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Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China

Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case... Pet. Sci. (2015) 12:218–231 DOI 10.1007/s12182-015-0030-0 OR IGINAL PAPER Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China 1 1 1 1 • • • • Cong-Sheng Bian Wen-Zhi Zhao Hong-Jun Wang Zhi-Yong Chen 1 2 1 3 • • • • Ze-Cheng Wang Guang-Di Liu Chang-Yi Zhao Yun-Peng Wang 1 1 1 • • Zhao-Hui Xu Yong-Xin Li Lin Jiang Received: 7 May 2014 / Published online: 22 April 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Tight sand gas is an important unconventional accumulation in the uplift period is found in the gas gas resource occurring widely in different petroleum reservoir analysis of the above two basins. Firstly, natural basins. In coal-bearing formations of the Upper Triassic in gas discharged in the uplift period has a lighter carbon the Sichuan Basin and the Carboniferous and Permian in isotope ratio and lower maturity than that formed in the the Ordos Basin, coal measure strata and tight sandstone burial period, belonging to that generated at the early stage constitute widely distributed source–reservoir assemblages of source rock maturity, and is absorbed and stored in coal and form the basic conditions for the formation of large measure strata. Secondly, physical simulation experiment tight sand gas fields. Similar to most tight gas basins in results at high-temperature and high-salinity inclusions, North America, the Sichuan, and Ordos Basins, all expe- and almost actual geologic conditions confirm that sub- rienced overall moderate uplift and denudation in Meso- stantial gas charging and accumulation occurred in the Cenozoic after earlier deep burial. Coal seam adsorption uplift period of the coal measure strata of the two basins. principles and actual coal sample simulation experiment Diffusive flow is the main mode for gas accumulation in 12 3 results show that in the course of strata uplift, pressure the uplift period, which probably reached 56 9 10 m in drops and desorption occurs in coal measure strata, re- the uplift period of the Xujiahe Formation of the Sichuan sulting in the discharge of substantial free gas. This ac- Basin, compensating for the diffusive loss of gas in the gas counts for 28 %–42 % of total gas expulsion from source reservoirs, and has an important contribution to the for- rocks. At the same time, the free gases formerly stored in mation of large gas fields. The above insight has promoted the pores of coal measure source rocks were also dis- the gas resource extent and potential of the coal measure charged at a large scale due to volumetric expansion re- tight sand uplift area; therefore, we need to reassess the sulting from strata uplift and pressure drop. Based on areas formerly believed unfavorable where the uplift scale experimental data, the gas totally discharged in the uplift is large, so as to get better resource potential and explo- period of Upper Paleozoic in the Ordos Basin, and Upper ration prospects. Triassic Xujiahe Formation in the Sichuan Basin is calcu- 8 3 2 lated as (3–6) 9 10 m /km . Geological evidence for gas Keywords Sichuan Basin  Ordos Basin  Tight sand gas Stratigraphic uplift  Coal measure  Hydrocarbon accumulation mechanism  Diffusion & Cong-Sheng Bian bcs_1981@petrochina.com.cn Research Institute of Petroleum Exploration & Development, 1 Introduction CNPC, Beijing 100083, China College of Geosciences, China University of Petroleum, Tight sand gas resources are widely distributed in the world Beijing 102249, China (Masters 1979; Law 2002; Holditch 2006). Statistics show Guangzhou Institute of Geochemistry, Chinese Academy of that tight sand gas has been discovered or predicted in 70 Sciences, Guangzhou 510640, Guangdong, China basins in North America, Europe, and the Asian-Pacific 12 3 region, with a resource extent of about 210 9 10 m , Edited by Jie Hao 123 Junggar Basin Pet. Sci. (2015) 12:218–231 219 showing huge potential in exploration and development Zhang et al. 2008) and the San Juan Basin (Ayers 2002)in (British Petroleum Company 2012). The United States has the United States, the Ordos and Sichuan Basins in China. the highest annual gas yield from tight sandstone at present As well, in the formation and development history of these 8 3 in the world, reaching 1.75 9 10 m in 2010, about 29 % petroleum basins, the strata were usually deposited and of total gas production of the country (British Petroleum buried, followed by uplifting at a later stage. The study by Company 2012). In China, the Zhongba tight gas field was Tissot and Welte (1984) shows that natural gas generated discovered in the western Sichuan Basin in 1971. With the in gas source rocks due to high temperature in the process progress in exploration technologies in recent years, new of burial was either discharged, migrated, and accumulated discoveries are continually being made. Two major tight in the reservoirs to form gas pools or was retained in the gas provinces in the Sichuan and Ordos Basins and five gas source rocks to form shale gas and coalbed methane. breakthrough areas in the Kuche deep zone of the Tarim There are limited publications at home and abroad re- Basin have been found so far (Fig. 1), with technically garding natural gas migration and accumulation under recoverable gas resources of 8–11 TCM, and annual tight uplift tectonic settings (Tian et al. 2007). It is usually be- gas yield more than 25.6 BCM in 2011 (Li et al. 2012; Zou lieved that strata denudation and faulting are likely to occur et al. 2012). Unconventional gas (i.e., tight gas and shale in the course of uplifting, resulting in damage to gas gas) is increasingly becoming one of the dominant re- reservoirs and dissipation of the gas (Hao et al. 1995; sources in the global natural gas industry, and more efforts Zhang et al. 1999). Based on studies of tight sand gas are being put into tight gas exploration in many countries accumulation in coal-bearing strata in recent years, we (Kuuskraa and Bank 2003; Smith et al. 2009; Zeng 2010; have found that tight gas from coal measure strata not only Dai et al. 2012; He et al. 2013). can form gas reservoirs in the course of deposition and Statistics show that tight gas resources worldwide burial, but also can accumulate in the moderate uplift mostly originate from coal measure source rocks (Dai et al. process if the regional seal has not been destroyed (Bian 2012), e.g., the Piceance Basin (Johnson and Rice 1990; et al. 2009; Zhao et al. 2010). This paper mainly takes gas 70°E 90° 110° 130° 0 1000km 40°N Songliao Basin Tarim Basin Tuha Basin 30° Bohai Bay Basin Ordos Basin Sichuan Basin 20° Tight gas fields name˖ 1.Dabei 2.Sulige 3.Wushenqi Major tight-gas bearing basin 4.Yulin 5.Zizhou 6.Guang'an 7.Anyue 8.Hechuan 9.Bajiaochang Tight-gas Tight gas field South China bearing area Sea Islands Fig. 1 Major tight sand gas basins in China 123 220 Pet. Sci. (2015) 12:218–231 fields of the Upper Triassic Xujiahe Formation in the Formations and the Upper Triassic Xu2, Xu4, and Xu6 Sichuan Basin and the Upper Paleozoic in the Ordos Basin Member tight sands, respectively (Zhang et al. 2009a), with as examples to discuss this geological process. porosities of 4 %–10 %, permeabilities of 0.01–1 mD and thicknesses of hundreds to a thousand meters. They closely and widely contact the source rocks, comprising a very 2 Geological setting favorable source–reservoir assemblage horizontally, which has laid the foundation for widespread gas accumulation in Several large gas fields with reserves more than 100 BCM the reservoirs and then the discovery of large scale reserves have been successively discovered in the Upper Paleozoic (Figs. 2, 3). Permo-Carboniferous in the Ordos Basin and the Upper Stratigraphic burial history shows that the tight sand gas Triassic Xujiahe Formation in the central Sichuan Basin zones in both the Ordos Basin and the Sichuan Basin all (Dai et al. 2012; Zhao et al. 2013) in recent years. By the experienced deep burial before the Late Cretaceous (Zhao end of 2011, large gas fields like Sulige, Wushenqi, and et al. 2005, 2010), with maximum burial depths up to Yulin had been discovered in Upper Paleozoic of the Ordos 3500–4000 m and 4500–6000 m, respectively. From the Basin, with proved gas reserves (including basically proved Late Cretaceous to this day, overall tectonic uplift and 12 3 reserves) approaching 3.0 9 10 m and gas-bearing area strata denudation occurred in these two basins, and the 4 2 about 2.0 9 10 km . Large gas fields like Guang’an, hydrocarbon generation of the source rocks stopped. The Hechuan, and Anyue had been discovered in the Upper denuded thickness is 800–1500 and 1500–2500 m, re- Triassic Xujiahe Formation of the Sichuan Basin, with spectively, and the buried depth of tight gas zones of these 8 3 proved gas reserves approaching 6000 9 10 m and gas- two basins is 1800–4000 m at present. Fortunately, uplift bearing area of more than 3000 km . Tight sand gas re- and denudation have not resulted in the damage to the sources of the two basins account for more than two-thirds regional seal. of the present total tight gas resources in China (Dai et al. 2012). A set of continental clastic coal-bearing formations 3 Theoretical model and experiment covering the whole basin is developed in the Upper Pa- leozoic of the Ordos Basin and the Xujiahe Formation of Substantial gas infusion is a prerequisite for gas accumu- the Sichuan Basin, respectively, and the source rocks lation in the uplift period of tight sand formation. Coal mainly consist of coal seams and coaly mudstone with high petrography (Wang et al. 1995; Zhang et al. 2000) showed gas generation potential. The Upper Paleozoic source rocks that microfractures and cleats are well developed in coals, of the Ordos Basin are distributed in the Carboniferous especially in high-rank coals. Substantial free gas was Taiyuan Formation and Permian Shanxi Formation (Zhang stored in pores and microfractures at the time of substantial et al. 2009b), with coal seam thicknesses of 10–25 m on gas generating from coal seams in the burial period. This average, 40 m locally, almost covering the whole basin expanded volumetrically due to the pressure drop in the stably. Coal measure mudstones have thicknesses of uplift period, and thus together with the desorbed and 60–130 m, almost the same distribution as the coal seams, liberated coal seam gas, became the important gas source and are usually 200-m thick in the west. The Upper Tri- for gas accumulation in the uplift period (Cui et al. 2005; assic Xujiahe Formation source rocks of the Sichuan Basin Zhao et al. 2010). Starting from the discussion of a theo- are distributed in the Xu1, Xu3, and Xu5 Members, with retical model of coal seam adsorption–desorption, by coal seam thicknesses of 5–15 m on average, distributed means of thermal simulation and gas accumulation stably horizontally. Coal measure mudstones have thick- simulation experiments of actual coal samples, the geolo- nesses of 100–800 m, thinning out from the western to the gical process of gas accumulation in the uplift period of central Sichuan Basin, basically covering the whole basin coal measure strata is demonstrated. (Zhao et al. 2011). Thermal evolution history shows that substantial gas generation and expulsion occurred in the 3.1 Theory of adsorption of gas in coal seams source rocks of the two basins in the geological history. and the experimental model The thermal evolution of source rocks has been at highly mature and overmature stages up to now, with cumulative Adsorption refers to the attachment of atoms or molecules 8 3 2 gas generating strength of (20–40) 9 10 m /km and of one substance on the surface of another substance 8 3 2 (40–100) 9 10 m /km , respectively. This can provide (Busch et al. 2003; Bae and Bhatia 2006). The adsorption abundant gas sources for large-scale gas accumulation in behavior of coal was observed in coal mining in the middle the tight sand reservoirs of the two large gas provinces. of the last century. After having successfully produced Their reservoirs are the Permian Shanxi and Shihezi coalbed methane, the United States made an in-depth study 123 Shan1-2 He5-8 Member Formation Upper Shihezi Lower Shihezi Shanxi Taiyuan Series Permian Carboniferous Xu6 Xu5 Xu4 Xu3 Xu2 Xu1 Xujiahe Upper Triassic He1-4 Tai1-2 Pet. Sci. (2015) 12:218–231 221 St rat um Source rock 0 40 80km GR Lithology RT andgas reservoir Hangjinqi Dongsheng Etuokeqi Shenmu Wushenqi Sulige Yulin Etuokeqianqi Zizhou Dingbian Jingbian Wuqi Ansai Yan’an Huanxian Fuxian Qingyang Huanglong Tongchuan Gas generation strength, Effective sandbody Gas reservoir of 8 3 2 10 m /km thickness, m the upper Paleozoic Fig. 2 Composite Upper Paleozoic stratigraphic section and coalbeds and He8 reservoirs, Ordos Basin Source rock Stratum GR Lithology RT and gas Series Form.Member reservoir Jurassic Zhenzhuchong Jiange Tongjiang 025 50km Yilong Mianzhu Nanchong Liangping Chengdu Guang'an Ya'an Shizhu Hechuan Zizhong Chongqing Leshan Gas reservoir Yibin Reservoir thickness of Xu4 Member, m 8 3 2 Gas generation intensity in Xu3 and Xu5 Members, 10 m /km Fig. 3 Xujiahe stratigraphic section showing gas source rocks and reservoirs, Sichuan Basin 5 222 Pet. Sci. (2015) 12:218–231 on the adsorptive action of coal seams (Radovic et al. 1997; measured experimentally represents the maximum value Clarkson and Bustin 1999). Coal is universely believed at and then declines, i.e., there is a large difference between present to be a porous medium with a big surface area. The the measured apparent adsorption capacity and the absolute adsorption of gas by coal is a physical adsorption process; adsorptive capacity. To help solve this problem, some the adsorption energy (heat) is small, the adsorption rate is correction models for calculating the adsorptive capacity fast, and the adsorption and desorption procedure is re- are introduced (Haydel and Kobayashi 1967; Murata et al. versible (Radovic et al. 1997; Clarkson and Bustin 1999). 2001), and a relatively common method is as follows: Therefore, the adsorptive force of coal to gas molecules is N ¼ N ð1  q =qÞð2Þ ab ap free ad intermolecular force, i.e., there are both adsorption equi- where N represents absolute adsorptive capacity, N is librium and adsorption heat (energy). Previous studies ab ap excessive adsorptive capacity or apparent adsorptive ca- (Mavor et al. 1990; Mukhopadhyay and Macdonald 1997; pacity, q is free gas density under equilibrium condi- Crosdale et al. 1998) show that the adsorption by coal free tions, and q is the adsorbed gas density. seams is affected by many factors like coal composition, ad Because of equipment limitations, most coal adsorption temperature and pressure, gas properties, and coal rank. experiments are conducted at temperatures less than 40 C Under geological conditions, with the variation of buried and pressures less than 15 MPa, and the relation between depth of coal seams, the impact of temperature and pres- pressure and adsorptive capacity is basically measured at sure on it is maximum. Experimental results show that one temperature. There are few experimental studies with pressure is proportional to adsorptive capacity, whereas the both temperature and pressure varying. Cui et al. (2005) influence of temperature is the reverse (Zhao et al. 2001). derive a characteristic curve for coal-adsorbed methane Many researchers (Anderson et al. 1966; Ruppel et al. based on adsorption potential theory and adsorption ex- 1974; Yang and Saunders 1985) have conducted many the- periments under high temperature and high pressure and oretical and experimental studies on the adsorptive action of believe that this characteristic curve has uniqueness, i.e., coal seams and established many theoretical models and there is only a unique peak value in adsorptive capacity, mathematical expressions related to adsorptive capacity. based on which a new adsorption model of coal is derived These include the monomolecular layer adsorption model and can be used to estimate the adsorptive capacity at and Langmuir’s equation, multimolecular layer adsorption different temperature–pressure conditions. model and BET’s equation, Freundlieh’s equation, Polo- myi’s adsorption potential theory, micropore filling theory, lnV ¼ AT½2:7lnT  lnP  12:6603þ B ð3Þ and Dubinin-Astak-hov’s equation (Sang et al. 2005; Su et al. where A and B stand for adsorption constant and can be 2008). However, it is difficult to accurately describe the obtained from experimental adsorptive data of coal at a adsorptive properties of coal using a certain isothermal ad- given temperature. The above equation can be used to sorption line or theoretical model due to the presence of calculate the adsorptive capacity of coal at any temperature micropores in coal. It is generally believed that gas is ad- and pressure. sorbed on the surface of coal in a monomolecular state under geological conditions; therefore, a monomolecular layer 3.2 Simulation experiment of hydrocarbon adsorption model and Langmuir’s equation are widely used expulsion due to coal seam uplift and pressure to describe the adsorptive features of coal and have become a drop classical theory maturely used in the development of coalbed methane for the moment. The model is described as follows: For the sake of understanding gas desorption due to tem- V  P perature and pressure drop under approximately real geo- V ¼ ð1Þ P þ P logical conditions, a specially designed autoclave was used to simulate the gas discharge process of coal seams at the where V represents the adsorptive capacity of coal, V is the Langmuir volume, P is Langmuir pressure, and P is time of dropping of temperature and pressure. Two and five sets of samples were selected from the Upper Triassic pressure. Xujiahe Formation of the Sichuan Basin and the Upper With increasing research, it is found that under subsur- Paleozoic of the Ordos Basin, repectively, to conduct the face high temperature and pressure conditions (pressure experiments. The deep burial and gas generation process of [15 MPa, temperature[80 C) (Gregory and Karen 1986; coal seams was simulated by external heating and pres- Mavor et al. 1990), multimolecular layer adsorption of gas surizing; when gas generation and expulsion reached occurs in coal seams. As well, methane becomes a super- equilibrium in the autoclave, the temperature and pressure critical gas under high pressure, and its density increases were reduced, and the gas discharge from coal was ob- significantly, resulting in a change of adsorptive capacity. This results in the phenomenon that the adsorptive capacity served. The experimental results are listed in Table 1 and 123 Pet. Sci. (2015) 12:218–231 223 Table 1 Gas expulsion of coals under different temperature and pressure conditions Coal sample Process Temperature, Pressure, Gas Total gas Staged gas Staged C MPa generation generation generation gas generation and expulsion, and expulsion and expulsion and expulsion mL rate, mL/g rate, mL/g ratio, % Xu3 member Heating and pressurizing 420 105 75 48.8 32.7 67 Cooling and depressurizing 320 50 37 16.1 33 Xu6 member Heating and pressurizing 420 105 575 40 28.8 72 Cooling and depressurizing 320 50 225 11.3 28 Liaohe lignite Heating and pressurizing 450 104 475 19.4 11.9 61.3 Cooling and depressurizing 350 60 300 7.5 38.7 Xianfeng lignite Heating and pressurizing 450 6 850 73.3 42.5 58 Cooling and depressurizing 350 2 615 30.8 42 Xianfeng lignite Heating and pressurizing 450 8.2 950 79.5 47.5 59.7 Cooling and depressurizing 350 4 640 32.0 40.3 Taiyuan Formation Heating and pressurizing 450 4.1 826 60.8 41.3 67.9 Cooling and depressurizing 350 2.4 390 19.5 32.1 Shanxi Formation Heating and pressurizing 450 4.4 975 70.3 48.8 69.4 Cooling and depressurizing 350 1 430 21.5 30.6 show that when the temperature and pressure of coals taken natural gas in the formation was approximated by pure from the Xujiahe Formation are reduced from 420 C and methane. A 3D high-temperature and high-pressure phy- 105 MPa to 320 C and 50 MPa, respectively (corre- sical simulation device independently designed by RIPED sponding to uplifting of formation from 4000 to 2000 m), was used for experiment. the desorbed and discharged gas is 11–16 mL/g, account- In the experiment (Table 2), the model was evacuated, ing for 28 %–33 % of total gas expulsion of coals. When then water was injected, and overburden pressure was ap- the temperature and pressure of coals from the Upper Pa- plied to simulate the water discharge process of formations leozoic of the Ordos Basin are reduced from 450 C and due to burial and compaction. When the overburden pres- 104–4 MPa to 350 C and 60–1 MPa, respectively (cor- sure was increased to 7 MPa, the fluid pressure reached responding to uplifting the formation from 4000–6000 m to 1.63 MPa at 60 C, and the formation was saturated with less than 3000 m), the desorbed and discharged gas is water and was in a near-equilibrium state. Then, methane 7.5–32 mL/g, accounting for 31 %–42 % of total gas ex- was injected into the bottom of the model to simulate the pulsion, showing that quite a lot of gas is desorbed and gas generation process. In the course of gas injection, water discharged during the temperature and pressure decline was continually discharged from the outlet. When the ex- process of coal seams in the uplift period. periment had been conducted for about 20 h, gas started to appear at the outlet, showing that after the gas source rock 3.3 Physical simulation of gas accumulation had been saturated with adsorbed gas, free gas started to migrate. When the gas flowing out of the outlet reached Physical simulation is an important method to study and 10 mL/min and the water yield decreased significantly, we reproduce geological processes (Zeng and Jin 2002; Zhao stopped the gas injection. This simulates the uplift and et al. 2006). To better study the gas accumulation process pressure drop process, and the gas desorption and gas ex- of coal measure tight sand formations in the uplift period, pulsion processes were observed. To better replicate the an experiment was conducted to simulate gas migration subsurface environment, we stopped any operation for 24 h and accumulation under strata uplift and pressure drop after having ceased gas injection, to ensure the equilibra- settings. To prove the existence of gas desorption and ex- tion of gas filling and adsorption. This simulates the ces- pulsion from coal measure formations in its uplift and sation of gas generation after the source rock reached its pressure drop, a physical simulation experiment model was maximum buried depth. Subsequently, the overburden designed based on the configuration relation of source pressure, fluid pressure, and temperature of the model were rocks and reservoirs of the Upper Triassic Xujiahe For- reduced, and at 175–182 h later, gas flowed out of the mation in the Sichuan Basin. Coal samples were crushed to outlet in an episodic type. This process corresponds to that 150–200 mesh, sandstone was replaced by glass beads, and after the formation had changed from burial to uplift and 123 Pressure measuring point Gas cylinder 224 Pet. Sci. (2015) 12:218–231 Table 2 Experiment process and phenomena Step Process and phenomena Step Process and phenomena 1 Installed model under dry and water free conditions 7 Increased injection pressure, gas started to appear at outlet 2 Evacuated for 12 h 8 When the outflow rate of methane reached 10 mL/min, the injection pressure was 2 MPa, closed the gas inlet 3 Slowly injected water into model from the bottom until 9 The gas outflow rate gradually decreased below 0.1 mL/min, and the the internal pressure is at 1 MPa uniformly fluid pressure decreased gradually too 4 Allowed fluid pressure to be 1.63 MPa, increased 10 When no further gas flowed out of the outlet, stood still for 24 h overburden pressure to 7 MPa, and heated the model to 60 C 5 Injected methane at pressure more than 1.63 MPa, water 11 Gradually reduced the overburden pressure to 2 MPa, the fluid pressure flow rate at outlet increased gradually did not change; reduced temperature to 30 C, recorded the gas flow rate at outlet 6 Water flow rate decreased gradually after having been 12 Gradually reduced the fluid pressure of model to 1.63 MPa, the increased to 40 mL/h overburden pressure and temperature did not change, recorded the gas flow rate at outlet (a) Relief valve Back pressure valve Gas collecting facility c c Metering Pressure system sensor A: Coarse sandstone Gas entry B: Coal C: Carbonaceous mudstone Pressure sensor Gas meter D: Mudstone Reduce overburden Reduce fluid 40 8 (b) pressure pressure Gas injection 30 6 Water producing rate Gas producing rate Overburden pressure Fluid pressure 20 4 10 2 0 0 0 60 120 180 200 Time, h Fig. 4 Experimental device (a) and results (b) of simulating gas migration and accumulation in uplift and pressure drop environment denudation when the temperature and pressure of the for- highly compressed in deep strata suffered from desorption mation fluid had dropped. Desorption occurred in the coal and bulk expansion due to pressure drop in the course of measure formation, gas was liberated, and substantial gas uplift. This generated power to force the gas to escape migrated (Fig. 4a, b). outward from the source rocks, and the gas can still largely migrate and accumulate. The existence of this process can enlarge the gas exploration realm to the ‘‘poor’’ gas accu- 4 Experimental results and analysis mulation area, i.e., gas reservoirs can still be found in the uplift area where gas reservoirs were formerly not believed The above experiment shows that under uplift settings, to develop. although the gas generation process of gas source rocks had The gas discharged from different thickness coal seams stopped, the substantial gas adsorbed on the particle sur- in the two basins due to temperature and pressure drop in faces inside the source rocks and free in the pores and the course of moderate uplift at late stage can be Volume, mL Pressure, MPa 2 Pet. Sci. (2015) 12:218–231 225 obtained from about 300 wells in the two basins, as shown 0 40 80 km in Figs. 5 and 6. Observed from these two figures, the gas Hangjinqi Dongsheng expulsion area formed by pressure drop and desorption of coal seams of both the Ordos Basin and the Sichuan Basin Etuokeqi 4 2 in the uplift period can reach (15–18) 9 10 km . The gas Shenmu 8 3 2 expulsion strength is basically (2–8) 9 10 m /km , and Wushenqi Sulige the high value areas have corresponded well to the dis- Yulin Etuokeqianqi covered gas fields, showing that the uplift period has pro- vided important gas supply for large-scale gas Dingbian Jingbian Zizhou accumulation in the two large gas provinces. Wuqi Ansai Yan’an Huanxian 5 Geological evidence Fuxian Qingyang 5.1 Fluid inclusions Huanglong Fluid inclusions provide important means to study the gas Tongchuan accumulation process (Wang and Tian 2000; Rossi et al. Gas reservoir of upper Paleozoic 2002;Lu 2005). The study of inclusions from the Xujiahe Gas release strength in uplifting period Formation reservoir in the central Sichuan Basin (Fig. 7; Table 3) showed that two stages of hydrocarbon inclusions Fig. 5 Gas release strength in the uplift period of the Upper 8 3 2 Paleozoic, Ordos Basin (unit: 910 m /km ) can be clearly identified based on their occurrence and fluorescent display characteristics. The first stage of hy- quantitatively estimated based on the simulation ex- drocarbon inclusions was developed in the early stage of periment results (Table 1) and the coal seam adsorption– quartz overgrowth, mainly occurring at the inner side of desorption equations (Eqs. 2, 3). The strength of desorption quartz overgrowth or along micro-fracture planes in the and discharge of gas from coal seams in the uplift period is early diagenesis of quartz grains, orange-red or light obtained based on calculation and statistics of data brownish yellow in fluorescent light (upper part of Fig. 7), 025 50 km Jiange Chengkou Tongjiang Yilong Mianzhu Santai Nanchong Liangping Chengdu Guang'an Hechuan Shizhu Ya'an Dazu Chongqing Leshan Yibin Gas reservoir Gas release strength in uplifting period, 8 3 2 10 m /km Fig. 6 Gas release strength in the uplift period of the Xujiahe coal measures and carbonaceous mudstones 5 226 Pet. Sci. (2015) 12:218–231 Fig. 7 Occurrence & fluorescent light features for fluid inclusions in Xu 2 Formation, Penglai area, Central Sichuan Basin. a/b-fluid inclusions in the inner side of quartz overgrowth or the micro-fissures, formed at early stage of diagenesis, orange-red or light brown in fluorescent light; c/ d-fluid inclusions distributed in belts along micro-fissures cutting quartz grains, formed at the late stage of diagenesis, light blue or light green in fluorescent light indicating that earlier heavy hydrocarbon exists in the in- the source rocks started to become mature, generate sub- clusions, and the homogenization temperature peak of the stantial gas and enter the reservoir stage. The salinity of associated brine inclusions is at 85–95 C (Table 3). The high-temperature inclusions is higher (16 %–22 %) and second stage hydrocarbon inclusions were developed after should be the record of the middle and late stages of the quartz overgrowth. They are mainly distributed in belts diagenesis when the salinity increased with the substantial along post-diagenesis micro-fissures cutting quartz grains discharge of formation water, and the organic matter was and are strong light blue and light bluish green in discharged at mature and highly mature stages. The age of fluorescent light (lower part of Fig. 7). Laser Raman the former probably corresponds to the period before and spectra showed that the main components of these inclu- after the end of the Jurassic and that of the latter corre- sions are methane and higher hydrocarbons, and the ho- sponds to the uplift period occurring at the end of Creta- mogenization temperature peak of the associated brine ceous. It is discovered by studying the gas generation and inclusions is between 110 and 130 C (Table 3). This expulsion history of Xujiahe Formation in central Sichuan indicates that two stages of gas accumulation took place in Basin that there are a total of two stages of gas expulsion, the central Sichuan Basin. It is discovered by further ana- migration, and accumulation: one stage occurred earlier, lysis (Table 3) that the salinity of low temperature inclu- corresponding to the substantial gas generation period of sions is lower (2 %–13 %), reflecting that the salinity of gas source rocks, and the other stage occurred later, cor- formation water was lower in this period, and they are the responding to tectonic uplift period. The two stages of gas products of the early and middle stages of diagenesis when accumulation are both characterized by large-scale 123 Pet. Sci. (2015) 12:218–231 227 Table 3 Occurrence and test data of fluid inclusions from Xu2 Member, Penglai area Occurrence in mineral deposit Distribution Type of fluid Size, Gas Single Homogenization Salinity, pattern of inclusions lm liquid phase temperature, C wt%NaCl inclusions ratio, % At dust lane and Zonal Hydrocarbon bearing 3 9 5 B5 Liquid 89 2.90 inside of quartz brine inclusions Zonal 4 9 7 B5 Liquid 90 3.06 overgrowth Zonal 6 9 9 B5 Liquid 92 3.06 Zonal 3 9 3 B5 Liquid 90 13.7 Zonal 2 9 6 B5 Liquid 89 13.6 Zonal 3 9 4 B5 Liquid 92 13.6 Zonal 4 9 4 B5 Liquid 90 13.7 Zonal and lineal 10 9 12 B5 Liquid 95 4.96 Zonal and lineal 2 9 12 B5 Liquid 94 5.71 Zonal 26 9 15 B5 Liquid 92 6.74 Zonal 15 9 16 B5 Liquid 92 6.88 Zonal 6 9 8 B5 Liquid 93 6.88 Along micro-fissures cutting Zonal Hydrocarbon 2 9 6 B5 Liquid 116 20.2 quartz grains and overgrowth, bearing brine Zonal 3 9 6 B5 Liquid 118 20.2 formed at late stage inclusions Zonal 1 9 7 B5 Liquid 125 20.2 of diagenesis Zonal 1 9 4 B5 Liquid 125 20.2 Zonal 3 9 7 B5 Liquid 117 20.1 Zonal and lineal 10 9 15 B5 Liquid 128 16.9 Zonal and lineal 2 9 3 B5 Liquid 126 17.0 Zonal and lineal 4 9 6 B5 Liquid 130 17.0 Zonal and lineal 3 9 4 B5 Liquid 129 16.9 Zonal 3 9 4 B5 Liquid 115 22.4 Zonal 4 9 5 B5 Liquid 119 20.0 Zonal 2 9 3 B5 Liquid 116 20.0 Zonal 2 9 10 B5 Liquid 124 20.1 Zonal 5 9 10 B5 Liquid 129 20.1 accumulation, the former resulted from gentle structures in Carboniferous coal seams in the Sulige region exceeds the central Sichuan Basin, whereas the latter resulted from 2.0 %; however, the gas in some regions is wetter, with the overall tectonic uplift (Zhao et al. 2010) (Fig. 8). C /(C –C ) coefficient being up to 86 %, and the carbon 1 1 5 isotope ratio of methane is lighter, -29.96 to -36.45 %, 5.2 Geochemical features showing that the gas accumulated in the Upper Paleozoic gas field mainly originates from adsorbed gas desorption and Experimental results show that with the decline of pressure, free gas expansion in the source kitchen in the uplift period, the carbon isotope ratios of hydrocarbon gas discharged from whereas the gas adsorbed inside source rocks mainly comes coal seams become lighter and lighter, and the C /(C –C ) from the early and middle mature stage of source rocks, 1 1 5 coefficient becomes lower and lower. The gas generated at having lower maturity. Therefore, it is normal for it to be the late stage of formation burial is discharged first, having different from the current maturity of the source rocks. higher maturity, whereas the gas adsorbed in coal seams at Moreover, such carbon isotope lightening gradually be- early stage of formation burial is discharged last, having comes apparent from the Sulige gas field in the west to the lower maturity (Table 4). The geochemical features of gas in Yulin gas field in the east of the Ordos Basin, which is sig- the western Ordos Basin show that (Dai et al. 2005) the nificantly related to the fact that the strata uplift gradually composition and carbon isotope ratios do not match the increased from 800 m in the west to 1400 m in the east at the maturity of the coal measure source rocks. The lower carbon late stage of the basin tectonics. This is because within a isotope ratio of methane and the lower C /(C –C ) coeffi- certain range, the larger the uplift, the more gas is desorbed 1 1 5 cient constitute a contradiction with the higher maturity of from the coal measure, resulting in a large proportion of gas source rocks. For instance, the maturity (R %) of Permo- accumulated in the gas field in the uplift period (Fig. 9). 123 228 Pet. Sci. (2015) 12:218–231 Hechuan WE Moxi Penglai Source rock (Caprock) Reservoir End of the middle Jurassic Source rock Source rock (Caprock) Reservoir End of the late Cretaceous Source rock Source rock (Caprock) Reservoir Himalayan Period Mudstone Coal Sandstone Gas reservoir Source rock Water layer Fault Fracture Migration direction Fig. 8 Gas accumulation model of the Xujiahe Formation in the central Sichuan Basin Table 4 Composition and carbon isotope of gas discharged by coal under different temperature and pressure conditions 13 13 Coal sample Process Temperature, Gas yield, Gas, C , C , C /(C –C ) d C , % d C , % 1 2? 1 1 5 1 2 C mL/g % % % coefficient Liaohe lignite Heating and pressurizing 450 19.4 6.9 3.7 3.2 0.6 -34.4 -25.7 Cooling and depressurizing 350 17.5 8.7 8.8 0.5 -40.6 -29.9 Xianfeng lignite Heating and pressurizing 450 79.5 40.1 22.8 17.3 0.6 -31.8 -26.0 Cooling and depressurizing 350 9.2 6.2 3 0.7 -34.5 -27.4 Taiyuan Formation Heating and pressurizing 450 60.8 72.7 54.6 18.2 0.8 -30.1 -24.5 Cooling and depressurizing 350 68.2 48.9 19.3 0.7 -34.6 -24.6 Shanxi Formation Heating and pressurizing 450 70.25 76.3 61.6 14.7 0.8 -31.5 -23.0 Cooling and depressurizing 350 71 59.4 11.6 0.8 -33.4 -23.8 This also suggests that the composition and carbon carbon isotope ratio of methane should be -32 to isotope ratios of the Xujiahe Formation gas in the central -36 %. In addition, the carbon isotopes of ethane and Sichuan Basin are inconsistent with the maturity of the propane are also characterized by lightening, only the source rocks. For instance, the carbon isotope ratio of the lightening amplitude decreases gradually. The ex- Xu2 Member methane in the Hechuan region is lighter perimental results show that such gas with low maturity (-39 to -42 %), but the maturity R of the lower coal was possibly formed and stored in the coal measure measure source rocks is 1.1–1.3, belonging to the sub- source rocks at the early stage, but it was discharged, stantial gas generation stage of coal measure source became free gas, and accumulated due to tectonic uplift rocks. Based on the statistical data from Dai (1992), the at the late stage (Fig. 9). 123 Pet. Sci. (2015) 12:218–231 229 7 3 2 -50 flow reached 0.8 9 10 m /(km Ma), and the charge 12 3 volume approached 56 9 10 m , whereas the bulk flow charge of gas mainly occurred in the burial period of the 12 3 basin, with a charge volume of 127 9 10 m , and in the -40 + + whole gas generation and accumulation history, the gas ++ 12 3 + diffusion loss volume was estimated to be 135 9 10 m . + + As a result, the gas diffusion loss cannot be compensated + + only by bulk flow charge, and it is hard to form the dis- covered Xujiahe Formation TCM scale gas field. There- -30 fore, the diffusion charge in the uplift period of formation effectively compensates for the diffusion loss of gas and CH contributes more to the efficient gas accumulation and Coal gas data counted by Dai (1992 preservation of large tight sand gas fields. -20 The concept of gas accumulation in the uplift period has Data from Sulige gas field important theoretical and practical significances. Firstly, it Data from Yulin gas field breaks the conventional view that the uplift period is un- Data from Xujiahe gas field favorable for gas accumulation, and secondly it promotes -10 the gas resource extent and potential of the tectonic uplift 0.40 0.60 0.80 1.00 1.30 1.50 2.00 3.00 4.00 5.00 area. On this basis, many large uplift areas formerly be- R , % lieved to be unfavorable for exploration are reassessed, and Fig. 9 Carbon isotope ratio versus R of Xujiahe Formation gas in o the area of favorable gas exploration provinces has been the Sichuan Basin and upper Paleozoic gas in the Ordos Basin and increased. typical coal-formed gas 6 Main gas accumulation pattern and geological 7 Discussion significance of uplift period accumulation On the basis of analyzing the coal seam adsorbed and It was generally believed that diffusion was one of the desorbed gas model, this paper uses a great deal of thermal major factors damaging gas reservoirs, and studies show simulation and physical simulation experiments to that diffusion is an important mode for gas to migrate with demonstrate the geological process of gas accumulation in molecular motion (Nelson and Simmons 1992; Lu and the uplift period of coal-bearing formations. As well, a lot Connell 2007; Lu et al. 2008; Korrani et al. 2012). With of fluid inclusions and geochemical evidence are available increasing study of low porosity and permeability reservoir for the analysis of real gas fields, showing that recognition gas, especially on tight gas, diffusion migration is believed of gas accumulation due to pressure drop and desorption as an important way for the gas to migrate and accumulate from coal-bearing formations in the uplift period has im- in tight reservoirs (Liu et al. 2012; Wang et al. 2014). (Liu portant theoretical and practical bases and is of great sig- et al. 2012) point out that the bulk flow under source– nificance to promoting the resource potential of coal- reservoir pressure differentials and the diffusion flow under bearing strata uplift areas. However, the high-pressure hydrocarbon concentration differential are two important ([20 MPa) coal seam adsorbed and desorbed gas mod- ways for gas migration and infusion. Experiments and el needs further study, for the adsorption state of methane geological analysis show that at the stage of strata uplift, at high pressure has possibly changed. The ordinary coal- gas generation in source rocks stopped, and the source– bed methane adsorption and desorption simulation ex- reservoir pressure differential dropped gradually. The periments are mainly undertaken at pressures of less than combined action of desorption of coal seam adsorbed gas 20 MPa, and it is important to conduct higher pressure and expansion of free gas in the original pores of the coal (20–40 MPa) adsorption–desorption gas tests in the future. seams significantly increased the gas concentration inside the source rocks and provided power for the gas to diffuse and migrate from source rocks to reservoirs. 8 Conclusions On the basis of geological analysis of the Xujiahe For- mation gas reservoirs in the Sichuan Basin, we calculated (1) The adsorption–desorption principle of coal seam bulk flow charge, diffusion flow charge, and diffusion loss gas emission and simulation experiment results in tight sand gas reservoirs. The results show that in the confirms that pressure drop and desorption occur in strata uplift period, the average charge rate of diffusion the uplift process of coal measure formation, and the δ C (PDB), ‰ 230 Pet. Sci. 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Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China

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Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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Pet. Sci. (2015) 12:218–231 DOI 10.1007/s12182-015-0030-0 OR IGINAL PAPER Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China 1 1 1 1 • • • • Cong-Sheng Bian Wen-Zhi Zhao Hong-Jun Wang Zhi-Yong Chen 1 2 1 3 • • • • Ze-Cheng Wang Guang-Di Liu Chang-Yi Zhao Yun-Peng Wang 1 1 1 • • Zhao-Hui Xu Yong-Xin Li Lin Jiang Received: 7 May 2014 / Published online: 22 April 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Tight sand gas is an important unconventional accumulation in the uplift period is found in the gas gas resource occurring widely in different petroleum reservoir analysis of the above two basins. Firstly, natural basins. In coal-bearing formations of the Upper Triassic in gas discharged in the uplift period has a lighter carbon the Sichuan Basin and the Carboniferous and Permian in isotope ratio and lower maturity than that formed in the the Ordos Basin, coal measure strata and tight sandstone burial period, belonging to that generated at the early stage constitute widely distributed source–reservoir assemblages of source rock maturity, and is absorbed and stored in coal and form the basic conditions for the formation of large measure strata. Secondly, physical simulation experiment tight sand gas fields. Similar to most tight gas basins in results at high-temperature and high-salinity inclusions, North America, the Sichuan, and Ordos Basins, all expe- and almost actual geologic conditions confirm that sub- rienced overall moderate uplift and denudation in Meso- stantial gas charging and accumulation occurred in the Cenozoic after earlier deep burial. Coal seam adsorption uplift period of the coal measure strata of the two basins. principles and actual coal sample simulation experiment Diffusive flow is the main mode for gas accumulation in 12 3 results show that in the course of strata uplift, pressure the uplift period, which probably reached 56 9 10 m in drops and desorption occurs in coal measure strata, re- the uplift period of the Xujiahe Formation of the Sichuan sulting in the discharge of substantial free gas. This ac- Basin, compensating for the diffusive loss of gas in the gas counts for 28 %–42 % of total gas expulsion from source reservoirs, and has an important contribution to the for- rocks. At the same time, the free gases formerly stored in mation of large gas fields. The above insight has promoted the pores of coal measure source rocks were also dis- the gas resource extent and potential of the coal measure charged at a large scale due to volumetric expansion re- tight sand uplift area; therefore, we need to reassess the sulting from strata uplift and pressure drop. Based on areas formerly believed unfavorable where the uplift scale experimental data, the gas totally discharged in the uplift is large, so as to get better resource potential and explo- period of Upper Paleozoic in the Ordos Basin, and Upper ration prospects. Triassic Xujiahe Formation in the Sichuan Basin is calcu- 8 3 2 lated as (3–6) 9 10 m /km . Geological evidence for gas Keywords Sichuan Basin  Ordos Basin  Tight sand gas Stratigraphic uplift  Coal measure  Hydrocarbon accumulation mechanism  Diffusion & Cong-Sheng Bian bcs_1981@petrochina.com.cn Research Institute of Petroleum Exploration & Development, 1 Introduction CNPC, Beijing 100083, China College of Geosciences, China University of Petroleum, Tight sand gas resources are widely distributed in the world Beijing 102249, China (Masters 1979; Law 2002; Holditch 2006). Statistics show Guangzhou Institute of Geochemistry, Chinese Academy of that tight sand gas has been discovered or predicted in 70 Sciences, Guangzhou 510640, Guangdong, China basins in North America, Europe, and the Asian-Pacific 12 3 region, with a resource extent of about 210 9 10 m , Edited by Jie Hao 123 Junggar Basin Pet. Sci. (2015) 12:218–231 219 showing huge potential in exploration and development Zhang et al. 2008) and the San Juan Basin (Ayers 2002)in (British Petroleum Company 2012). The United States has the United States, the Ordos and Sichuan Basins in China. the highest annual gas yield from tight sandstone at present As well, in the formation and development history of these 8 3 in the world, reaching 1.75 9 10 m in 2010, about 29 % petroleum basins, the strata were usually deposited and of total gas production of the country (British Petroleum buried, followed by uplifting at a later stage. The study by Company 2012). In China, the Zhongba tight gas field was Tissot and Welte (1984) shows that natural gas generated discovered in the western Sichuan Basin in 1971. With the in gas source rocks due to high temperature in the process progress in exploration technologies in recent years, new of burial was either discharged, migrated, and accumulated discoveries are continually being made. Two major tight in the reservoirs to form gas pools or was retained in the gas provinces in the Sichuan and Ordos Basins and five gas source rocks to form shale gas and coalbed methane. breakthrough areas in the Kuche deep zone of the Tarim There are limited publications at home and abroad re- Basin have been found so far (Fig. 1), with technically garding natural gas migration and accumulation under recoverable gas resources of 8–11 TCM, and annual tight uplift tectonic settings (Tian et al. 2007). It is usually be- gas yield more than 25.6 BCM in 2011 (Li et al. 2012; Zou lieved that strata denudation and faulting are likely to occur et al. 2012). Unconventional gas (i.e., tight gas and shale in the course of uplifting, resulting in damage to gas gas) is increasingly becoming one of the dominant re- reservoirs and dissipation of the gas (Hao et al. 1995; sources in the global natural gas industry, and more efforts Zhang et al. 1999). Based on studies of tight sand gas are being put into tight gas exploration in many countries accumulation in coal-bearing strata in recent years, we (Kuuskraa and Bank 2003; Smith et al. 2009; Zeng 2010; have found that tight gas from coal measure strata not only Dai et al. 2012; He et al. 2013). can form gas reservoirs in the course of deposition and Statistics show that tight gas resources worldwide burial, but also can accumulate in the moderate uplift mostly originate from coal measure source rocks (Dai et al. process if the regional seal has not been destroyed (Bian 2012), e.g., the Piceance Basin (Johnson and Rice 1990; et al. 2009; Zhao et al. 2010). This paper mainly takes gas 70°E 90° 110° 130° 0 1000km 40°N Songliao Basin Tarim Basin Tuha Basin 30° Bohai Bay Basin Ordos Basin Sichuan Basin 20° Tight gas fields name˖ 1.Dabei 2.Sulige 3.Wushenqi Major tight-gas bearing basin 4.Yulin 5.Zizhou 6.Guang'an 7.Anyue 8.Hechuan 9.Bajiaochang Tight-gas Tight gas field South China bearing area Sea Islands Fig. 1 Major tight sand gas basins in China 123 220 Pet. Sci. (2015) 12:218–231 fields of the Upper Triassic Xujiahe Formation in the Formations and the Upper Triassic Xu2, Xu4, and Xu6 Sichuan Basin and the Upper Paleozoic in the Ordos Basin Member tight sands, respectively (Zhang et al. 2009a), with as examples to discuss this geological process. porosities of 4 %–10 %, permeabilities of 0.01–1 mD and thicknesses of hundreds to a thousand meters. They closely and widely contact the source rocks, comprising a very 2 Geological setting favorable source–reservoir assemblage horizontally, which has laid the foundation for widespread gas accumulation in Several large gas fields with reserves more than 100 BCM the reservoirs and then the discovery of large scale reserves have been successively discovered in the Upper Paleozoic (Figs. 2, 3). Permo-Carboniferous in the Ordos Basin and the Upper Stratigraphic burial history shows that the tight sand gas Triassic Xujiahe Formation in the central Sichuan Basin zones in both the Ordos Basin and the Sichuan Basin all (Dai et al. 2012; Zhao et al. 2013) in recent years. By the experienced deep burial before the Late Cretaceous (Zhao end of 2011, large gas fields like Sulige, Wushenqi, and et al. 2005, 2010), with maximum burial depths up to Yulin had been discovered in Upper Paleozoic of the Ordos 3500–4000 m and 4500–6000 m, respectively. From the Basin, with proved gas reserves (including basically proved Late Cretaceous to this day, overall tectonic uplift and 12 3 reserves) approaching 3.0 9 10 m and gas-bearing area strata denudation occurred in these two basins, and the 4 2 about 2.0 9 10 km . Large gas fields like Guang’an, hydrocarbon generation of the source rocks stopped. The Hechuan, and Anyue had been discovered in the Upper denuded thickness is 800–1500 and 1500–2500 m, re- Triassic Xujiahe Formation of the Sichuan Basin, with spectively, and the buried depth of tight gas zones of these 8 3 proved gas reserves approaching 6000 9 10 m and gas- two basins is 1800–4000 m at present. Fortunately, uplift bearing area of more than 3000 km . Tight sand gas re- and denudation have not resulted in the damage to the sources of the two basins account for more than two-thirds regional seal. of the present total tight gas resources in China (Dai et al. 2012). A set of continental clastic coal-bearing formations 3 Theoretical model and experiment covering the whole basin is developed in the Upper Pa- leozoic of the Ordos Basin and the Xujiahe Formation of Substantial gas infusion is a prerequisite for gas accumu- the Sichuan Basin, respectively, and the source rocks lation in the uplift period of tight sand formation. Coal mainly consist of coal seams and coaly mudstone with high petrography (Wang et al. 1995; Zhang et al. 2000) showed gas generation potential. The Upper Paleozoic source rocks that microfractures and cleats are well developed in coals, of the Ordos Basin are distributed in the Carboniferous especially in high-rank coals. Substantial free gas was Taiyuan Formation and Permian Shanxi Formation (Zhang stored in pores and microfractures at the time of substantial et al. 2009b), with coal seam thicknesses of 10–25 m on gas generating from coal seams in the burial period. This average, 40 m locally, almost covering the whole basin expanded volumetrically due to the pressure drop in the stably. Coal measure mudstones have thicknesses of uplift period, and thus together with the desorbed and 60–130 m, almost the same distribution as the coal seams, liberated coal seam gas, became the important gas source and are usually 200-m thick in the west. The Upper Tri- for gas accumulation in the uplift period (Cui et al. 2005; assic Xujiahe Formation source rocks of the Sichuan Basin Zhao et al. 2010). Starting from the discussion of a theo- are distributed in the Xu1, Xu3, and Xu5 Members, with retical model of coal seam adsorption–desorption, by coal seam thicknesses of 5–15 m on average, distributed means of thermal simulation and gas accumulation stably horizontally. Coal measure mudstones have thick- simulation experiments of actual coal samples, the geolo- nesses of 100–800 m, thinning out from the western to the gical process of gas accumulation in the uplift period of central Sichuan Basin, basically covering the whole basin coal measure strata is demonstrated. (Zhao et al. 2011). Thermal evolution history shows that substantial gas generation and expulsion occurred in the 3.1 Theory of adsorption of gas in coal seams source rocks of the two basins in the geological history. and the experimental model The thermal evolution of source rocks has been at highly mature and overmature stages up to now, with cumulative Adsorption refers to the attachment of atoms or molecules 8 3 2 gas generating strength of (20–40) 9 10 m /km and of one substance on the surface of another substance 8 3 2 (40–100) 9 10 m /km , respectively. This can provide (Busch et al. 2003; Bae and Bhatia 2006). The adsorption abundant gas sources for large-scale gas accumulation in behavior of coal was observed in coal mining in the middle the tight sand reservoirs of the two large gas provinces. of the last century. After having successfully produced Their reservoirs are the Permian Shanxi and Shihezi coalbed methane, the United States made an in-depth study 123 Shan1-2 He5-8 Member Formation Upper Shihezi Lower Shihezi Shanxi Taiyuan Series Permian Carboniferous Xu6 Xu5 Xu4 Xu3 Xu2 Xu1 Xujiahe Upper Triassic He1-4 Tai1-2 Pet. Sci. (2015) 12:218–231 221 St rat um Source rock 0 40 80km GR Lithology RT andgas reservoir Hangjinqi Dongsheng Etuokeqi Shenmu Wushenqi Sulige Yulin Etuokeqianqi Zizhou Dingbian Jingbian Wuqi Ansai Yan’an Huanxian Fuxian Qingyang Huanglong Tongchuan Gas generation strength, Effective sandbody Gas reservoir of 8 3 2 10 m /km thickness, m the upper Paleozoic Fig. 2 Composite Upper Paleozoic stratigraphic section and coalbeds and He8 reservoirs, Ordos Basin Source rock Stratum GR Lithology RT and gas Series Form.Member reservoir Jurassic Zhenzhuchong Jiange Tongjiang 025 50km Yilong Mianzhu Nanchong Liangping Chengdu Guang'an Ya'an Shizhu Hechuan Zizhong Chongqing Leshan Gas reservoir Yibin Reservoir thickness of Xu4 Member, m 8 3 2 Gas generation intensity in Xu3 and Xu5 Members, 10 m /km Fig. 3 Xujiahe stratigraphic section showing gas source rocks and reservoirs, Sichuan Basin 5 222 Pet. Sci. (2015) 12:218–231 on the adsorptive action of coal seams (Radovic et al. 1997; measured experimentally represents the maximum value Clarkson and Bustin 1999). Coal is universely believed at and then declines, i.e., there is a large difference between present to be a porous medium with a big surface area. The the measured apparent adsorption capacity and the absolute adsorption of gas by coal is a physical adsorption process; adsorptive capacity. To help solve this problem, some the adsorption energy (heat) is small, the adsorption rate is correction models for calculating the adsorptive capacity fast, and the adsorption and desorption procedure is re- are introduced (Haydel and Kobayashi 1967; Murata et al. versible (Radovic et al. 1997; Clarkson and Bustin 1999). 2001), and a relatively common method is as follows: Therefore, the adsorptive force of coal to gas molecules is N ¼ N ð1  q =qÞð2Þ ab ap free ad intermolecular force, i.e., there are both adsorption equi- where N represents absolute adsorptive capacity, N is librium and adsorption heat (energy). Previous studies ab ap excessive adsorptive capacity or apparent adsorptive ca- (Mavor et al. 1990; Mukhopadhyay and Macdonald 1997; pacity, q is free gas density under equilibrium condi- Crosdale et al. 1998) show that the adsorption by coal free tions, and q is the adsorbed gas density. seams is affected by many factors like coal composition, ad Because of equipment limitations, most coal adsorption temperature and pressure, gas properties, and coal rank. experiments are conducted at temperatures less than 40 C Under geological conditions, with the variation of buried and pressures less than 15 MPa, and the relation between depth of coal seams, the impact of temperature and pres- pressure and adsorptive capacity is basically measured at sure on it is maximum. Experimental results show that one temperature. There are few experimental studies with pressure is proportional to adsorptive capacity, whereas the both temperature and pressure varying. Cui et al. (2005) influence of temperature is the reverse (Zhao et al. 2001). derive a characteristic curve for coal-adsorbed methane Many researchers (Anderson et al. 1966; Ruppel et al. based on adsorption potential theory and adsorption ex- 1974; Yang and Saunders 1985) have conducted many the- periments under high temperature and high pressure and oretical and experimental studies on the adsorptive action of believe that this characteristic curve has uniqueness, i.e., coal seams and established many theoretical models and there is only a unique peak value in adsorptive capacity, mathematical expressions related to adsorptive capacity. based on which a new adsorption model of coal is derived These include the monomolecular layer adsorption model and can be used to estimate the adsorptive capacity at and Langmuir’s equation, multimolecular layer adsorption different temperature–pressure conditions. model and BET’s equation, Freundlieh’s equation, Polo- myi’s adsorption potential theory, micropore filling theory, lnV ¼ AT½2:7lnT  lnP  12:6603þ B ð3Þ and Dubinin-Astak-hov’s equation (Sang et al. 2005; Su et al. where A and B stand for adsorption constant and can be 2008). However, it is difficult to accurately describe the obtained from experimental adsorptive data of coal at a adsorptive properties of coal using a certain isothermal ad- given temperature. The above equation can be used to sorption line or theoretical model due to the presence of calculate the adsorptive capacity of coal at any temperature micropores in coal. It is generally believed that gas is ad- and pressure. sorbed on the surface of coal in a monomolecular state under geological conditions; therefore, a monomolecular layer 3.2 Simulation experiment of hydrocarbon adsorption model and Langmuir’s equation are widely used expulsion due to coal seam uplift and pressure to describe the adsorptive features of coal and have become a drop classical theory maturely used in the development of coalbed methane for the moment. The model is described as follows: For the sake of understanding gas desorption due to tem- V  P perature and pressure drop under approximately real geo- V ¼ ð1Þ P þ P logical conditions, a specially designed autoclave was used to simulate the gas discharge process of coal seams at the where V represents the adsorptive capacity of coal, V is the Langmuir volume, P is Langmuir pressure, and P is time of dropping of temperature and pressure. Two and five sets of samples were selected from the Upper Triassic pressure. Xujiahe Formation of the Sichuan Basin and the Upper With increasing research, it is found that under subsur- Paleozoic of the Ordos Basin, repectively, to conduct the face high temperature and pressure conditions (pressure experiments. The deep burial and gas generation process of [15 MPa, temperature[80 C) (Gregory and Karen 1986; coal seams was simulated by external heating and pres- Mavor et al. 1990), multimolecular layer adsorption of gas surizing; when gas generation and expulsion reached occurs in coal seams. As well, methane becomes a super- equilibrium in the autoclave, the temperature and pressure critical gas under high pressure, and its density increases were reduced, and the gas discharge from coal was ob- significantly, resulting in a change of adsorptive capacity. This results in the phenomenon that the adsorptive capacity served. The experimental results are listed in Table 1 and 123 Pet. Sci. (2015) 12:218–231 223 Table 1 Gas expulsion of coals under different temperature and pressure conditions Coal sample Process Temperature, Pressure, Gas Total gas Staged gas Staged C MPa generation generation generation gas generation and expulsion, and expulsion and expulsion and expulsion mL rate, mL/g rate, mL/g ratio, % Xu3 member Heating and pressurizing 420 105 75 48.8 32.7 67 Cooling and depressurizing 320 50 37 16.1 33 Xu6 member Heating and pressurizing 420 105 575 40 28.8 72 Cooling and depressurizing 320 50 225 11.3 28 Liaohe lignite Heating and pressurizing 450 104 475 19.4 11.9 61.3 Cooling and depressurizing 350 60 300 7.5 38.7 Xianfeng lignite Heating and pressurizing 450 6 850 73.3 42.5 58 Cooling and depressurizing 350 2 615 30.8 42 Xianfeng lignite Heating and pressurizing 450 8.2 950 79.5 47.5 59.7 Cooling and depressurizing 350 4 640 32.0 40.3 Taiyuan Formation Heating and pressurizing 450 4.1 826 60.8 41.3 67.9 Cooling and depressurizing 350 2.4 390 19.5 32.1 Shanxi Formation Heating and pressurizing 450 4.4 975 70.3 48.8 69.4 Cooling and depressurizing 350 1 430 21.5 30.6 show that when the temperature and pressure of coals taken natural gas in the formation was approximated by pure from the Xujiahe Formation are reduced from 420 C and methane. A 3D high-temperature and high-pressure phy- 105 MPa to 320 C and 50 MPa, respectively (corre- sical simulation device independently designed by RIPED sponding to uplifting of formation from 4000 to 2000 m), was used for experiment. the desorbed and discharged gas is 11–16 mL/g, account- In the experiment (Table 2), the model was evacuated, ing for 28 %–33 % of total gas expulsion of coals. When then water was injected, and overburden pressure was ap- the temperature and pressure of coals from the Upper Pa- plied to simulate the water discharge process of formations leozoic of the Ordos Basin are reduced from 450 C and due to burial and compaction. When the overburden pres- 104–4 MPa to 350 C and 60–1 MPa, respectively (cor- sure was increased to 7 MPa, the fluid pressure reached responding to uplifting the formation from 4000–6000 m to 1.63 MPa at 60 C, and the formation was saturated with less than 3000 m), the desorbed and discharged gas is water and was in a near-equilibrium state. Then, methane 7.5–32 mL/g, accounting for 31 %–42 % of total gas ex- was injected into the bottom of the model to simulate the pulsion, showing that quite a lot of gas is desorbed and gas generation process. In the course of gas injection, water discharged during the temperature and pressure decline was continually discharged from the outlet. When the ex- process of coal seams in the uplift period. periment had been conducted for about 20 h, gas started to appear at the outlet, showing that after the gas source rock 3.3 Physical simulation of gas accumulation had been saturated with adsorbed gas, free gas started to migrate. When the gas flowing out of the outlet reached Physical simulation is an important method to study and 10 mL/min and the water yield decreased significantly, we reproduce geological processes (Zeng and Jin 2002; Zhao stopped the gas injection. This simulates the uplift and et al. 2006). To better study the gas accumulation process pressure drop process, and the gas desorption and gas ex- of coal measure tight sand formations in the uplift period, pulsion processes were observed. To better replicate the an experiment was conducted to simulate gas migration subsurface environment, we stopped any operation for 24 h and accumulation under strata uplift and pressure drop after having ceased gas injection, to ensure the equilibra- settings. To prove the existence of gas desorption and ex- tion of gas filling and adsorption. This simulates the ces- pulsion from coal measure formations in its uplift and sation of gas generation after the source rock reached its pressure drop, a physical simulation experiment model was maximum buried depth. Subsequently, the overburden designed based on the configuration relation of source pressure, fluid pressure, and temperature of the model were rocks and reservoirs of the Upper Triassic Xujiahe For- reduced, and at 175–182 h later, gas flowed out of the mation in the Sichuan Basin. Coal samples were crushed to outlet in an episodic type. This process corresponds to that 150–200 mesh, sandstone was replaced by glass beads, and after the formation had changed from burial to uplift and 123 Pressure measuring point Gas cylinder 224 Pet. Sci. (2015) 12:218–231 Table 2 Experiment process and phenomena Step Process and phenomena Step Process and phenomena 1 Installed model under dry and water free conditions 7 Increased injection pressure, gas started to appear at outlet 2 Evacuated for 12 h 8 When the outflow rate of methane reached 10 mL/min, the injection pressure was 2 MPa, closed the gas inlet 3 Slowly injected water into model from the bottom until 9 The gas outflow rate gradually decreased below 0.1 mL/min, and the the internal pressure is at 1 MPa uniformly fluid pressure decreased gradually too 4 Allowed fluid pressure to be 1.63 MPa, increased 10 When no further gas flowed out of the outlet, stood still for 24 h overburden pressure to 7 MPa, and heated the model to 60 C 5 Injected methane at pressure more than 1.63 MPa, water 11 Gradually reduced the overburden pressure to 2 MPa, the fluid pressure flow rate at outlet increased gradually did not change; reduced temperature to 30 C, recorded the gas flow rate at outlet 6 Water flow rate decreased gradually after having been 12 Gradually reduced the fluid pressure of model to 1.63 MPa, the increased to 40 mL/h overburden pressure and temperature did not change, recorded the gas flow rate at outlet (a) Relief valve Back pressure valve Gas collecting facility c c Metering Pressure system sensor A: Coarse sandstone Gas entry B: Coal C: Carbonaceous mudstone Pressure sensor Gas meter D: Mudstone Reduce overburden Reduce fluid 40 8 (b) pressure pressure Gas injection 30 6 Water producing rate Gas producing rate Overburden pressure Fluid pressure 20 4 10 2 0 0 0 60 120 180 200 Time, h Fig. 4 Experimental device (a) and results (b) of simulating gas migration and accumulation in uplift and pressure drop environment denudation when the temperature and pressure of the for- highly compressed in deep strata suffered from desorption mation fluid had dropped. Desorption occurred in the coal and bulk expansion due to pressure drop in the course of measure formation, gas was liberated, and substantial gas uplift. This generated power to force the gas to escape migrated (Fig. 4a, b). outward from the source rocks, and the gas can still largely migrate and accumulate. The existence of this process can enlarge the gas exploration realm to the ‘‘poor’’ gas accu- 4 Experimental results and analysis mulation area, i.e., gas reservoirs can still be found in the uplift area where gas reservoirs were formerly not believed The above experiment shows that under uplift settings, to develop. although the gas generation process of gas source rocks had The gas discharged from different thickness coal seams stopped, the substantial gas adsorbed on the particle sur- in the two basins due to temperature and pressure drop in faces inside the source rocks and free in the pores and the course of moderate uplift at late stage can be Volume, mL Pressure, MPa 2 Pet. Sci. (2015) 12:218–231 225 obtained from about 300 wells in the two basins, as shown 0 40 80 km in Figs. 5 and 6. Observed from these two figures, the gas Hangjinqi Dongsheng expulsion area formed by pressure drop and desorption of coal seams of both the Ordos Basin and the Sichuan Basin Etuokeqi 4 2 in the uplift period can reach (15–18) 9 10 km . The gas Shenmu 8 3 2 expulsion strength is basically (2–8) 9 10 m /km , and Wushenqi Sulige the high value areas have corresponded well to the dis- Yulin Etuokeqianqi covered gas fields, showing that the uplift period has pro- vided important gas supply for large-scale gas Dingbian Jingbian Zizhou accumulation in the two large gas provinces. Wuqi Ansai Yan’an Huanxian 5 Geological evidence Fuxian Qingyang 5.1 Fluid inclusions Huanglong Fluid inclusions provide important means to study the gas Tongchuan accumulation process (Wang and Tian 2000; Rossi et al. Gas reservoir of upper Paleozoic 2002;Lu 2005). The study of inclusions from the Xujiahe Gas release strength in uplifting period Formation reservoir in the central Sichuan Basin (Fig. 7; Table 3) showed that two stages of hydrocarbon inclusions Fig. 5 Gas release strength in the uplift period of the Upper 8 3 2 Paleozoic, Ordos Basin (unit: 910 m /km ) can be clearly identified based on their occurrence and fluorescent display characteristics. The first stage of hy- quantitatively estimated based on the simulation ex- drocarbon inclusions was developed in the early stage of periment results (Table 1) and the coal seam adsorption– quartz overgrowth, mainly occurring at the inner side of desorption equations (Eqs. 2, 3). The strength of desorption quartz overgrowth or along micro-fracture planes in the and discharge of gas from coal seams in the uplift period is early diagenesis of quartz grains, orange-red or light obtained based on calculation and statistics of data brownish yellow in fluorescent light (upper part of Fig. 7), 025 50 km Jiange Chengkou Tongjiang Yilong Mianzhu Santai Nanchong Liangping Chengdu Guang'an Hechuan Shizhu Ya'an Dazu Chongqing Leshan Yibin Gas reservoir Gas release strength in uplifting period, 8 3 2 10 m /km Fig. 6 Gas release strength in the uplift period of the Xujiahe coal measures and carbonaceous mudstones 5 226 Pet. Sci. (2015) 12:218–231 Fig. 7 Occurrence & fluorescent light features for fluid inclusions in Xu 2 Formation, Penglai area, Central Sichuan Basin. a/b-fluid inclusions in the inner side of quartz overgrowth or the micro-fissures, formed at early stage of diagenesis, orange-red or light brown in fluorescent light; c/ d-fluid inclusions distributed in belts along micro-fissures cutting quartz grains, formed at the late stage of diagenesis, light blue or light green in fluorescent light indicating that earlier heavy hydrocarbon exists in the in- the source rocks started to become mature, generate sub- clusions, and the homogenization temperature peak of the stantial gas and enter the reservoir stage. The salinity of associated brine inclusions is at 85–95 C (Table 3). The high-temperature inclusions is higher (16 %–22 %) and second stage hydrocarbon inclusions were developed after should be the record of the middle and late stages of the quartz overgrowth. They are mainly distributed in belts diagenesis when the salinity increased with the substantial along post-diagenesis micro-fissures cutting quartz grains discharge of formation water, and the organic matter was and are strong light blue and light bluish green in discharged at mature and highly mature stages. The age of fluorescent light (lower part of Fig. 7). Laser Raman the former probably corresponds to the period before and spectra showed that the main components of these inclu- after the end of the Jurassic and that of the latter corre- sions are methane and higher hydrocarbons, and the ho- sponds to the uplift period occurring at the end of Creta- mogenization temperature peak of the associated brine ceous. It is discovered by studying the gas generation and inclusions is between 110 and 130 C (Table 3). This expulsion history of Xujiahe Formation in central Sichuan indicates that two stages of gas accumulation took place in Basin that there are a total of two stages of gas expulsion, the central Sichuan Basin. It is discovered by further ana- migration, and accumulation: one stage occurred earlier, lysis (Table 3) that the salinity of low temperature inclu- corresponding to the substantial gas generation period of sions is lower (2 %–13 %), reflecting that the salinity of gas source rocks, and the other stage occurred later, cor- formation water was lower in this period, and they are the responding to tectonic uplift period. The two stages of gas products of the early and middle stages of diagenesis when accumulation are both characterized by large-scale 123 Pet. Sci. (2015) 12:218–231 227 Table 3 Occurrence and test data of fluid inclusions from Xu2 Member, Penglai area Occurrence in mineral deposit Distribution Type of fluid Size, Gas Single Homogenization Salinity, pattern of inclusions lm liquid phase temperature, C wt%NaCl inclusions ratio, % At dust lane and Zonal Hydrocarbon bearing 3 9 5 B5 Liquid 89 2.90 inside of quartz brine inclusions Zonal 4 9 7 B5 Liquid 90 3.06 overgrowth Zonal 6 9 9 B5 Liquid 92 3.06 Zonal 3 9 3 B5 Liquid 90 13.7 Zonal 2 9 6 B5 Liquid 89 13.6 Zonal 3 9 4 B5 Liquid 92 13.6 Zonal 4 9 4 B5 Liquid 90 13.7 Zonal and lineal 10 9 12 B5 Liquid 95 4.96 Zonal and lineal 2 9 12 B5 Liquid 94 5.71 Zonal 26 9 15 B5 Liquid 92 6.74 Zonal 15 9 16 B5 Liquid 92 6.88 Zonal 6 9 8 B5 Liquid 93 6.88 Along micro-fissures cutting Zonal Hydrocarbon 2 9 6 B5 Liquid 116 20.2 quartz grains and overgrowth, bearing brine Zonal 3 9 6 B5 Liquid 118 20.2 formed at late stage inclusions Zonal 1 9 7 B5 Liquid 125 20.2 of diagenesis Zonal 1 9 4 B5 Liquid 125 20.2 Zonal 3 9 7 B5 Liquid 117 20.1 Zonal and lineal 10 9 15 B5 Liquid 128 16.9 Zonal and lineal 2 9 3 B5 Liquid 126 17.0 Zonal and lineal 4 9 6 B5 Liquid 130 17.0 Zonal and lineal 3 9 4 B5 Liquid 129 16.9 Zonal 3 9 4 B5 Liquid 115 22.4 Zonal 4 9 5 B5 Liquid 119 20.0 Zonal 2 9 3 B5 Liquid 116 20.0 Zonal 2 9 10 B5 Liquid 124 20.1 Zonal 5 9 10 B5 Liquid 129 20.1 accumulation, the former resulted from gentle structures in Carboniferous coal seams in the Sulige region exceeds the central Sichuan Basin, whereas the latter resulted from 2.0 %; however, the gas in some regions is wetter, with the overall tectonic uplift (Zhao et al. 2010) (Fig. 8). C /(C –C ) coefficient being up to 86 %, and the carbon 1 1 5 isotope ratio of methane is lighter, -29.96 to -36.45 %, 5.2 Geochemical features showing that the gas accumulated in the Upper Paleozoic gas field mainly originates from adsorbed gas desorption and Experimental results show that with the decline of pressure, free gas expansion in the source kitchen in the uplift period, the carbon isotope ratios of hydrocarbon gas discharged from whereas the gas adsorbed inside source rocks mainly comes coal seams become lighter and lighter, and the C /(C –C ) from the early and middle mature stage of source rocks, 1 1 5 coefficient becomes lower and lower. The gas generated at having lower maturity. Therefore, it is normal for it to be the late stage of formation burial is discharged first, having different from the current maturity of the source rocks. higher maturity, whereas the gas adsorbed in coal seams at Moreover, such carbon isotope lightening gradually be- early stage of formation burial is discharged last, having comes apparent from the Sulige gas field in the west to the lower maturity (Table 4). The geochemical features of gas in Yulin gas field in the east of the Ordos Basin, which is sig- the western Ordos Basin show that (Dai et al. 2005) the nificantly related to the fact that the strata uplift gradually composition and carbon isotope ratios do not match the increased from 800 m in the west to 1400 m in the east at the maturity of the coal measure source rocks. The lower carbon late stage of the basin tectonics. This is because within a isotope ratio of methane and the lower C /(C –C ) coeffi- certain range, the larger the uplift, the more gas is desorbed 1 1 5 cient constitute a contradiction with the higher maturity of from the coal measure, resulting in a large proportion of gas source rocks. For instance, the maturity (R %) of Permo- accumulated in the gas field in the uplift period (Fig. 9). 123 228 Pet. Sci. (2015) 12:218–231 Hechuan WE Moxi Penglai Source rock (Caprock) Reservoir End of the middle Jurassic Source rock Source rock (Caprock) Reservoir End of the late Cretaceous Source rock Source rock (Caprock) Reservoir Himalayan Period Mudstone Coal Sandstone Gas reservoir Source rock Water layer Fault Fracture Migration direction Fig. 8 Gas accumulation model of the Xujiahe Formation in the central Sichuan Basin Table 4 Composition and carbon isotope of gas discharged by coal under different temperature and pressure conditions 13 13 Coal sample Process Temperature, Gas yield, Gas, C , C , C /(C –C ) d C , % d C , % 1 2? 1 1 5 1 2 C mL/g % % % coefficient Liaohe lignite Heating and pressurizing 450 19.4 6.9 3.7 3.2 0.6 -34.4 -25.7 Cooling and depressurizing 350 17.5 8.7 8.8 0.5 -40.6 -29.9 Xianfeng lignite Heating and pressurizing 450 79.5 40.1 22.8 17.3 0.6 -31.8 -26.0 Cooling and depressurizing 350 9.2 6.2 3 0.7 -34.5 -27.4 Taiyuan Formation Heating and pressurizing 450 60.8 72.7 54.6 18.2 0.8 -30.1 -24.5 Cooling and depressurizing 350 68.2 48.9 19.3 0.7 -34.6 -24.6 Shanxi Formation Heating and pressurizing 450 70.25 76.3 61.6 14.7 0.8 -31.5 -23.0 Cooling and depressurizing 350 71 59.4 11.6 0.8 -33.4 -23.8 This also suggests that the composition and carbon carbon isotope ratio of methane should be -32 to isotope ratios of the Xujiahe Formation gas in the central -36 %. In addition, the carbon isotopes of ethane and Sichuan Basin are inconsistent with the maturity of the propane are also characterized by lightening, only the source rocks. For instance, the carbon isotope ratio of the lightening amplitude decreases gradually. The ex- Xu2 Member methane in the Hechuan region is lighter perimental results show that such gas with low maturity (-39 to -42 %), but the maturity R of the lower coal was possibly formed and stored in the coal measure measure source rocks is 1.1–1.3, belonging to the sub- source rocks at the early stage, but it was discharged, stantial gas generation stage of coal measure source became free gas, and accumulated due to tectonic uplift rocks. Based on the statistical data from Dai (1992), the at the late stage (Fig. 9). 123 Pet. Sci. (2015) 12:218–231 229 7 3 2 -50 flow reached 0.8 9 10 m /(km Ma), and the charge 12 3 volume approached 56 9 10 m , whereas the bulk flow charge of gas mainly occurred in the burial period of the 12 3 basin, with a charge volume of 127 9 10 m , and in the -40 + + whole gas generation and accumulation history, the gas ++ 12 3 + diffusion loss volume was estimated to be 135 9 10 m . + + As a result, the gas diffusion loss cannot be compensated + + only by bulk flow charge, and it is hard to form the dis- covered Xujiahe Formation TCM scale gas field. There- -30 fore, the diffusion charge in the uplift period of formation effectively compensates for the diffusion loss of gas and CH contributes more to the efficient gas accumulation and Coal gas data counted by Dai (1992 preservation of large tight sand gas fields. -20 The concept of gas accumulation in the uplift period has Data from Sulige gas field important theoretical and practical significances. Firstly, it Data from Yulin gas field breaks the conventional view that the uplift period is un- Data from Xujiahe gas field favorable for gas accumulation, and secondly it promotes -10 the gas resource extent and potential of the tectonic uplift 0.40 0.60 0.80 1.00 1.30 1.50 2.00 3.00 4.00 5.00 area. On this basis, many large uplift areas formerly be- R , % lieved to be unfavorable for exploration are reassessed, and Fig. 9 Carbon isotope ratio versus R of Xujiahe Formation gas in o the area of favorable gas exploration provinces has been the Sichuan Basin and upper Paleozoic gas in the Ordos Basin and increased. typical coal-formed gas 6 Main gas accumulation pattern and geological 7 Discussion significance of uplift period accumulation On the basis of analyzing the coal seam adsorbed and It was generally believed that diffusion was one of the desorbed gas model, this paper uses a great deal of thermal major factors damaging gas reservoirs, and studies show simulation and physical simulation experiments to that diffusion is an important mode for gas to migrate with demonstrate the geological process of gas accumulation in molecular motion (Nelson and Simmons 1992; Lu and the uplift period of coal-bearing formations. As well, a lot Connell 2007; Lu et al. 2008; Korrani et al. 2012). With of fluid inclusions and geochemical evidence are available increasing study of low porosity and permeability reservoir for the analysis of real gas fields, showing that recognition gas, especially on tight gas, diffusion migration is believed of gas accumulation due to pressure drop and desorption as an important way for the gas to migrate and accumulate from coal-bearing formations in the uplift period has im- in tight reservoirs (Liu et al. 2012; Wang et al. 2014). (Liu portant theoretical and practical bases and is of great sig- et al. 2012) point out that the bulk flow under source– nificance to promoting the resource potential of coal- reservoir pressure differentials and the diffusion flow under bearing strata uplift areas. However, the high-pressure hydrocarbon concentration differential are two important ([20 MPa) coal seam adsorbed and desorbed gas mod- ways for gas migration and infusion. Experiments and el needs further study, for the adsorption state of methane geological analysis show that at the stage of strata uplift, at high pressure has possibly changed. The ordinary coal- gas generation in source rocks stopped, and the source– bed methane adsorption and desorption simulation ex- reservoir pressure differential dropped gradually. The periments are mainly undertaken at pressures of less than combined action of desorption of coal seam adsorbed gas 20 MPa, and it is important to conduct higher pressure and expansion of free gas in the original pores of the coal (20–40 MPa) adsorption–desorption gas tests in the future. seams significantly increased the gas concentration inside the source rocks and provided power for the gas to diffuse and migrate from source rocks to reservoirs. 8 Conclusions On the basis of geological analysis of the Xujiahe For- mation gas reservoirs in the Sichuan Basin, we calculated (1) The adsorption–desorption principle of coal seam bulk flow charge, diffusion flow charge, and diffusion loss gas emission and simulation experiment results in tight sand gas reservoirs. The results show that in the confirms that pressure drop and desorption occur in strata uplift period, the average charge rate of diffusion the uplift process of coal measure formation, and the δ C (PDB), ‰ 230 Pet. Sci. 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