
Payne and Ortoleva, 2001), as well as the gas sorption, migration, diffusion, pooling and accumulation processes (
Scott, 2002;
Yao et al., 2009;
Cai et al., 2011;
Karacan and Goodman, 2012;
Song et al., 2012;
Vishal et al., 2015;
Xu et al., 2015). The comprehensive analysis and restoration of these dynamic processes is critical for the understanding of gas enrichment characteristics in coal reservoir. Basin-scale investigations on the CBM reservoir formation history have been performed by many scholars in different sedimentary basins (
Boreham et al., 1998;
Johnson and Flores, 1998;
Payne and Ortoleva, 2001;
Faiz et al., 2007;
Wei et al., 2007, 2008, 2010;
Alsaab et al., 2009;
Sachsenhofer et al., 2012;
Cai et al., 2014;
Yan et al., 2015,
2021b).
In this research, the burial history, geothermal evolution history and hydrocarbon generation history of deep buried CBM reservoir in Daning-Jixian Block were simulated for clarification of the geological control effects on the dynamic processes of CBM accumulation and the enrichment characteristic of current gas in different tectonic domains. The research results have a crucial significance for exploration and development of deep CBM in this area.
2 Geological settings
Daning-Jixian Block located in the eastern margin of Ordos Basin. The structural contour map of the No. 8 coal seam is shown in Fig.1. Tectonically, the Daning-Jixian Block is a monocline which inclines to west (Fig.1). According to tectonics shape feature and fault distribution, Daning-Jixian Block is divided into four structural domains, i.e., the eastern slope zone (B1), Puxian depression zone (B2), Guyi-Yaoqu anticline and fault zone (B3) and western slope belt (B4).
Block B1 locates in the south-east of study area. Morphologically, block B1 is a northwest-dipping monocline with an inclination angle of 3°–6°. Block B2 is a depression, formed by extrusion, which locates in the north-east of study area. Some small normal faults are developed in this domain. Block B3 is a northeast–southwest structural belt, which locates in the middle of study area. The Xueguan-Yukou thrust fault is distributed in this domain, which is accompanied by several small thrust faults. Affected by the traction of the Xueguan-Yukou reverse fault, a long-axis anticline was formed in this domain with structural amplitude of 80–120 m and an axis length of about 40 km. Block B4 is a gentle slope zone in the west of study area, with a dip angle of 0.3°–2.5°.
The strata preserved in Daning-Jixian Block include rocks of the Archaeozoic, Proterozoic, Cambrian, Ordovician, Carboniferous, Permian, Triassic, and Quaternary Systems (Fig.2). Multi coal seams (No.1–8) are developed in Carboniferous-Permian coal-bearing strata (Fig.2). The fluvial-deltaic environmental No. 5 coal of Shanxi Formation and shallow-marine environmental No. 8 coal of Taiyuan Formation are the main target coal seams for CBM production in this area, with thickness of 0.5–8 m and 3–9.8 m, respectively. The vitrinite reflectance (Ro, max, %) of organic matter in these two coal seams is about 1.37%–3.40%, belonging to the coal rank of middle-volatile bituminous, semi-anthracite and anthracite.
3 Method and data set
3.1 Sample and experiments
Two coal samples were collected from fresh coal mining face in underground coal mines which adjoin our study area (Fig.1). To minimize the influence of tectonic deformation and macroscopic coal type on the experimental results, primary structural and semi-bright coals were selected for experiments. Samples were crushed and sieved to a size range of 0.18–0.25 mm (i.e., 60–80 mesh), and moisture-equilibrium treatment was completed. Then, a series of isothermal adsorption experiments were carried out at 35°C, 45°C, and 55°C under an equilibrium pressure of up to 20 MPa. Combining with the simulated results of coal reservoir conditions (pressure and temperature) evolution, these isotherms were used to characterize the adsorption capacity evolution character of coal reservoir during uplift process.
3.2 Simulation process
Data of 13 exploration wells were used to simulate the evolution of coal reservoir. The fundamental data that includes proximate analysis, coal seam burial depth, thickness, vitrinite reflectance, gas content and others were used for numerical simulation. Tab.1 provides an example of the input database of the Well J2.
The dynamic equilibrium model used in our previous work (
Yan et al., 2015,
2021b) which proposed by
Wei et al. (2007,
2010), was also used in this study for the simulation of reservoir geological evolution history. The tectonic subsidence history, geothermal field evolution history and methane generation features were simulated by using a commercial computer software program PetroMod 2016. More details can be found in our previous work (
Yan et al., 2015,
2021b).
4 Result and discussion
4.1 Tectonic and subsidence evolution
In the Archaean and Proterozoic Era, as a part of the North China Platform, the Ordos Basin was in a relatively stable stage. In the Early Paleozoic Cambrian and Ordovician periods, the study area was in neritic sedimentary environment (Philip et al., 2009), a large number of marine strata were deposited in this period. Then, the study area experienced a long-term depositional gap from the Early Paleozoic to Late Paleozoic, resulting in the lack of Late Ordovician, Silurian, Devonian and Early Carboniferous strata. After that, coal-bearing strata were deposited in the period of the Middle and Late Carboniferous–Early Permian. These coal-bearing strata that enriched organic went through a series of burial and uplift processes (Fig.3 and Fig.4), which can be divided into six stages.
4.1.1 Stage I: Middle-Late Carboniferous and Permian Periods
The study area changed from uplifting to subsidence stage due to Hercynian crustal orogeny. The fluvial-deltaic environmental Shanxi Formation and shallow-marine environmental Taiyuan Formation coal-bearing strata were deposited in this period (Fig.4(a)). The deposition speed was 3.3–12.1 m/Ma and 4.4–13.9 m/Ma, respectively. The sedimentary thickness was 25–91 m and 63.9–202.5 m, respectively (Fig.5(a) and Fig.5(b)). In the Carboniferous Period, the sedimentary center was located near wells J25, J34, and J13 in eastern study area. The deposition speed increased from west to east, and, the coal-bearing strata thickness of Taiyuan Formation gradually increased from west to east. Then, the sedimentary center shifted southward, in the region near Well J47, with a sedimentary thickness of more than 200 m. The deposition thickness was thinner in central study area, ranging from 60 m to 80 m (Fig.5(b)). At the end of Early Permian Period, the total thickness of coal-bearing strata gradually thickened from north-west to south-east, and the maximum thickness reached about 250 m near Well J47 in the south-east (Fig.5(c)). Then, the subsidence speed increased to approximately 17.61–31.86 m/Ma in the Late Permian Period, and the sedimentary thickness was 588–1064 m in the Shihezi and Shiqianfeng Periods (Fig.6(a)). During this stage, the sedimentary center was located near Puxian, and the sedimentary thickness reached more than 1000 m. At the end of this evolution period, the burial depth of coal-bearing strata generally increased from south-west to north-east, and reached the maximum burial depth of about 1200 m in Puxian area (Fig.6(b)).
4.1.2 Stage II: Early and Middle Triassic Periods
Abundant river-lake face red clastic rocks i.e., Liujiagou Formation, Heshanggou Formation, Zhifang Formation and Yanchang Formation were deposited successively during the Early and Middle Triassic periods. The deposition speed was about 37.1–45.6 m/Ma, and the deposition thickness ranged from 1760 m to 2160 m. Thicker strata were deposited in the areas where thinner strata deposited in the previous evolution stage (Fig.6(b), Fig.7(a)). At the end of this stage, the burial depth of the Taiyuan Formation increased gradually from south-east to north-west in study area (Fig.4(b); Fig.7(b)). In Puxian region, where the previous sedimentary strata were relatively thicker, the buried depth reached more than 3000 m (Fig.4(b) and Fig.7(b)).
4.1.3 Stage III: Late Triassic Period to Late Jurassic Period
In the Late Triassic Period, a south-north compressive stress field was formed by the collision between the North China Plate and the Yangtze Plate during the Indosinian Orogeny (Fig.4(c)). Affected by this compressive stress, the study area was uplifted and eroded with eroding speed was about 26.4–32.9 m/Ma. The eroding thickness was about 370–460 m (Fig.8(a)). The Yanchang Formation was eroded in this period (Fig.4(c)). At the end of Late Triassic, the buried depth of coal reservoir is about 2260–2660 m (Fig.8(b)). In the following Early Jurassic Period, the study area experienced short-term deposition (Fig.4(d)). Then, in the Middle and Late Jurassic Periods, a northwest–southeast compressive stress field was formed by the collision between the Paleo-Pacific Plate and the Eurasian Plate during the Early Yanshanian Orogeny (Fig.3). Affected by this compressive stress, the study area was uplifted and eroded again (Fig.4(e)), with eroding speed and the eroding thickness was about 7.6–12 m/Ma and 230–360 m, respectively (Fig.9(a)). The anticlinal structural belt and the fracture system were also formed under this compressive stress in central region of study area (Fig.4(e)) (
Yang et al., 2013). The tectonic pattern of the study area was basically formed in this evolution stage (Fig.9(b)).
4.1.4 Stage IV: the Early Cretaceous Period
During the fourth stage, subsidence occurred throughout the study area as a result of the Middle Yanshanian Orogeny (Fig.4(f)). The deposition thickness reached about 700–1100 m in Puxian region (
Niu et al., 2010). The Permo-Carboniferous coal-bearing strata were deep buried again. The burial depth reached the maximum value about more than 3000 m. Increasing magma-heat events occurred as a result of the Middle Yanshanian Orogeny (Fig.4(f)) (
Cao et al., 2018;
Li et al., 2022).
4.1.5 Stage V: the Late Cretaceous period
In this stage, the study area was uplifted and eroded again due to the uplift of the Lvliang Mountain Orogeny (Fig.4(g)). The south-east part of study area uplifted most and the whole block was inclined to west in this stage (
Wang et al., 2013). The eroding speed and the eroding thickness were about 14.3–23.6 m/Ma and 920–1520 um, respectively (Fig.10(a)). At the end of this stage, the burial depth of the Taiyuan Formation increased gradually from the central area to the surrounding area in study area (Fig.4(g); Fig.10(b)).
4.1.6 Stage VI: Cenozoic Era and Quaternary
During the Cenozoic Era, the regional stress field changed to northeast–southwest direction. Affected by this variation of regional stress field, some reverse faults were transformed into normal faults in the Puxian area (Fig.4(h)) (e.g., the region of Well J25) (
Tian et al., 2012). The structural framework of the study area was basically formed (Fig.11(a), 11(b)). During this period, the research area continued to be uplifted and eroded (Fig.3). The erosion thickness was approximately 540–900 m, with a speed of 8.5–14.1 m/Ma (Fig.11(a)). The sedimentary strata eroded most in the south-east region. During the last evolution period (i.e. Quaternary period), a set of mudstones was deposited in study area, with a deposition rate of 3–84 m/Ma and a deposition thickness of 0–170 m (Fig.4(i) and Fig.12(a)). The deposition thickness decreased from the middle to the surrounding area. Then, the burial depth characteristic of the Taiyuan Formation and tectonic pattern of the study area were formed (Fig.12(b)).
4.2 Geothermal evolution and coal maturation
The thermal evolution history of the study area can be divided into six stages according to the subsidence and paleogeothermal gradient evolution history (Fig.13).
During stage I (306.5–251 Ma), geothermal gradient was 2.8°C/100 m. The coal reservoir was buried shallowly, and the maturity of organic matter was relatively low (Fig.13). In stage II (251–203.6 Ma), due to the rapid subsidence, the coal reservoir temperature rose up to 114°C–119.6°C at the end of the Middle Triassic (Fig.14(a)). Coal-forming organic matter had undergone long-time plutonic metamorphism. The vitrinite reflectance of coal organic matter reached about 0.73%–0.77% (Fig.14(b)). In stage III (203.6–145.5 Ma), the temperature of the coal reservoir decreased because of the tectonic movement occurring during the Late Triassic–Jurassic (Fig.13). Then, during the stage IV (145.5–130 Ma), coal-bearing strata was buried deeply again. Affected by regional thermal metamorphism, the temperature of coal reservoir rapidly raised to about 210°C–310°C (Fig.15(a)). The maximum vitrinite reflectance of coal organic matter reached 1.60%–3.4% (Fig.15(b)). Later the study area was uplifted rapidly and suffered denudation, and the geothermal gradient gradually returned to normal. For the last two stages, the geothermal field reverted to a lower level, and the geo-temperature substantially decreased (Fig.13). Coal organic matter stopped maturing during those two stages.
4.3 CBM accumulation process
In each evolution stage, the
in situ gas content of the coal seam was mainly affected by the synthetic effects of coal adsorption capacity and reservoir conditions (pressure and temperature). An experimental simulation method that modified from Bustin and Bustin (
2008) was used to investigate the evolutionary process of CBM accumulation in our previous works (
Yan et al., 2015,
2021b). In this work, a similar experimental simulation work was also conducted. The coal adsorption capacity increases whiles the pressure and temperature of coal reservoir decrease (i.e., the uplifting process) (Fig.16). Tab.2 shows the adsorption capacity evolution characteristics in the domains of B1 and B2 during the uplifting process in Cenozoic Era and Quaternary Period.
The experimental results shown in Fig.16 and Tab.2 were used to simulate the CBM accumulation processes and the formation history combining with the results from simulation using the dynamic equilibrium mode. The output of the simulation includes many evolution characteristics of reservoir parameters (i.e., burial depth; cumulative gas production; cumulative gas diffusion; Ro; in situ gas content at each geological time; and gas production, diffusion and retention features during each geological evolutionary stage). Tab.3 provides an example of the simulation results for simulation nodes that contain Well J2. The evolution of the CBM reservoir of the No. 8 coal is shown in Fig.17. The evolution history of CBM reservoir formation in the study area can also be divided into six stages in accordance with the subsidence (Fig.3) and paleo-geothermal (Fig.13) evolution history.
The first stage is the main formation period of coal-bearing strata (306.5–251 Ma). In this period, the coal reservoirs were buried shallowly and the maturity of organic matter was low. Hydrocarbon generation was mainly consisted of biogenic gas which almost completely escaped due to the shallow depth and absence of an effective local reservoir cap. The second stage (251–203.6 Ma) is the rapidly burial stage. With the increasing of coal reservoir temperature, coal-forming organic matter suffered long-term plutonic metamorphism. The first significant hydrocarbon (i.e., primary plutonic metamorphism CBM) generation occurred in this stage. A few amounts of this primary CBM were stored in coal seam, while most was dissipated through diffusion. Subsequently, in the third stage (203.6–145.5 Ma), affected by the tectonic movement from the Late Triassic to Jurassic, the coal-bearing strata are in an uplifting and fluctuant condition. The coal reservoir temperature decreased a lot, and the coalification was stagnant while hydrocarbon generation stopped. In the fourth stage (145.5–130 Ma), the coal seam temperature rose rapidly affected by regional thermal metamorphism effect (Fig.13 and Fig.15(a)). Coal-forming organic matter rapidly matured, and huge amount of secondary thermogenic gas generated accompanied with strong gas diffusion. During stage V (130−65.5 Ma), geothermal gradient gradually returned to normal, the maturation of organic matter and generation of hydrocarbon ceased. During the last stage VI (65.5 Ma–now), a large amount of gas was diffused due to further eroded of the overlying strata, which was controlled by the Himalayan tectonic movement.
4.4 Hydrocarbon system evolution and geological control effects
There is one hydrocarbon system in Daning-Jixian area (Fig.18). During the dynamic evolution histories of hydrocarbon accumulation (i.e., generation, expulsion, and migration processes), the organic matter experienced twice hydrocarbon generation and accumulation processes, which happened in the Early-Middle Indosinian and Middle Yanshanian (Fig.18). The source rocks and reservoirs are Carboniferous Taiyuan Formation and Permian Shanxi Formation, while the late Permian strata works as seal rocks providing a suitable condition for gas preservation. The Hercynian movement, Indosinian movement, Yanshanian movement and Himalayan movement sequential affect the evolution process of coal-bearing strata. As showed in Fig.18, the CBM accumulation process (e.g., gas generation, accumulation and expulsion processes) was controlled by five tectonic events in study area, which led to the differential distribution of gas-bearing property at present.
During stage I, the generated gas is mainly composed of biogenic gas (306.5–251 Ma). The amount of gas generated was minimal and almost diffused due to poor storage and preserve conditions.
During stage II (251−203.6 Ma), the organic matter experienced primary thermogenic gas generation and accumulation processes. At this stage, the coal-bearing strata generated gas stably in the whole region and the huge thick strata which covered the coal reservoir provided a good capping condition. The accumulated dissipation and preserved gas content were in range of 13.8–15.8 m3/t and 5.8–6.6 m3/t, respectively (Fig.19(a) and Fig.19(b)). The regional distribution characteristic of accumulated dissipation and gas content was basically consistent with the distribution characteristic of coal reservoir maturity (Fig.14(b)), which indicated a controlling mechanism of gas generation to gas dissipation and preservation.
The third stage is a CBM escaping stage (203.6–145.5 Ma). Influenced by the Late Triassic Indosinian movement (the first tectonic event) the study area was uplifted and denudated, which caused the paleo-geothermal temperature decreased and hydrocarbon generation stopped. Affected by this uplifting mechanism, the gas that generated in the early stage further diffused. During this stage, the amount of gas diffusion was about 0.5–1.5 m3/t (Fig.20(a)). By the end of the Triassic period, the gas content distribution characteristic was basically inherited from the previous period, and the gas content was about 4.5–6.1 m3/t (Fig.20(b)).
In the Early Jurassic, the study area experienced short-term deposition (in the early stage III). The hydrocarbon generation process was still stagnant as a response of relatively low paleo-geothermal. Then, during the Middle and Late Jurassic periods, affected by the Early Yanshanian movement (the second tectonic event), the study area was uplifted and eroded as a result of strong NW-SE compressive stress field. During this stage, the primary CBM generated in the stage II was further diffused. The total escaping amount was 3.0–6.2 m3/t (Fig.21(a)). Controlled by the compressive stress field, a regional anticlinal structural belt and a series of faults were formed in the middle of the region. The gas diffusion intensity in block B3 was stronger than other regions influenced by the development of these tectonic structures (e.g. the region of well J3). At the end of Jurassic, the gas content in the study area was 0–2.6 m3/t, showing a trend of increasing from central region to north-western and south-western (Fig.21(b)). Generally, the tectonic activities resulted in the regional gas escape divergence in this evolution stage, which caused the lower gas content in the central anticline fault zone than the other areas.
After the gas diffusion period (Stage III), the third critical tectonic event occurred in the Middle Yanshanian Orogeny (stage IV: 145–130 Ma).
In this stage, the magma-heat event resulted in an abnormal geothermal field and accelerated the coal maturation process. Hydrocarbon generation and diffusion reached their respective peak value over the entire geological evolutionary history. The heating center was in south-west of research area and radiated from south-west to north-east, and the vitrinite reflectance distribution and gas generation shown the same regularity. The total accumulated gas volume was about 106–265 m
3/t (Fig.17). As the result of regional tensile stress field in this evolution period, the reverse faults that developed in the region of anticlinal-fault structural belt were transformed into normal faults (
Wang et al., 2014). These opening faults system led to a large amount of gas escaped in this region. By the end of the Early Cretaceous, the accumulative amount of gas escaping was about 70–250 m
3/t (Fig.22(a)). The final gas content of the study area was about 18–62 m
3/t by the end of this stage (Fig.22(b)). The gas that preserved in the central anticlinal fault tectonic zone was less than other regions as a result of differential diffusion process.
During the stage V (130–65.5 Ma), the study area was uplifted by the Lvliang Mountains Orogeny (fourth tectonic event in the Late Yanshanian Orogeny), which resulted in the termination of coal thermal maturation and hydrocarbon generation (Stage V in Fig.13). During this stage, the study area inclined to the west under the influence of south-east to north-west compressive stress. The uplifting amplitude gradually decreased from south-east to north-west. A large number of gases that preserved during the early evolution stages escaped, especially in the strong uplifting region. The total gas diffusion was about 6–46 m3/t in this stage (Fig.23(a)). The gas content was 11–35 m3/t at the end of this stage (Fig.23(b)). Significantly, as shown in Fig.23(a), this stage had stronger influence on the gas diffusion process. Influenced by differentiated uplifting, gases in the B1 block were drastically escaped while the gas escape in B2 block was relatively moderate (Fig.23(a)). Besides, because the lower gas content retained in the early stage, the gas diffusion was weaker in B3 block during this stage. In terms of the B4 block, it had higher gas content after this stage (Fig.23(b)) because of its lower escaping amount in stage IV and V.
During the last stage (65.5 Ma–now), the study area continued to be uplifted and eroded affected by the Himalayan tectonic movement, the CBM that preserved in the early stage was further diffused. At about 23 Ma, the fifth critical tectonic event caused a north-east to south-west compressive stress, which was nearly parallel to the axial direction of the reverse fault that formed in the early evolution stages. As a result of this stress field, the properties of some local thrust faults were reversed (e.g., some local thrust faults in the Puxian region) (
Tian et al., 2012). Affected by the development of these open fault system, large quantities of gas were escaped. The gas saturation and gas content decreased rapidly in the east part of the study area (B2 domain, e.g. Well J25), while the gas diffusion in other areas (B1, B3, and B4) was relatively lower (Fig.24(a), Tab.2). In general, the combining effects of basin uplifting and tectonic inversion (the formation of extensional faults) controlled the gas content and dissipation characteristics of reservoirs in this evolution period. The total amount of gas diffusion was about 1.5–15.5 m
3/t in this stage (Fig.24(a)), and the current gas content was about 6–22 m
3/t in the whole region at the end of this stage (Fig.24(b)).
Generally, the coal reservoir formation evolution process is a complex and continuous dynamic changing process, and multi-factors coupling effect finally leads to the distribution diversity of the current gas in coal reservoir. The distribution characteristics of CBM are controlled by hydrocarbon generation, gas preserved condition, buried depth, tectonic activities (i.e. experienced five critical tectonic events) and other geological conditions (e.g., hydrogeological characteristics) in each evolution stage.
5 Conclusions
In this paper, the tectonic subsidence history, geothermal field evolution history and methane accumulation features of Daning-Jixian block were studied by using numerical simulation. The main controlling factors of the current gas content heterogeneous distribution in the study area were clarified. The following three understandings have been formed.
1) The dynamic accumulation process of CBM is mainly controlled by three tectonic activities: the Indosinian Orogeny, the Yanshanian Orogeny and the Himalayan Orogeny.
2) The evolution of CBM reservoir can be divided into six stages: i. shallow-buried biogenic gas stage; ii. the primary thermogenic gas generation stage; iii. The turbulence uplifting and diffusion stage; iv. the secondary thermal gas generation stage; v. monolithic uplifting gas escape stage; vi. structural inversion and further CBM diffusion stage.
3) Five tectonic events were the main reason that controls the complex and continuous dynamic change process of CBM accumulation during different evolution periods, and led to the differential distribution of gas content. i. The first tectonic event (the Late Triassic) led to the cessation of plutonic metamorphism and the dissipation of biogas and primary thermal gas that had been preserved during the early evolution stages. ii. The second tectonic event (the Middle and Late Jurassic) led to the differential escape of primary CBM, especially in B3 block. iii. The third tectonic event involved the Middle Yanshanian magma-heat events (the Early Cretaceous), which dominated the distribution of the secondary CBM generation and gas diffusion combing with the influence of an opening tectonic structural system. iv. The fourth tectonic event controlled differentiated uplifting (the Late Cretaceous) resulted in drastically escape of CBM in B1 block than other tectonic domains. v. The fifth tectonic event, the Neogene Himalayan structural inversion, caused the reversion of some local thrust faults and the low gas content in the B1 block.
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