Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 1,

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1 Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 1, A comparative study of the characteristics of coalbed methane reservoirs in the Zhina region, Guizhou Province and the Southern Qinshui Basin, Shanxi Province, China Song Li* and Dazhen Tang The Coalbed Methane Reservoir Laboratory, National Engineering Center, China University of Geosciences, Beijing , China lisong8585@gmail.com tang@cugb.edu.cn *Corresponding author Abstract: In order to explore the coalbed methane recovery potential of the Zhina region in the Southwest China, this paper compared the physical properties of coal reservoirs in the Zhina region and the successfully developed Southern Qinshui Basin. The results show that the pore structures of the coal samples in the two areas are dominated by adsorption pores, which is favourable for the accumulation of CBM. However, coal reservoir in the tectonically stable Southern Qinshui Basin has better homogeneity, while coal reservoir in the tectonic-intense Zhina region has higher heterogeneity. In addition, coal in the Southern Qinshui Basin which is mainly dominated by fresh-water swamp and forests swamp facies has high vitrinite contents, and better adsorption capacities. Moreover, coal reservoir affected from the whole-region magmatic metamorphism in the Southern Qinshui Basin has better physical property and homogeneity than coal reservoir in the Zhina region that only regionally affected by magma metamorphism. [Received: February 19, 2012; Accepted: July 15, 2012] Keywords: adsorption capacity; seepage capacity; high rank coal; coal reservoir; China. Reference to this paper should be made as follows: Li, S. and Tang, D. (2014) A comparative study of the characteristics of coalbed methane reservoirs in the Zhina region, Guizhou Province and the Southern Qinshui Basin, Shanxi Province, China, Int. J. Oil, Gas and Coal Technology, Vol. 7, No. 1, pp Biographical notes: Song Li is a PhD student in the School of Energy Resources from China University of Geosciences, Beijing. He received his Master in Energy Geology and Engineering and Bachelor in Petroleum Exploration Engineering from the same university. Currently, he focuses on coalbed methane geology and exploration, especially in the characterisation of coal sedimentology and the coal reservoir. Copyright 2014 Inderscience Enterprises Ltd.

2 96 S. Li and D. Tang Dazhen Tang is a Professor in the School of Energy Resources from China University of Geosciences, Beijing. Currently, he is the person in charge of the Coalbed Methane Reservoir Laboratory of National Engineering Center. He is also the Academic Leader in coalbed methane geology and exploration in China University of Geosciences, Beijing. His research interests include coalbed methane geology, organic petrology, petroleum systems, etc. 1 Introduction The worldwide coalbed methane (CBM) exploitation programs focus primarily on medium and low rank coals, however, high rank coals are generally the main targets for surface CBM recovery in China. Because high rank coal usually has a higher gas content, so the regions with high rank coal reservoirs are the most likely districts for Chinese CBM recovery in the future. Now China attaches an unprecedented importance to CBM exploration and development due to the breakthrough of the commercial program of CBM development in the Southern Qinshui Basin of North China. Chinese CBM recovery activities have an extremely uneven geographical distribution, and most of them are concentrated in North China. The Zhijin and Nayong (Zhina) region, known as the largest anthracite coal producing area in Guizhou Province, locating at the Southwest of China, is rich in coal and CBM resources, and has the highest potential for high rank CBM recovery in the Southwest China (Figure 1). However, in recent years, CBM exploration and development in this region has not been profitable. The reasons for this include the complex geological conditions as well as the lack of basic research. The component of CBM in the Zhina region and Southern Qinshui Basin are similar, both are dominated by methane (content higher than 98%), with a trace of ethane and minor amounts (content lower than 1%) of N 2 and CO 2 (Gu, 2002; Shi et al., 2007; Li et al., 2012). Differences in the coal reservoir physical properties are the major differences between the two areas, and reservoir properties are the prerequisites in determining the possibility of CBM exploration and its development potential. In this paper, various testing methods were used to evaluate the size, shape, distribution and connectivity of pores and fractures in the two regions. Moreover, looking forward to the theoretical and practical breakthroughs in the Southwest China CBM exploration and development, the gas adsorption and seepage capacities of coal reservoirs in the Zhina region and Southern Qinshui Basin were compared. Coal reservoir physical properties essentially include gas content and permeability. Gas content reflects the CBM adsorption capacity of the coal reservoir, and permeability reflects the pore and fracture system of the coal reservoir. Therefore, research on the physical property of the coal reservoir would reveal the pore and fracture system s influence on the concentration, migration and output of CBM. Coal is a porous media that includes pores, microfractures, and cracks (Clarkson and Bustin, 1999; Gilman and Beckie, 2000; Karacan and Okandan, 2001; Mastalerz et al., 2008). Coal pores have been classified into micropores (less than 10 nm in diameter), transition pores ( nm in diameter), mesopores (100 1,000 nm in diameter), and macropores (greater than 1000 nm in diameter) by previous researchers. Macropores and mesopores (also named together as seepage pores) are the main gas flow paths during CBM recovery, however, transition pores and micropores (also named together as adsorption pores) are the main

3 A comparative study of the characteristics of coalbed methane reservoirs 97 place for concentration of CBM (Fu et al., 2001; Lin et al., 2000; Shi and Durucan, 2005; Yao et al., 2009). Figure 1 Location and geological structure of the study areas (see online version for colours) km Changzhi Anze GH Wulumuqi Mabi WY Taiyuan Beijing GP Gaoping Qinshui Duanshi DS TA Guiyang XH Yangcheng HC JC Jincheng FX LS DSY HF Nayong HJG ZK Zhijin ZH NF QS Syncline Fault Study area Anticline City Sample points Puding km Liuzhi Anshun Many researchers have studied the CBM adsorption capacity from the angle of coal rank, coal quality and coal maceral composition, etc. (Crosdale et al., 1998; Laxminarayana and Crosdale, 1999, 2002; Mastalerz et al., 2004; Unsworth et al., 1989; Sang et al., 2005; Su et al., 2005; Yao and Liu, 2007; Zhang and Yang, 1999). Some recent studies have characterised various pores, microfractures, cleats, fractures, and their influences on permeability of coal (Gamson et al., 1993; Radlinski et al., 2004; Su et al., 2001; Solano- Acosta et al., 2007; Wang et al., 1996; Xu et al., 2005; Yao et al., 2008). Almost all the world s CBM bearing basins which have been commercially developed have been dominated by magmatic metamorphism; thus, the multi-stage magmatic metamorphism s influences on the CBM recovery and reservoir properties have drawn extensive attention (Yang and Tang, 2000). The rapid warming caused by magma intrusion made abundance re-generated methane. The CBM generation in the San Juan Basin; Raton Basin, USA; Gunnar Great Basin, Australia; and Qinshui Basin, China, etc., are good examples for this viewpoint (Ayers, 2002; Cooper et al., 2007; Gurba and Weber, 2001; Liu et al., 2005; Tang et al., 2004; Wu et al., 2005; Wang, 2007). In addition, some researchers have pointed out that the chemical and physical structure evolution of the coal reservoir is the essential control of gas content in different

4 98 S. Li and D. Tang coalification stages, and the depositional environment has significant influence on permeability and heterogeneity of the coal reservoir (Qin et al., 1999; Tang et al., 2000). 2 Samples and tests A total of 14 samples were collected from the Zhina region in South China, eight of which were located in the Diaoshuiyan coal mine (DSY), the Hongfa coal mine (HF), the Huangjiagou coal mine (HJG), the Fuxing coal mine (FX), the Liangshan coal mine (LS), the Niufang coal mine (NF), the Zhihe coal mine (ZH), and the Qingshan coal mine (QS), and the remaining six samples came from a drill hole in Zhijin County. Meanwhile, another 14 coal samples were obtained from the Southern Qinshui Basin, including the Duanshi coal mine (DS-1 and DS-2), the Houcun coal mine (HC-1 and HC-2), the Tangan coal mine (TA-1 and TA-2), the Wangyun coal mine (WY), the Gaohe coal mine (GH), the Jincheng region (JC-1, JC-2, JC-3 and JC-4), the Gaoping district (GP) and the Xihe area (XH). The collected samples fully account for the geographical distribution and the main coal seams (Table 1). Table 1 Result data of laboratory test of samples from the Zhina region and Southern Qinshui Basin Districts Sample no. R o (%) Zhina region Coal maceral composition V (%) I (%) M (%) Φ (%) K (md) DSY HF HJG FX LS NF ZH QS ZK ZK ZK ZK ZK ZK Average value Notes: Because some samples cannot be made into cylindrical cores, their pore and permeability parameters were not included in this table. Samples DSY, FX and LS have fractures. V: vitrinite; I: inertinite; M: mineral; Φ: porosity; K: permeability.

5 A comparative study of the characteristics of coalbed methane reservoirs 99 Table 1 Result data of laboratory test of samples from the Zhina region and Southern Qinshui Basin (continued) Districts Sample no. R o (%) Southern Qinshui Basin Coal maceral composition V (%) I (%) M (%) Φ (%) K (md) DS DS HC HC TA TA WY GH JC JC JC JC GP XH Average value Notes: Because some samples cannot be made into cylindrical cores, their pore and permeability parameters were not included in this table. Samples DSY, FX and LS have fractures. V: vitrinite; I: inertinite; M: mineral; Φ: porosity; K: permeability. The laboratory tests of the coal samples include vitrinite reflectance measurements, coal maceral analyses, fracture analyses, characteristics of porosity and permeability tests, nuclear magnetic resonance (NMR) experiment, mercury intrusion test, low-temperature N 2 isotherm adsorption/desorption analyses method, and methane isothermal adsorption measurement (samples on training equilibrium-water). The instruments used in this research including Laborlxe 12 POL Fluorescence Microscope, Ultrapore-200A Helium Porometer, Ultraperm TM 200 Permeameter, Rec Core 2500 Nuclear Magnetic Resonance Measuring Equipment, Automatic Mercury Injection Apparatus 9410, Quadrasorb SI Specific Surface Area Instrument, and IS-100 High Pressure Isotherm Instrument. In order to verify the consistency and repeatability, the experiments were performed twice. 3 Results and discussions 3.1 Adsorption capacity Since CBM are mainly adsorbed in the pore system, the CBM adsorption capacity not only affects the gas content, but also has a decisive impact on the recovery of CBM. The adsorption pores in the coal reservoir are the pores of which diameters are less than 100 nm, which include micropores and transition pores. To explore the difference between the concentration of CBM in the Zhina region and Southern Qinshui Basin, the

6 100 S. Li and D. Tang NMR experiment, low-temperature N 2 isotherm adsorption/desorption method, and methane isothermal adsorption measurement were used Characteristics of adsorption pores structures The NMR results show that coal samples in the Zhina region and Southern Qinshui Basin have similar T 2 spectrum morphology (Figure 2) with their prominent peaks locating at low T 2 region and the sub-peaks at high T 2 region (some samples do not have sub-peaks). The prominent peaks and the sub-peaks are located at T 2 = 0.8 ms and T 2 = 50 ms, respectively. And the prominent peaks are much higher than the sub-peaks, which indicates that the coal reservoirs in the two areas mainly consist of absorption pores, while seepage pores and fractures are poorly developed. That is resulted from high rank coal which is dominated by vitrinite that mainly consists of micropores and transition pores, whereas the inertinite which typically consists of macropores and mesopores is relatively low. And it is a major property of high rank coal reservoir. Generally speaking, the adsorption pores structures are favourable for the accumulation of CBM in both the Zhina region and Southern Qinshui Basin; moreover, the nuclear magnetic signal of coal samples in the Southern Qinshui Basin (DS-1 and HC-1) is higher than those in the Zhina region (LS and QS), which indicates that coal samples in the Southern Qinshui Basin have higher adsorption pore contents, and thus better adsorption capacities. Figure 2 NMR test data of coal samples (see online version for colours) The low-temperature N 2 isotherm adsorption/desorption method is very effective in differentiating the transition pores and the micropores. The test result shows the BET special surface area (S BET ) of the coal reservoir in the Zhina region ranges between and m 2 /g, with an average value of m 2 /g; the BJH total pore volume (V BJH ) varies from to ml/g with an average value of ml/g; the average pore diameter (D PORE ) is about nm (Table 2). However, S BET of the coal reservoir in the Southern Qinshui Basin ranges between and m 2 /g, with an average value of m 2 /g, and it is lower than that of the Zhina region. The average V BJH value and pore diameter are ml/g and nm respectively, and they are higher than those of the Zhina region. The contents of transition pores are higher than micropores in the Southern Qinshui Basin, while in the Zhina region the adsorption pores are dominated by micropores, and the contents of micropores are all more than 50%. The micropores mainly contribute to the S BET, while the transition pores have a fundamental influence on

7 A comparative study of the characteristics of coalbed methane reservoirs 101 the V BJH and D PORE. Those are the main reasons why the S BET is higher in Zhina region, while the V BJH and D PORE are higher in the Southern Qinshui Basin. Table 2 Result data of low-temperature N 2 isotherm adsorption/desorption test Districts Sample no. S BET (m 2 /g) V BJH (ml/g) Pores proportions (vol.%) (nm) P tran P micro D PORE Zhina region DSY HF HJG FX LS NF ZH QS Average value Southern DS Qinshui Basin DS HC HC SH TA WY GH Average value Notes: S BET : the BET special surface area; V BJH : the BJH total pore volume; D PORE : average pore diameter; P tran : transition pores; P micro : micropores CBM adsorption capacity The Langmuir parameters of isotherm adsorption experiments are usually used to evaluate the adsorption capacity of the coal reservoir. Langmuir volume and Langmuir pressure are commonly used Langmuir parameters. Langmuir volume is the maximum adsorption volume of the CBM reservoirs, a representative of the methane adsorption capacity of coal. Langmuir pressure is the pressure when the actual adsorption volume reaches 50% of the maximum adsorption volume, a representation of the degree of how easy the adsorption process takes place. The Langmuir volume of raw coal in the Zhina region ranges from to m 3 /t with an average value of m 3 /t; while the Langmuir volume of the raw coal in the Southern Qinshui Basin varies from to m 3 /t with an average value of m 3 /t (Figure 3). The Langmuir pressure in the Zhina region is about 2.08~2.74 MPa with an average value of 2.68 MPa; and the Langmuir pressure in the Southern Qinshui Basin is about 2.10~3.74 MPa with an average value of 2.81 MPa. The CBM adsorption capacities of the two regions are relatively high. Generally speaking, when the Langmuir volume is high and the Langmuir

8 102 S. Li and D. Tang pressure is greater than 2 MPa, the conditions will be conductive to high recovery of CBM, so the two areas both possess high-yielding conditions. Figure 3 Isotherm curves of various samples (see online version for colours) 3.2 Seepage capacity The macropores, mesopores and fractures are the major controlling factors of the seepage capacity, and their characteristics have a significant impact on the permeability and the output of CBM. In this paper, the mercury intrusion method and the optical microscopic method were used to describe the seepage pores structures and the characteristics of the fractures. Also, the traditional pore and fracture parameters of coal reservoirs in the two regions were compared. Thus, the differences between the seepage capacities of coal reservoirs in the Zhijin region and Southern Qinshui Basin can be analysed Characteristics of the seepage pore structures The mercury intrusion method can quantitatively get the parameters of the pore whose diameter is larger than 3.75 nm, so this method has certain advantages in testing the pore structures of seepage pores. The characteristic parameters include expulsion pressure, the mercury intrusion saturation and extrusion efficiency. Generally, the lower the expulsion pressure and the higher the mercury intrusion saturation and extrusion efficiency are, the better the coal reservoir is. The mercury intrusion saturation of coal reservoirs in the Zhina region and Southern Qinshui Basin is generally low, which also shows that pores structures are mainly composed by micropores and transition pores, resulting in difficulties of the mercury vapor entering the seepage pores (Table 3). Mercury extrusion efficiency in the Zhina region is lower than that in the Southern Qinshui Basin. The pore structure of the coal reservoir in the Zhina region has a typical bimodal distribution feature, and it means that the micropores and transition pores are dominant in the pore structure, and the macropores are seldom but the mesopores are the least developed. This kind of pore structure can easily lead to the bottleneck or discontinuity problem, which reduces the mercury extrusion efficiency and permeability, making it difficult for the mercury steam to flow out of coal. Expulsion pressure of most samples in the Zhina region are lower than that of the Southern Qinshui Basin, since the macropores and mesopores contents in the Zhina region is higher than those in the Southern Qinshui Basin.

9 A comparative study of the characteristics of coalbed methane reservoirs 103 Table 3 Result data of mercury intrusion test of samples from the Zhina region and Southern Qinshui Basin Districts Sample no. V in (%) E ex (%) P ex Content of pores (%) (MPa) P macro P meso P tran / P micro Curve type Zhina region DSY C HF D HJG B FX B LS C NF A ZH C QS D Average value Southern DS B Qinshui Basin DS B HC B HC B TA B WY B GH B JC A JC B JC B JC B GP B XH B Average value Notes: V in : maximum intrusion saturation; E ex : extrusion efficiency; P ex : expulsion pressure; P macro : macropores; P meso : mesopores; P tran : transition pores; P micro : micropores. From the statistics analyses of the mercury intrusion/extrusion of the coal samples from the Southern Qinshui Basin and the Zhina region, three typical seepage pore models are listed below. Sample NF the typical sample in type A, is characterised by a good pore structure, and the maximum intrusion saturation is up to 68% with a general extrusion efficiency of about 40%, while the displacement pressure are normally below 1 MPa. Connectivity between pores and fractures are good in type A. B is represented by DS-1 sample, and it is characterised by high displacement pressure and extrusion efficiency (about 70%), as well as the very low maximum intrusion saturation value about 20%. The well-developed micropores and transition pores are the direct reasons for the low intrusion saturation, in addition, the high mercury extrusion efficiency indicates the good connectivity between the seepage pores. C with a low displacement pressure is represented by sample HJG, and the mercury intrusion saturation and extrusion efficiency are both about 35%. The pore structure of such type has a typical

10 104 S. Li and D. Tang bimodal distribution feature, because the mesopores are the least developed in coal. This kind of pore structure can easily lead to the bottleneck or discontinuity problem, which reduces the permeability. B is the main seepage pore model in the Southern Qinshui Basin, and this kind of pore structure is favourable for the output of CBM; however, in the Zhina region, all the three types are developed. The coal reservoir in the Zhina region has higher heterogeneity, and the condition is unfavourable for the surface recovery of CBM Characteristics of fractures Fractures have a fundamental influence on the permeability of the coal reservoir. Using optical microscopy can compare the size and morphology of the fractures in the two districts directly. The fractures in study areas are divided into four types A, B, C and D. A with width (W) > 5 um and length (L) > 10 mm are fractures which can be distinguished clearly in macroscopical view. B with W > 5 um and 10 mm > L > 1 mm are continuous and long fractures. C with W < 5 um and 1 mm > L > 300 um are the intermittent fractures. And type D with W < 5 um and L < 300 um are short fractures. The fracture frequency of the coal reservoir in the Southern Qinshui Basin ranges from 10 to 236 per 9 cm 2 with an average value of 64 (Table 4). C is the leading fracture type, followed by type D and type A, and type B are the least developed one in the Southern Qinshui Basin. The fracture frequency of the coal reservoir in the Zhina region varies from 15 to 583 per 9 cm 2 with a larger average value of 90. D is the uppermost fracture type, followed by type C, and almost no type A and type B in this region. The fracture frequency of sample LS is up to 583 per 9 cm 2, which may have some connection with the faults in the vicinity. Since the tectonic setting is the main controlling factor of the development of fractures, the tectonic-intense Zhina region has a higher fracture density than the tectonic-stable Southern Qinnan Basin has. Table 4 Districts Zhina region Result data of fracture analyses of samples from the Zhina region and Southern Qinshui Basin Sample no. Fractures numbers (per 9 cm 2 ) Fractures proportions (%) A, B C D Sum A, B C D NF LS FX HJG HF ZK ZK ZK ZK ZK ZK Average value

11 A comparative study of the characteristics of coalbed methane reservoirs 105 Table 4 Result data of fracture analyses of samples from the Zhina region and Southern Qinshui Basin (continued) Districts Southern Qinshui Basin Sample no. Fractures numbers (per 9 cm 2 ) Fractures proportions (%) A, B C D Sum A, B C D HC HC DS DS GH JC JC JC JC GP XH Average value Characteristics of porosity and permeability The porosity ranges from 1.8 to 9.5% with an average value of 4.3% in the Zhina region, and the permeability varies from 0.01 to 3.56 md. While in the Southern Qinshui Basin, the porosity ranges from 1.1 to 8.1% with an average value of 4.1%, and the permeability ranges between 0.01 and 0.84 md. The porosity of the two regions is similar, but there are a certain differences in permeability. The permeability of the coal reservoir in the Southern Qinshui Basin is generally low with all the tested values under 1 md. While the permeability range of coal samples in the Zhina region spreads much wider. The permeability of sample DSY, FX and LS is higher than 3 md, and the permeability values of the other samples are equal to that of the Southern Qinshui Basin (Figure 4). Figure 4 Permeability values of coal samples (see online version for colours)

12 106 S. Li and D. Tang 3.3 Analysis of the difference of coal reservoir characteristics and impacting factors Regional tectonic conditions Regional tectonic conditions have a prominent impact on the physical property of the coal reservoir. The main coal seams of the two regions experienced Indosinian, Yanshanian and Himalayan Movements. The Southern Qinshui Basin is located in the tectonically stable region in the western North China Craton. Uplifting, erosion, incompetent folds and faults were the main expression of the tectonic movements since the Indosinian Movement, and finally a large multiple syncline was formed. In the Yanshanian stage, the North China platform continued uplift, resulting in the deletion of the Cretaceous rocks. The major form of the Himalayan Movement is the transformation of previous structures, which finally resulted in the present structural configuration (Zhao, 2009). Areal tectonic was steady in the coal-accumulating period, and later tectonic deformation was weak. Consequently, the development of wide folds is the main characteristic of this region, with fewer faults, the coal distribution is continuous with gentle occurrence, and the primary structures of coal have been preserved. Therefore, the coal samples in the Southern Qinshui Basin have similar pore and fracture system, less reservoir heterogeneity, and less discrete porosity and permeability parameters. Table 5 Coal reservoir pressure of some drill hole in the Zhina region and Southern Qinshui Basin Districts Drill hole Coal seam Pressure gradient Zhina region J Southern Qinshui Basin JS HG HG HG Source: Modified from Qin et al. (2008) and Wu et al. (2008) While the Zhina region located in the southwestern margin of the Yangtze paraplatform, and folded zone which includes steep anticlines and gentle synclines was formed in this region, resulting from the large scale uplift in the Indosinian stage and the transformation of the major faults and folds in the Himalayan period. The Upper Permian coal-bearing strata in some steep anticline axis have been eroded, whereas most of the strata in the syncline have been preserved. Therefore, the primary coal controlling structures in the Zhina region are the large synclines and multiple synclines (Gu, 2002). With extensive

13 A comparative study of the characteristics of coalbed methane reservoirs 107 tectonic activities, and development of faults and folds, the coal seams have undergone superposition of multi-phase transformation. Not only were the coal structures greatly deformed, but also the physical property of the coal reservoir experienced fundamental changes, which increased the coal reservoir heterogeneity. The pressure gradient of the coal reservoir in the Southern Qinshui Basin varies from 0.55 to 0.94 MPa/100 m, with an average value of 0.81 MPa/100 m; in the Zhina region, the pressure gradient of the coal reservoir varies from 0.98 to 2.54 MPa/100 m, with an average value of 1.77 MPa/100 m (Table 5). Compared with the Southern Qinshui Basin, the pressure gradient of the coal reservoir in the Zhina region is significantly higher with a higher dispersion degree, which probably is the major reason for the strong heterogeneity in the Zhina region Coal facies Coal reservoirs formed in different sedimentary environments have different physical properties. Coal deposited in fresh-water swamp and forest swamp has high vitrinite content, and its pore structure is relatively favourable. This kind of coal reservoir has high permeability since vitrinite is prone to form fractures. However, coal which is deposited in drained swamp has lower vitrinite content, leading to poor development of the pore and fracture system. Therefore, its porosity and permeability is very low. Coal facies directly determines the coal macerals composition, and also controls the pore and fracture system of coal reservoir indirectly. This paper relies on various indices, including the gelification index (GI), tissue preservation index (TPI), ratio between vitrinite and inertinite (V/I), and wood index (WI) to reflect the peat accumulation information in the Zhina region and in the Southern Qinshui Basin (Table 6). Coal in the Zhina region is mainly deposited in forest swamp and drained swamp, while coal in the Southern Qinshui Basin is chiefly dominated by fresh-water swamp and forests swamp facies. Because the vitrinite content is higher in the Southern Qinshui Basin, the development of micropores and transition pores as well as the CBM adsorption capacities are good in the Southern Qinshui Basin than those in the Zhina region. Table 6 Parameters of coal facies in the Zhina region and Southern Qinshui Basin Districts Sample no. GI TPI V/I WI Coal facies type Zhina region HF Drained swamp HJG Forest swamp FX Forest swamp NF Forest swamp ZK Forest swamp ZK Forest swamp ZK Drained swamp ZK Drained swamp ZK Forest swamp ZK Forest swamp

14 108 S. Li and D. Tang Table 6 Parameters of coal facies in the Zhina region and Southern Qinshui Basin (continued) Districts Sample no. GI TPI V/I WI Coal facies type Southern Qinshui Basin DS Fresh-water swamp DS Fresh-water swamp HC Forest swamp HC Forest swamp JC Fresh-water swamp JC Fresh-water swamp JC Fresh-water swamp JC Fresh-water swamp GP Forest swamp XH Forest swamp Coalification Analyses of the organic matter s thermal and maturation history of coal reservoirs in the Zhina region and in the Southern Qinshui Basin show the thermal evolution has experienced two stages. They are pre-yanshanian and Yanshanian periods. Burial metamorphism was dominant in the pre-yanshanian stage, while magmatic metamorphism was dominant in the Yanshanian period, and the present distribution of coal rank in the two regions is the result of burial metamorphism and magmatic metamorphism. Before the Yanshanian period, the Southern Qinshui Basin had a normal geothermal gradient (3 C/100 m), and the burial metamorphism could have turned coal into fat coal or coking coal stage (Fan, 2001). In the Yanshanian period, the overlying strata in this region suffered certain erosion resulting from the intense tectonic events. At the same time, the uplift of the Moho surface and the intrusion of magma formed a high geo-temperature field, which sped up the coal evolution and turned fat coal into dry coal or anthracite rapidly (Jie and Li, 2000; Lin and Su, 2007). Because the Zhina region had a higher geothermal gradient of 4 C per 100 m, the burial temperature of coal reservoir before the Yanshan magmatic metamorphism was about 185 C, and the burial metamorphism could have turned coal into the dry coal stage. In the Yanshanian period this area was little affected by magmatic activities, except for the regional fault zones (Chen et al., 2010). Coal samples in the fault zone not only have a high degree of metamorphism, but also have better permeability, which is quite different from other samples and resulted in the strong heterogeneity in this area. There are three deep faults in the Hercynian orogenic stage. The northern Nayong-Wengan fault, the eastern Guiyang-Shizong fault, and the western Shuicheng-Ziyun fault (Tian, 2008). The sample DSY which is located near the Shuicheng-Ziyun fault and samples FX and LS which are located near the Nayong-Wengan fault have relatively high metamorphism and permeability, as their max values are larger than 2.5%, the permeability values are 42.6, 12.5, and 3.56 md, respectively. This indicates that the upwelling of the magmatic hydrothermal fluids along the fractures greatly increased the temperature of coals near the fault zone, and thus increased the coal metamorphism degree; the extensive tectonic

15 A comparative study of the characteristics of coalbed methane reservoirs 109 activities near the fault zone also resulted in the development of pores and fractures, which greatly improved the permeability of coal reservoir. The maximum vitrinite reflectance (R o, max ) in the Zhina region ranges from 1.64 to 3.31%, while in the Southern Qinshui Basin it varies from 1.60 to 3.43%, and the high metamorphic grade is a shared character in both study areas. There is a significant correlation between the porosity and the coal rank of the coal reservoir. The porosity was firstly reduced as the R o, max rising (when R o, max ranges from 1.60% to 2.40%), and then it increased with the climb of R o, max (when max is between 2.40% and 3.43%) (Figure 5). Burial metamorphism was dominant when R o, max is between 1.60% and 2.40%. With the increase of burial depth, the primary porosity of coal reservoir sharply declined when the compaction degree of coal increased. Therefore, the porosity decreased with the increase of R o, max. Coal-bearing strata were uplifted by later tectonic movement, and burial metamorphism stopped. However, magmatic metamorphism in the Yanshanian period turned the shallow buried coal into anthracite coal. The baking heat of magma resulted in the volatilisation of a large number of volatile substances from coal, which not only formed a large number of densely clustered round-shaped or tubular pores but also led to the increase of molecular spacing and variation of the order and directivity of the coal macromoleculars arrangement with the increase of the structural defects. The later uplift and erosion of the coal seams resulted in less pressure than that in the burial metamorphism period, which is conducive to the preservation of the pores system. Thus, when R o, max is between 2.40% and 3.31%, the porosity values of the coal reservoir show an increasing trend. Figure 5 Connection between porosity and vitrinite reflectance (see online version for colours) 4 Conclusions 1 The pore structures of samples in the Zhina region and the Southern Qinshui Basin are dominated by adsorption pores, and the Langmuir volume and Langmuir pressure are relatively high. The adsorption capacity is generally good in the two regions, and the two areas both possess high-recoverability. The seepage pores are not quite developed in the samples of the Southern Qinshui Basin and Zhina region, and the mercury intrusion saturation and extrusion efficiency are generally low. The pore structures in the Southern Qinshui Basin are conducive to the recovery of CBM, while in the Zhina region the pore structure of some samples can easily lead to the

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