Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China

Journal of Natural Gas Science and Engineering 23 (2015) 524e537

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Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China Yue Chen a, b, *, Dazhen Tang a, b, Hao Xu a, b, Yong Li a, b, Yanjun Meng a, b a b

School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China Coal Reservoir Laboratory of National Engineering Research Center of CBM Development & Utilization, Beijing 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 November 2014 Received in revised form 2 February 2015 Accepted 15 February 2015 Available online 10 March 2015

Significant progress has been made in coalbed methane (CBM) exploration and development in the eastern margin of the Ordos Basin where nearly 2000 CBM wells have been drilled, achieving a maximum gas production rate of 16000 m3/d by 2013. The geological evolution of the eastern Ordos Basin plays an important role in the CBM formation. This study is focused on the interrelationship between structural geology, gas accumulation and production characteristics of the No. 4 þ 5 coal in the Permian Shanxi Formation as well as the No. 8 þ 9 coal in the Carboniferous Taiyuan Formation. This research is based on the data collected from CBM production wells and coal samples from coalmines and exploration wells. The results show that thermogenic gas is the dominant CBM source in the study area and there are two significant generation periods, the coalification in the Triassic and the magmatic thermometamorphism during the Yanshan movement. Combining the structure background and hydrogeological conditions, the monoclinic-hydraulic sealing model was proposed as the representative CBM enrichment model. Different types of structures are also classified, and their influence on the CBM accumulation is discussed. Compressional structures formed during the Yanshan movement are conducive to CBM enrichment and retention; however, the tensional structures formed during the Himalaya movement may have led to CBM dissipation. Combining the structural effect on the CBM production with CBM exploration and development practices in the study area, the following three types of high gas production models are summarized: updip of the monocline, the axial part of the anticline or nose structure, and the structural high far from the normal fault. © 2015 Elsevier B.V. All rights reserved.

Keywords: Ordos Basin Coalbed methane Structure Accumulation Production

1. Introduction Recently, the development of coalbed methane (CBM) from high-rank coals has achieved significant success in the southern Qinshui Basin in China (Su et al., 2005; Cai et al., 2011; Tao et al., 2012). CBM resources in medium-rank coal are abundant in China, especially in the Ordos Basin (Tang et al., 2004; Yao et al., 2008; Li and Zhang, 2013; Meng et al., 2014). The successful development of CBM in the San Juan and Black Warrior Basin in the United States (Murray, 1996; Ayers, 2002; Pashin, 2010; Tong et al., 2014) has shown that low to medium-rank coal reservoirs are of significant importance in CBM development. The eastern margin of the Ordos Basin CBM field occurs along the Yellow River, with a

* Corresponding author. China University of Geosciences (Beijing), Beijing 100083, China. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.jngse.2015.02.018 1875-5100/© 2015 Elsevier B.V. All rights reserved.

length of 560 km from north to south, a width of 50e200 km from east to west and an area of 25,000 km2. The coal resources within this area occur at depths above 1500 m, with the gas resources estimated to be 9  1012 m3 (Jie, 2010). It has become the second largest industrial development CBM field after the Qinshui Basin in China. By the end of 2013, the eastern margin of the Ordos Basin CBM field contained nearly 2000 drilled wells, among which the highest gas production rate of a single vertical well exceeded 6000 m3/d and that a single horizontal well achieved16,000 m3/d. Gas contents and production from CBM wells in different regions show significant differences. Different structural characteristics in different regions not only control the shape, continuity and permeability of the coal seams but also have a direct influence on the CBM generation, migration, accumulation and production (Song et al., 2013). Different parts of the same structure may have different stress properties, positively or negatively influencing the CBM retention or even development (Ayers, 2002). Pashin and Groshong Jr. (1998) have stated the effects of extensional stress

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were benefited to improve permeability and compressive stress was favorable for CBM retention. Groshong Jr. et al. (2009) have reported the water production rate was high near normal faults but the trend was opposite close to fold-thrust belts. CBM composition and the methane isotopic variation have been reported by Li et al. (2014a), partly showing the CBM generation and evolution history. Other reports have primarily focused on a small part of the eastern Ordos Basin and are limited to the relationships between gas contents and structural properties, e.g., the Hancheng area (Yao et al., 2014) and the Liulin area (Li et al., 2014c, 2014e). However, the structural evolution and its influence on CBM generation, accumulation and production have not been studied in detail. The goal of this study was to reveal structural effects on CBM generation, accumulation and field development over the entire area of the eastern margin of the Ordos Basin, and to establish certain typical CBM accumulation and high gas production models controlled by structures. 2. Tectonic setting The Ordos Basin, a stable polycyclic sedimentary basin formed on the North China Craton, is located in North China covering an area of 250,000 km2 and contains the second largest accumulation of coal resources in China (Xu et al., 2012; Yang et al., 2013). The basin is divided into seven structural units (Xue et al., 2011; Tang et al., 2012) (Fig. 1). The eastern margin of the Ordos Basin is a NeS striking and west trending monocline within the following three tectonic units: the Yimeng uplift, Jinxi fold and Weibei uplift from north to south (Jiang et al., 2012; Wang et al., 2013; Fig. 2). A series of large-scale folds along the NeS and NEeSW trending and the less developed faults has been observed in the area, suggesting

Fig. 1. The distribution of tectonic units in the Ordos Basin.

Fig. 2. The tectonic map of the eastern margin of the Ordos Basin.

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a transitive structural type (Yao et al., 2009). The eastern margin of the Ordos Basin underwent three tectonic movements (the Indo movement, the Yanshan movement and the Himalaya movement) after the Carboniferous-Permian coal forming period (Zhang and Wu, 2013; Li et al., 2014b). During the Indo movement period, the collision between the Yangtze plate and the North China plate as a result of NeS compressive stress formed a few compressive structures in the large-scale monoclinic (Li et al., 2014d). During the following Yanshan movement period, a series of NEeSW trending reverse faults and folds were formed due to NWeSE comprehensive stress. Meanwhile, magmatic intrusions caused a significant increase in the coal thermal maturity. The Zijin mountain alkaline rock outcrops at the junction of the Linxian and Xingxian are primarily shallow intrusive and volcanic facies, and the isotopic age of its syenite-porphyry is 125 million years. This is a strong evidence of magmatic activity (Tang et al., 2000). During the period of the Himalaya movement, the collision between the India plate and the Eurasian plate produced NEeSW compressive stress making the existing NEeSW trending compressive structures tensile to a certain extent (Fang et al., 2005). 3. Coal geology The primary coal seams for CBM development are the No. 4 þ 5 coal seam in the Permian Shanxi Formation and the No. 8 þ 9 coal seam in the Carboniferous Taiyuan Formation. These coals occur at a depth of 300e2600 m, with a thickness range of 0.5e30 m. The coals have a random vitrinite reflectance (Ro) ranging from 0.44% to 2.35% in most areas and a permeability range of 0.01e10 mD. The roof and floor of the main coal seams consist primarily of mudstone, sandy mudstone and sandstone (Jiang et al., 2012; Tang et al., 2012), which generally have low permeability and good sealing properties for CBM. And some limestone roof of the No. 8 þ 9 coal seam may have negative effects in the central and southern regions. Because the limestone roofs are usually also aquifers with groundwater activity that may dissolve gases and carry them out from CBM reservoir. 4. Methodology The data used in this study, such as coal burial depth, thickness, vitrinite reflectance, methane adsorption isotherm and gas content, were collected from the results of measurements and tests of the No. 4 þ 5 and the No. 8 þ 9 coal seams from 40 CBM wells. Vitrinite reflectance measurements were performed following ASTM D 2798-06 (2006) and ASTM D 2799-05a (2005a). Gas content data were obtained from gas content test reports of the CBM exploration wells, and measurements of the gas contents followed the Chinese National Standard GB/T19559-2004. Methane adsorption isotherm experiments on 14 coal samples were performed following the Chinese National Standard GB/ T19560-2004. All samples were crushed and sieved to a size range of 0.18e0.25 mm (60e80 mesh), and 100e125 g was weighed for the moisture-equilibrium treatment for at least four days. After these preparations, the adsorption isotherm experiments were performed at a temperature of 30  C and an equilibrium pressure up to 10 MPa. The data of geological events, the stratigraphic age, lithology, current burial depth and thickness, the abundance and types of organic matter, the erosion stratigraphic thickness resulting from tectonic uplift, and the paleo geothermal gradient were collected for the study of coal evolution in the study area. Then, the software of Basinmod was used to simulate the coal burial history, thermal evolution and hydrocarbon generation. The data of the stable carbon isotopes were obtained from the CBM test reports (Li et al.,

2014a; Li and Zhang, 2013). The stable isotopes of 13C/12C in the CBM were analyzed using the delta notation (d) relative to known standards of Pee Dee Belemnite (PDB). Gas and water production data are from the CBM development wells, which were on production for at least 200 days and reached a relatively stable production level. The structural controls on CBM accumulation and production were determined by examining structures with coal and gas properties. For further research, typical wells were chosen to examine the effects of the different structural parts on the gas content and production. 5. Results and discussions 5.1. Structural controls on the CBM reservoir 5.1.1. Structural controls on the coal rank, gas content and 13C/12C of CBM 5.1.1.1. Structural controls on coalification. The essence of coalification is a changing process of the coal physical and chemical structure caused by the pressure and temperature. The pressure and temperature are affected by the burial depth (Fang et al., 2005). Firstly, the subsidence range caused by tectonic activity controls the burial depth and the level of the coal metamorphism. Secondly, the abnormal thermal event of magmatic intrusion during the Yanshan movement is another critical factor resulting in the coal metamorphism. The present distribution characteristics of coal rank in the eastern margin of the Ordos Basin are controlled by the subsidence range and magmatic intrusion. During the Triassic, which is regarded as the significant burial period of the Late Paleozoic coal seams, the burial depth of the coal seams gradually increased to a maximum depth of 4000 m (Tang et al., 2000). During this period, the coal burial depth increased gradually from north to south with a maximum difference of 1500 m. Thus, the distribution of coal rank during the Triassic, controlled by the burial depth, increased from north to south. After the Indo Movement, with the basin retreating westward, the contours of Ro values rotated clockwise accordingly. In addition, the thermometamorphism caused by the magmatic activity during the Yanshan movement, superimposed on the previous coal maturity (Tang et al., 1992). Present Ro values of the Late Paleozoic coal gradually increase from the northeast to the southwest. For the No. 4 þ 5 coal seam in the Shanxi Formation, Ro values of the coal range primarily from 0.59% to 2.35%. However, Ro values are abnormally high (>4.0%) near the Zijin mountain and reduce around, affected by the Yanshanian magmatic activity. Igneous intrusion can affect coal metamorphism by cross cutting the coals with restricted halo or increasing regional geothermal gradient (Amijaya and Littke, 2006; Cooper et al., 2007; Moore et al., 2014). In the study area, the increase of geothermal gradient resulting from igneous intrusion could affect hundreds of square kilometers. In the north, such as Zhungeer, Hequ and Baode, Ro values are relatively low (0.59%e 0.81%). Ro values increase southwestward and can reach 1.56% in the middle, even reaching >2.0% in the south (Fig. 3). 5.1.1.2. Structural evolution controls on the hydrocarbon generation and adsorption capacity of coal. The hydrocarbon generation through coalification provides abundant gas resources for CBM reservoir. According to the coal burial and thermal evolution history in the study area, the coal seams have experienced two periods of hydrocarbon generation by thermal pyrolysis. The first period was in the Triassic with the burial depth of the coal seams increasing gradually pyrolysis of the organic material began, and the gas started to generate, corresponding to the initial period of thermal degradation. During this period, the gas generating

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Fig. 4. A burial and thermal history diagram of the Late Paleozoic coals of the eastern margin of the Ordos Basin.

potential increased from north to south controlled by the coal burial depth. The second period of hydrocarbon generation was during the Yanshan movement in the Jurassic-Early Cretaceous when magmatic intrusions caused coal thermometamorphism and secondary hydrocarbon generation (Tang et al., 2000). During this period, a significant amount of hydrocarbons were generated, particularly in the central regions of the study area, which provided a rich gas source supplementary to the CBM reservoir (Tang et al., 1999; Fig. 4). Coal metamorphism not only influenced the hydrocarbon generation but also the adsorption capacity of the coals. During the process of coal metamorphism, the physical and chemical structure, and the pore-fracture structure are significantly changed, possibly causing changes in the coal's adsorption (Yao et al., 2011; Gürdal and Yalçın, 2000). Based on the results of the adsorption isotherm experiments it can be observed that when the Ro values range from 0.5% to 1.9%, the Langmuir volume increased as the coal rank increased (Fig. 5). This was consistent with other previous studies (Moore, 2012; Laxminarayana and Crosdale, 1999). The results indicate that at higher the coal ranks, the greater the adsorption capacity of the coal over a certain Ro range, which would be more conducive to the retention of the CBM.

Fig. 3. A contour map of vitrinite reflectance of the No. 4 þ 5 coal seam along the eastern margin of the Ordos Basin.

Fig. 5. A graph showing the relationship between Langmuir volume and vitrinite reflectance of the coal samples studied.

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The gas content distribution shows a good correlation with the coal rank distribution in the study area. For example, gas content of the No. 4 þ 5 coal seam in the Shanxi Formation ranges from 1.5 to 16.8 m3/t. There are three areas special with gas contents >15 m3/t (i.e., the Sanjiao-Liulin, Daning-Jixian and Hancheng areas, Fig. 6). All of these areas are located in the middle or the south of the study area where the coal rank is relatively high. The contour map of the gas content shows it increases from NE to SW and this trend has a good correlation with the coal rank distribution characteristics. This indicates that the influence of the coal maturation caused by structural evolution on the hydrocarbon generation and adsorption capacity is crucial for CBM accumulation and retention. 5.1.1.3. Structural evolution controls on the 13C/12C and components of CBM. The d13C1 characteristics of CBM are effectively used to identify the gas origin (Scott et al., 1994; Liu et al., 1999; Sun et al., 2011) since the components and d13C1 characteristics of gases generated during the different stages of coalification vary significantly (Galimov, 1980; Law et al., 1993). In the early stage of coalification (Ro, max<0.5%), gases are generated via microbial fermentation (biogenic gas), is primarily CH4 that cannot be retained with shallow burial depth and macro-mesopore structures of coals (Moore, 2012; Li et al., 2013). Then thermogenic gases begin to be generated by thermal degradation (0.5% < Ro, max < 1.7%) and thermal pyrolysis (1.7% < Ro, max<2.5%) (Flores et al., 2008). During coalification, 12Ce12C chemical bonds break first followed by 12 Ce13C and 13Ce13C, resulting in d13C1 values for CH4 heavier as coalification continuing (Li et al., 2010). The d13C1 values for CH4 samples from the eastern margin of Ordos Basin range from 58.99‰ to 28.80‰ (Table 1), indicating that most of the CH4 is thermogenic, except for a few samples from Baode, which may be a mixture of thermogenic gas and secondary biogenic gas, which is in agreement with the findings of Tian et al. (2012a). Methane carbon isotope values become heavier from north to south in the study area in a similar pattern to the vitrinite reflectance (Fig. 7). The distribution of d13C1 for CH4 was influenced by the isotope fractionation as a result of coalification. Hydrodynamic effects and shallow burial depth of coals has most likely aided the generation of biogenic gas resulting in lighter d13C1 value for CH4 in the northern parts of the study area. When coalification passes the over mature stage (Ro, max > 2.5%), high temperature and pressure conditions gradually crack the pregenerated wet gas intoCH4. Hence, it can be expected that the proportion of CH4 in the CBM would increase and C2H6 would be reduced with the increasing maximum vitrinite reflectance. Results of gas molecular composition analyses show that CH4 is the primary component of the CBM, with percentages of ranging between 65.88% and 98.88% and an average of 90% (Table 1). However, the proportion of CH4 shows significantly regional variation, with a high percentage, generally over 90% to the south of the Liulin area, whereas it is generally <80% the north of the Liulin area. The regional variation of CH4 is consistent with the variation of coal maturity, indicating that the CH4 content of the CBM is controlled by the coal maturity of coalification. 5.1.2. The control of the structural properties on the CBM retention Different structures are formed by different tectonic stress fields and have different characteristics of internal tectonic stress distribution. Therefore, geological structures can affect the structure, occurrence and fracture development of the coal reservoir and its cap rock, and can affect the flow of the groundwater (Li et al., 2014c). 5.1.2.1. Monocline controls on gas content. As previously mentioned, gas contents of the coal seams in the study area increases from east to west, which is consistent with the monoclinic

Fig. 6. A contour map of gas content of the No. 4 þ 5 coal seam along the eastern margin of the Ordos Basin.

Y. Chen et al. / Journal of Natural Gas Science and Engineering 23 (2015) 524e537 Table 1 Bulk molecular composition, carbon isotopes of methane and vitrinite reflectance data for gas samples from the eastern margin of the Ordos Basin (Li et al., 2014a; Li and Zhang, 2013; modified). Site

Coal seam CH4 (%) CO2 (%) N2 (%) C2H6 (%) d13C1 (‰) Ro,

Baode Baode Baode Baode Baode Baode Baode Baode Baode Baode Baode Linxian Liulin Liulin Liulin Shilou Shilou Shilou Shilou Jixian Jixian Jixian Jixian Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng Hancheng

No.4 þ No.4 þ No.4 þ No.4 þ No.4 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.8 þ No.4 þ No.8 þ No.8 þ No.8 þ No.4 þ No.4 þ No.8 þ No.8 þ No.3 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.5 No.11 No.11

5 5 5 5 5 9 9 9 9 9 9 9 9 9 9 5 9 9 9 5 5 9 9

70.08 67.89 84.52 82.73 84.08 83.80 81.21 83.25 91.06 89.03 65.88 80.95 96.34 89.16 94.18 95.92 94.56 97.68 96.38 98.88 96.69 92.76 94.02 87.83 97.50 97.54 85.37 97.50 97.54 85.37 82.67 88.58 84.44 92.94 94.57 95.57 95.84 96.36 93.59 94.24 88.28

4.26 2.78 6.65 0.36 0.51 5.01 8.31 3.67 2.13 4.60 21.87 2.40 0.56 4.90 2.84 1.31 0.82 1.32 2.96 0.81 1.56 2.98 3.34 0.76 0.81 0.70 0.77 0.81 0.70 0.77 2.21 0.60 1.32 1.37 1.20 1.20 1.45 0.62 0.57 2.00 1.00

25.19 29.23 8.66 16.87 15.32 11.13 10.41 13.05 6.78 6.08 11.56 12.64 3.09 5.77 2.98 2.77 4.62 1.02 0.67 0.28 1.75 4.13 2.48 11.31 0.65 0.59 13.51 0.65 0.59 13.51 14.65 10.06 13.35 4.53 3.07 2.59 2.03 2.87 5.68 3.50 10.67

0.04 0.02 0.04 0.02 0.03 0.03 0.04 0.03 0.03 0.03 0.14 4.01 0.17

0.03 0.11 0.15 0.10 1.03 1.16 0.35 1.03 1.16 0.35 0.47 0.76 0.89 1.32 1.35 0.63 0.67 0.15 0.16 0.26 0.05

51.26 58.99 53.90 50.80 51.50 55.52 54.20 53.20 52.90 57.80 57.70 58.00 48.77 55.31 50.58 41.15 41.77 43.53 38.02 42.90 37.80 38.02 40.80 36.20 33.50 34.10 35.90 33.50 34.10 35.90 35.00 41.60 39.10 38.60 38.80 37.60 28.80 35.80 40.10 37.40 37.40

max

(%)

0.75 0.83 0.76 0.81 0.74 0.86 0.78 0.89 0.79 0.75 1.12 0.98 1.33 1.50 1.80 1.64 1.76 1.75 1.76 1.63 1.60 1.90 2.14 2.03 2.19 2.06 2.34 2.19 2.06 2.34 2.47 2.52 2.59 2.57 2.54 2.22 2.23 2.07 2.05 2.10 2.09

geological background. In addition, in the Sanjiao block, which is a NW dipping monocline located in the middle of the study area with simple tectonic conditions and rare faults development (Fig. 8). Six CBM wells located from east to west in the Sanjiao block were selected to compare variations in gas content. The well section

Fig. 7. A graph showing the relationship between d13C1 values for CH4 of the CBM and maximum vitrinite reflectance of the coal samples studied.

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(Fig. 9) indicates that from east to west, the gas content increases with the increasing coal burial depth. Well SJ-06 located in the west with a deeper burial depth and a higher reservoir pressure compared to the other wells has a high gas content of 12.85 m3/t. In contrast, well SJ-06 located in the east with a shallower burial depth and a lower reservoir pressure has a low gas content of only 0.5 m3/t. 5.1.2.2. Reverse fault and fold controls on the gas content. A series of NWeSE trending reverse faults and secondary folds that developed along the eastern margin of the Ordos Basin underwent the three large-scale tectonic movements. The syncline is typically dominated by compressive stress, and its hydrodynamic conditions typically belong to the stagnant zone as defined by Pashin (2005, 2007). The groundwater flows from the two wings of the syncline to the core, forming a favorable trap for CBM enrichment, combined with a thick cap above the coal seams in the syncline core area. To the north of the Hancheng block where certain small-scale synclines present, the contour map (Fig. 10) of gas contents indicate high gas contents primarily distributed near the synclines. The syncline core regions have certain favorable conditions for enrichment and retention, such as a deep burial depth, compressive stress field and rare fracture development. The effect of the anticlines on the CBM gas contents is more complex with different structural stress fields. The axial part of the anticline above the neutral plane along with the development of tensile fractures in the tensile stress field and the relatively shallow burial depth cannot effectively seal the CBM and may allow the gas to escape from the coal reservoir. This is demonstrated by looking at gas content variations of the EeW trending well section (Fig. 11) in the Daning-Jixan block. Gas contents at the axial part of the syncline are higher (Well J10; 18.76 m3/t) compared to those at the axial part of the anticline (Well J4; 12.60 m3/t). Reverse faults often develop along with the folds in a compressive stress field. For example, a series of NEeSW trending reverse faults and fold combinations developed in the Daning-Jixan block. Typically, the reverse fault plane has a good seal to prevent the gas from migrating through it, and the associated folds could become favorable traps for gas accumulation; thus, the structural combinations of reverse faults and the associated folds are favorable for the CBM enrichment, and they typically have high gas contents. In the middle of the Daning-Jixan block, gas contents near the reverse faults and folds are as high as >20 m3/t (Fig. 12), observably higher than other regions without the presence of reverse faults and folds. This gas content distribution pattern proves that a suitable combination of reverse faults and folds is favorable for the CBM enrichment and retention. 5.1.2.3. Normal fault controls on gas content. A normal fault can be characterized as a tensional fault, and its relative open fault plane typically becomes a good migration channel for gas. In addition, the part near the normal fault plane typically has relatively low stress that may cause a large amount of CBM to desorb from coal reservoir, resulting in a reduction in the gas content (Pashin and Groshong Jr., 1998; Wang et al., 1993; Smith, 1995). However, both of the sides distant from the normal fault plane typically form two high stress regions favorable for gas adsorption. The typical cases with these characteristics are the Yanchuanan block and the Hancheng block. For example, in the Yanchuannan block the NEeSW trending normal faults are the primary structures in this area, among which there are two relatively large-scale faults, the F09 and F10 faults, breaking the coal seam along with its roof and floor (Figs. 13 and 14). The two normal faults have a significant influence on the gas content distribution. Gas contents in the regions near the F09 and F10 faults are extremely low, with values approximately less than

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Fig. 8. A map showing the elevation of the top of the No. 4 þ 5 coal seam in the Sanjiao Block.

Fig. 9. A well section showing gas variation in different parts of the monocline in the Sanjiao Block.

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Fig. 12. A contour map showing the relationship between structure and gas content of the No. 4 þ 5 coal seam in the Daning-Jixian block.

Fig. 10. A contour map showing the relationship between structure and gas content of the No. 4 þ 5 coal seam of the northern area in the Hancheng block.

2 m3/t. However, further away from the two faults, gas contents gradually increase to 12 m3/t. The F12, F13 and F14 faults affect gas contents next to the F09 and F10 faults, and gas contents near the three faults are lower than their surrounding areas, with a reduced value of less than 8 m3/t (Fig. 14). Therefore, a normal fault can break the seal of the CBM reservoir causing gas to migrate and

escape through its fault plane. Present day gas contents could be very low in areas near normal faults. 5.2. Accumulation and enrichment model of the CBM As previously mentioned, CBM along the eastern margin of the Ordos Basin is dominated by thermogenic gas, except for a small amount of secondary biogenic gas present in the low rank coal in the north or along the eastern edge (Li and Zhang, 2013; Tian et al., 2012a). The coal underwent two primary gas generation periods;

Fig. 11. A well section showing gas content variation in different parts of the syncline and anticline in the Daning-Jixian block.

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Fig. 13. A northwest-southeast seismic section of the Yanchuannan block showing normal fault development.

Fig. 14. A contour map of gas contents distribution in the Yanchuannan block showing the relationship between gas contents and normal faults.

Fig. 15. A accumulation model for the CBM reservoir in the eastern margin of the Ordos Basin.

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Fig. 18. A contour map showing the distribution of gas production rates and groundwater hydraulic zones in the Baode block. Fig. 16. A contour map showing the distribution of gas production and groundwater hydraulic zones in the Liulin block.

the first period was in the Triassic as a result of the burial depth increasing, generating a maximum of 97.8 m3/t of gas. The second period was during the Yanshan movement in the Jurassic-early Cretaceous due to magmatic intrusions, generating a maximum of

Fig. 17. A diagram showing the variation of gas and water production rates from the CBM wells in different structural parts of a monocline in the Liulin block.

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Fig. 19. Graphs showing the variation of gas and water production rates from the CBM wells in different structural parts of a nose structure of the Baode block.

120 m3/t of gas (Tang et al., 2000). The CBM reservoir in the study area underwent accumulation during the Triassic and Jurassic-early Cretaceous; particularly, the compressive structures formed during the Yanshan movement provided a very favorable trap for gas accumulation. However, the NWeSE trending tensional stress field during the Himalaya movement had a tensional effect on the compressive structures, resulting in the escape of gas from the reservoir. The eastern margin of the Ordos Basin, as a whole large-scale monocline, combined with the hydrological conditions, can form the hydrodynamic seal for the CBM reservoir (Tian et al., 2012b). Meteoric water, supplied from the eastern outcrop of the coal seam or aquifer, flows from east to west along the stratigraphic tendency. With the flowing distance of the groundwater, the flowing speed of the groundwater gradually decreases, and the salinity increases, finally forming a weak runoff-stagnation area that can prevent the CBM from escaping along the updip direction and becomes a favorable area for CBM enrichment. From the eastern edge to the westward center of the Ordos Basin, the reservoir pressure and vitrinite reflectance increase, thus increasing the amount of the adsorbed CBM. Based on the geological characteristics of the study area, this study summarized a monoclinic-hydraulic sealing accumulation model as the typical CBM accumulation model (Fig. 15). 5.3. High gas production models and typical development cases Structure is a key factor affecting the gas and water production of the CBM wells (Pashin et al., 1995; Groshong Jr., 2004; Meng et al., 2014). Production data from CBM wells in the eastern margin of the Ordos Basin shows that the CBM wells located in the structural high part of the weak runoff-stagnant groundwater zone typically have the following characteristics: the reservoir pressure

Fig. 20. A contour map showing the distribution of gas and water production rates and groundwater hydraulic zones in the Hancheng block.

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Fig. 21. Graphs showing the variation of gas and water production rates from the CBM wells in different structural parts of a normal fault affecting zones of the Hancheng block.

drops rapidly through drainage, gas production increases rapidly, and water production remains low. These characteristics could maintain stable, high gas production for a relatively long period. Based on the production characteristics of these three types of high gas production models are presented: updip of the monocline, the axial part of the anticline or nose structure, and a structural high far from the normal fault. 5.3.1. High gas production model of updip of the monocline and a typical case The monocline, with a slight transformation intensity and rare development of faults or folds, is favorable for CBM development. The Liulin block is a typical case located in the middle of the study area near the edge of the basin (Fig. 2), with simple structural conditions, rare fault development, a shallow depth of 300e1100 m, permeability of 0.01e4.8 mD, and gas contents of 5.8e16.2 m3/t. The block is a SW trending monoclinic structure, with a burial depth that increases gradually from northeast to southwest, and the groundwater flows along the stratigraphic tendency from northeast to southwest. Hydrological conditions in the Liulin block can generally be regarded as the weak-runoff or stagnant zone (Gao et al., 2012; Meng et al., 2014; Li et al., 2014c), generally having a high gas content, except for a small area in the northeast (Fig. 16). The regional distribution of the gas production indicates that the CBM wells in the structural high part of the monoclinic updip, typically with a burial depth of <700 m in the east of the Liulin area, have a relatively high gas production rate. As an example, for the L well group gas production reached a maximum value of 3600 m3/d in a short drainage time of approximately 400 days; in contrast, gas production from the wells in the structural low part in the west area with a larger burial depth was lower. Well F13, which has a low gas production in the western area, only reached gas production rates of less than 100 m3/d after a drainage period of 400 days (Fig. 16). For further research, a well section of the L well group was built to understand the relationship between the variation in the gas production rate and the structural properties (Fig.17). This section shows

that gas production from the CBM wells in the monocline structure increased from the structural low to the structurally high updip. The gas production of well L-01 in the deepest part of the structure part of the well-connecting section was only 500 m3/d, whereas well L-04 produced approximately 2000 m3/d. However, the water production was generally less than 10 m3/d for all wells in the well section because the L well group was located in the structural high of the Liuling block which is considered a weak-runoff zone. 5.3.2. High gas production model of the axial part of the anticline or nose structure and a typical case The Baode block is a west dipping monocline in the northern part of the eastern margin of the Ordos Basin with the development of a nose structure in the northern area (Figs. 2 and 18). The total dissolved solids (TDS) of the groundwater ranges from 1000 to 5000 mg/L, increasing from the southeast to the northwest, and the hydraulic zones are successively a runoff zone, a weak-runoff zone and a stagnant zone. The major coal seam for CBM development is the No.4 þ 5 coal seam of the Shanxi Formation, with a burial depth of 400e1200 m, a thickness of 1.19e20.21 m, permeability of 0.4e12 mD and gas contents of 4e9 m3/t. The CBM wells with a relatively high gas production of over 2000 m3/d were primarily concentrated in the zone near the axial part of the nose structure. For example, well BD-1, located in the axial part of the nose structure, showed a high gas production of 6000 m3/d, a casing pressure of 2.55 MPa and a cumulative gas production of 184  104 m3 (Fig. 18). The major factors for the high gas production are as follows: first, the wells were located in the axial part of the nose structure, which was favorable for the decreasing reservoir pressure and the CBM desorption. Second, fractures developed to a certain extent in the axial part of the nose structure within a tensional stress field, which were beneficial for water drainage and gas migration and production. Finally, the axial part of the nose structure located in the weak-runoff or stagnant zone with a relatively weak hydraulic force was conducive to the CBM accumulation and retention. For further analysis, an evaluation of four wells in the different parts of

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the nose structure, BD-1, BD-2, BD-3 and BD-4, showed a gas production difference (Fig. 19). Well Bd-1 located in the axial part of the nose structure showed the highest gas production of 6000 m3/ d of the four wells and a relatively low water production of approximately 10 m3/d. The gas production of the wells in the well section showed a decreasing trend from the axial part of the nose structure to the wing, resulting in the lowest gas production rate of approximately 1000 m3/d and a relatively high water production of 20 m3/d from well BD-4 located in the deepest part of the wing. 5.3.3. High gas production model of a structural high far from the normal fault and a typical case The Hancheng block is located to the south of the eastern margin of the Ordos Basin, representing a typical case of the effect of normal faults on CBM development (Fig. 2). This block is a NWeSE dipping monocline with a NEeSW trending normal fault that developed in the northwest. The burial depth of the coal seam ranges from 350 to 800 m and its elevation decreases from the southeast to the northwest in the eastern region of the normal fault. Currently, there are 80 CBM development wells in this area with a drainage time of 3e8 years, a maximum gas production rate of 7600 m3/d, a maximum water production of 62.8 m3/d, and an average of 0.47e36.5 m3/d. The wells with a gas production rate of over 1500 m3/d are primarily located in the middle area in the groundwater stagnant zone with a low water potential and high mineralization. In contrast, the wells located in the northeast and southwest with a high water potential and low mineralization typically show low gas production rate, high water production rate and reservoir pressures that are difficult to decrease (Fig. 20). The well section shows that the gas production rate increases, and the water production rate decreases as the distance increases away from the normal fault. The wells near the fault typically show low gas production and high water production rates, such as well HC01 with no gas production and well HC02 with low gas production rate of 700 m3/d and high water production rate of 20 m3/ d (Fig. 21). However, wells HC03, HC04 and HC05, located in the monocline updip, reached gas production rates of 3000 m3/d, 2500 m3/d and 2600 m3/d, respectively, whereas their water production values were as low as approximately 1 m3/d. The variations in gas and water production rate indicate that the normal fault had a negative effect on CBM development due to groundwater forced to flow from the aquifer to the coal seam, resulting in a high water production and difficulty in decreasing the reservoir pressure. The wells distal to the normal fault were only slightly affected. In addition, the gas desorbed from the structural low could migrate along updip to supply wells in the structural high. Therefore, wells far from the normal fault in the structural high part of the monoclinic updip could achieve stable and sustained high gas production rates. The tensional fractures and normal faults developed during the Himalaya movement transformed the CBM reservoir, breaking the seal of the reservoir and causing the gas to escape, decreasing gas saturation to a certain extent. In additional, the coal seam may connect with nearby aquifers through normal faults and fractures, resulting in coal reservoir pressure decreasing difficultly. However, the tensional fractures and normal faults could increase the permeability of the coal seam, which was favorable for the gas and water migration and production, and was typically observed in a well near a normal fault having a high water production. 5.3.4. Mechanism of the high gas production from a CBM well in structural highs The gas production models and the cases above indicate that wells in structural highs within the groundwater weak-runoff or stagnant zone could easily achieve a high gas production rates based on the following points: (1) The groundwater weak-runoff

and stagnant zones are favorable for the CBM enrichment and retention and typically have high gas contents, providing a source with a large amount of CBM; however, the groundwater supplement is extremely slow, which is favorable for decreasing the reservoir pressure through drainage. (2) The groundwater replenishment of the structural high part is slow; thus, the reservoir pressure can be effectively decreased through a short drainage period, and a large amount of adsorbed CBM would desorb from the surface of the coal to be produced through the well, (3) Structural highs occur typically in a low stress field with tensional fractures that developed to improve permeability, which is favorable for the gas migration; in addition, a structural high represents late stage structural uplift of the basin where secondary cleats have developed well to provide channels for the CBM migration. (4) When tectonic stresses are released because of water drainage, the coal shrinks, and the fluid pressure decreases to form a low water potential zone in the stress released area, and then groundwater flows carrying gas from the high water potential zone to the low water potential zone. The result is that the absorbed gas of the structural low migrates to the structural high through groundwater flow and is produced through the CBM wells. 6. Conclusions (1) The structural evolution of the basin has influenced coal maturity, the internal structure of the coal and its reservoir capacity; in addition, it also controlled the CBM generation, accumulation and retention and affected the genetic types and components of the CBM. Structural properties influenced the CBM reservoir differently; for example, gas content increases as the burial depth increases along the trend of the monocline. The syncline and reverse fault with a compressive stress field are favorable for CBM enrichment and retention, whereas the axial part of the anticline with the development of fractures allows gas to escape and has a negative influence on the reservoir. The normal fault with developed tensional fractures can also result in gas escape. (2) The CBM of the eastern margin of the Ordos Basin is primarily of thermogenic origin that was generated and accumulated during the Triassic and Jurassic-Early Cretaceous. Upon combining the structural evolution with the hydrological conditions, gas content increases as the burial depth and groundwater mineralization increase. Thus, a monoclinic hydraulic accumulation model was used to describe the characteristics of the CBM reservoir. (3) The CBM wells in the structural highs with a weak hydrodynamic force typically showed high gas production rates, low water production rates and rapidly decreasing reservoir pressures. Based on the geological conditions and the CBM development cases, the following three types of high gas production models were presented: updip of the monocline, the axial part of the anticline and nose structure, and the structural high far from the normal fault. Acknowledgments We would like to thank the China United CBM National Engineering Research Center and PetroChina CBM Company Limited for providing CBM well data and coal samples for this study. This work was supported by the Major National Science and Technology Special Projects (Grant No. 2011ZX05038-001, 2011ZX05062-01), the Ministry of Land and Resources of the People's Republic of China special funds for scientific research on public causes (Grant No. 201311015), and the National Natural Science Foundation (NSFC) Project (Grant No. 41272175).

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