Study on Coalbed Methane Accumulation Characteristics and Favorable Areas in the Binchang Area, Southwestern Ordos Basin, China

International Journal of Coal Geology 95 (2012) 1–11

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International Journal of Coal Geology

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Study on coalbed methane accumulation characteristics and favorable areas in the Binchang area, southwestern Ordos Basin, China

H. Xu a,b,⁎, D.Z. Tang a,b, D.M. Liu a,b, S.H. Tang a,b, F. Yang c, X.Z. Chen a,b,W.Hea,b, C.M. Deng a,b a School of Energy Resources, China University of Geosciences, Beijing 100083, PR, China b Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development & Utilization, Beijing 100083, PR, China c Department of Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO 65401, USA article info abstract

Article history: The Binchang area of southwestern Ordos Basin is one of the most promising areas for low-rank coalbed Received 30 January 2012 methane (CBM) in China. This work investigates the Jurassic Yanan Formation coal and CBM geology and Received in revised form 1 February 2012 accumulation characteristics of CBM in the southwestern Ordos Basin based on data from 46 wells and labo- Accepted 6 February 2012 ratory measurements of 14 coal samples from 7 mines. The results show that coal rank in the Binchang area is Available online 14 February 2012 mainly sub-bituminous A and high-volatile C bituminous (0.46%–0.73%Ro). Coals are dominated by inertinite (14.7–85.6%); less abundant are vitrinite (8.5–77.7%) and liptinite (1.5–15.2%). Minerals are found only in Keywords: – Ordos basin small amounts (0.4 8.3%). Permeability is between 0.04 and 25.3 mD, and porosity ranges from 2.4% to Low-rank coal 20.1%. Most coal pores are less than 100 nm in diameter, making them favorable for gas adsorption but Coalbed methane unfavorable for gas permeability. Pore morphology is represented mainly by micro- and mesopores with a Accumulation characteristics well-connected and ink-bottle shaped (narrow throat and wide body) morphology. These coals are charac- Binchang area terized by a high adsorption volume of more than 3.0×10−3 ml/g. Methane isothermal adsorption measurements of 12 coal samples revealed that their maximum adsorption capacity (on a dry and ash-free basis) varies from 5.06 to 13.37 m3/t, depending on moisture content. However, under the inﬂuence of gas preservation conditions, the in-place gas content is generally 0.11–6.26 m3/t. Finally, based on a comprehensive analysis of coal thickness, gas content, hydrogeology conditions, roof, ﬂoor, and depth properties, this study indicated that the best prospective target areas for CBM production are forecasted to be the Tingnan and Dafosi areas, which are located in the syncline, central south part of the study area. © 2012 Elsevier B.V. All rights reserved.

1. Introduction the assessment of CBM potential (Long and Wang, 2005; Xu et al., 2010). Until now, however, the Binchang area has not had any com- Early coalbed methane (CBM) exploration and development in the mercially producing CBM wells, although there have been a number 1990s targeted middle and high-rank coals distributed throughout of companies exploring the area, conducting research and assessing the southern Qinshui Basin and the eastern Ordos Basin, China (Cai the size and nature of potential resources. Multidisciplinary and sys- et al., 2011; Li et al., 2011; Wei et al., 2010). However, in recent tematic studies on CBM are still needed. This paper presents a com- years, with the successful development of CBM in the Powder River prehensive study of coal geology, CBM reservoir properties, and Basin, Uinta Basin, and Raton Basin in the United States (Ayers, accumulation characteristics in the Binchang area to evaluate the 2002; Flores, 1998) and in the Surat Basin of Australia (Scott et al., potential for CBM production. 2007), low-rank deposits have also been shown to be economically signiﬁcant. China has a large amount of low-rank CBM resources 2. Geological setting (Wang, et al., 2009), which are becoming a focus of much research and offering a new ﬁeld of CBM exploration and development. 2.1. Tectonic setting The Binchang area in the southwestern Ordos Basin is one of the most potential areas for low-rank CBM in China. Previous investiga- The Ordos Basin is situated in central China with an area of tions of the CBM in this area have focused on the geological back- 250,000 km2 (Fig. 1). It is a stable polycyclic sedimentary basin, ground and coal geology (Lin et al., 2009; Wang et al., 2004)oron which was formed on the North China Craton (Tang et al., 2012; Xu et al., 2011). The Ordos Basin contains the second largest coal resources in China, next only to the North China Coal Basin. The ⁎ Corresponding author. Tel.: +86 10 82320973; fax: +86 10 82326850. coal-bearing deposits of the basin consist of Pennsylvanian, Permian, E-mail address: [email protected] (H. Xu). Triassic, and Jurassic strata (Dai et al., 2002, 2006; Wang, 1996).

Fig. 1. Location of the Binchang area in China (1—Gaojiapu exploration area, 2—Yadian exploration area, 3—Bindong exploration area, 4—Yangjiaping exploration area, 5—Mengcun mine field, 6—Hujiahe mine field, 7—Wenjiapo exploration area, 8—Yanjiahe mine area, 9—Tingnan mine area, 10—Xiaozhuang mine field, 11—Huoshizui mine, 12—Baizigou mine area, 13—Dafosi mine, 14—Xiagou mine, 15—Shuilian mine, 16—Hushengou mine, 17—Shizui mine, 18—Yangshan mine, 19—Jiangjiahe exploration area, 20—Liushicun mine, 21— Heigou mine; A-A' is the location of the hydrogeological profile in Fig. 13).

Abundant coal resources (shallower than 2000 m) occur in the in this area dip gently towards the northwest at an angle of less Permo-Pennsylvanian (421 Gt), Triassic (0.67 Gt), and Jurassic (119 than 10°. The area was structurally altered, with numerous broad Gt) sequences (Wang, 1996). and gently dipping anticlines and synclines with a W–E strike. The basin is divided into six structural units (Fig. 1): the Yimeng uplift, Weibei uplift, Western edge thrusting belt, Jinxi flexural fold 2.2. Coal-bearing strata and depositional environments belt, Tianhuan depression, and Yishan slope (Zhang et al., 2009). Tectonically, the Binchang area is located in the northern margin In the Binchang area, the main coal-bearing sequences occur in the of the Weibei uplift belt (Fig. 1), which is a monoclinal structure Middle Jurassic Yanan Formation (Nos. 1–4 coal seams), which con- where faults cover only a small area. The Jurassic coal-bearing strata tain 3 members (Lin et al., 2009). The upper member is composed H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11 3 of gray mudstone, siltstone, and thinly bedded coal, most of which divided into a large block (approximately, 15×15×15 cm), a moder- has been eroded. The middle member of the Yanan Formation is com- ate block (approximately, 5×5×5 cm) and small pieces for thin posed of gray mudstone, muddy siltstone, siltstone, carbonaceous sections. mudstone, and thin coal. The bottom of the middle member consists Mean vitrinite reflectance (%Ro) measurements and maceral an- of medium-grained sandstone. The lower member is composed of alyses (500 points) were performed on the same polished section sandstone, siltstone, mudstone, and gray-brown bauxite mudstone; of the coal samples using a Leitz MPV-3 photometer microscope, the central part contains a thick coal seam (Fig. 2). The most econom- according to ISO 7404.3-1994 (1994) and ISO 7404.5-1994 ically significant coal seam of the Yanan Formation is No. 4, which (1994), respectively. Twelve samples were analyzed for proximate occurs in the Lower member. This seam is 1.74–21.3 m thick (about analysis (Table 4), including ash yield, moisture content and vola- 9.23 m on average) and is thicker in the Dafosi and Tingnan mines tile matter, following Chinese national standard GB/T 212-2008 than that in other areas (Fig. 1). The burial depth of the No. 4 coal (2008), which correspond to the ISO 1171-1997 (1997), ISO 562- seam is about 1300 m in the northwestern area and 300 m in the 1998 (1998),andISO 11722-1999 (1999). However, the differ- southeastern area (Table 1). ences between GB/T 212-2008 (2008) and above ISO standards include: (1) the determination of moisture content by the air dry 3. Samples and experiments method was added, (2) the temperature is increased to 900± 10 °C in 3 min and lasted for 7 min during the determination of The data used in this paper, such as the coal thickness, vitrinite volatile matter in the Chinese national standard GB/T 212-2008 (2008); however, the temperature is increased to 900±5 °C in reflectance (%Ro,ran), coal burial depth, roof lithology, floor lithology, in-place gas content and hydro-geology, were collected from 46 4 min for ISO standard. For moisture content determination, all exploration wells. Both these data were analyzed and these wells coal samples were crushed and sieved to less than 0.2 mm, and were drilled by the Coal Geological Bureau of Shanxi Province. 1±0.1 g sample was then weighed for the aeration drying at a Based on these data (Tables 1, 2), the distribution and the character temperature of 105–110 °C for 1 h. of the CBM geology conditions in the Binchang areas were analyzed. Mercury porosimetry analyses were performed using an Autopore Fourteen fresh samples were taken directly from the working III 9420 instrument (Table 5), which automatically registers pressure, faces of seven underground mines in the Binchang area (Fig. 1; pore diameter, intrusion volume, and surface area. Before porosime- Table 3). Most of the coals collected were big blocks with a weight try analysis, all samples were dried at 75 °C for 48 h (Liu et al., about 2 kg. Samples were numbered, carefully packed, and were 2009). The implementation standard was the ISO 15901.1-2005 returned to the laboratory for experiments. Each of the samples was (2005). Due to unsuccessful sample preparation, the experiment of mercury porosimetry analyses of samples Yang-1 and Ya-1 were not carried out. A total of 12 samples (Table 6) were sieved to a maximum size of 3.35 mm, dried at 105 °C for 24 h in a vacuumed oven, sieved again to 0.23–0.45 mm, and then dried again (Zhang et al., 2010). Low-temperature nitrogen adsorption analysis was then carried out following the ISO 15901.2-2006 (2006) standard with a modified Micromeritics ASAP-2000 automated surface area analyzer. Accord- ing to Chinese standard GB/T 19560-2008 (2008), methane adsorption isotherm experiments on 12 samples were performed at the Gas Research Center, Langfang Branch of Research Institute of Petro- leum Exploration and Development. All coal samples were prepared by crushing and sieving to a size of 0.18–0.25 mm (60–80 mesh), and then 100–125 g samples were weighed for the moisture- equilibrium treatment. The moisture-equilibrium treatment for each sample was processed for at least four days. After these pretreat- ments, the coals were put into the sample cell of the IS-100 for the adsorption isotherm experiment. The experimental temperature and equilibrium pressure were 30 °C and up to 10 MPa (Yao et al., 2009). The gas-in-place data and the hydrologic and water data were collected by the authors from the Coal Geological Bureau of Shanxi Province.

4. Results and discussions

4.1. Geological characteristics of CBM reservoirs

4.1.1. Petrological composition and the rank of the Bincheng coals Coals from the Binchang area are mainly sub-bituminous A and high-volatile bituminous based on the ASTM D388-99 (2005). The vitrinite reﬂectance ranges from 0.46% to 0.73%, with a mean of 0.6% (Tables 1, 3). It decreases from the west to the east (Fig. 3) and has a positive relationship with the present burial depth of the coal seam. In the Binchang area, the maceral composition is dominated by inertinite (14.7–85.6%), followed by vitrinite (8.5–77.7%) and liptinite (1.5–15.2%). Minerals present in the coal are in small amounts Fig. 2. Stratigraphic column of Jurassic coal-bearing strata in the Binchang area. (0.4–8.3%) (Table 3). 4 H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11

Table 1 Coal thickness, depth, vitrinite reﬂectance, roof lithology, ﬂoor lithology and gas content of the Yanan formation No.4 coal seam in the Binchang area.

3 Mine area Well Coal thickness (m) Ro,ran (%) Depth (m) Roof lithology Floor lithology In-place gas content (m /t) Mengcun M1 10.23 0.66 868.42 Silty Mudstone Muddy Siltstone 0.26 Gaojiapu G1 10 0.71 799.46 Silty Mudstone Muddy Siltstone n G2 5.6 0.73 1361.75 Silty Mudstone Muddy Siltstone n Bindong B1 7.5 0.62 486.26 Silty Mudstone Muddy Siltstone n Dafosi D1 16.29 n 309.96 Mudstone Mudstone 2.81 D2 16.91 n 550.58 Mudstone Mudstone 3.72 D3 19.38 0.56 391.92 Silty Mudstone Mudstone 3.01 D4 18.29 n 555.95 Silty Mudstone Mudstone 4.1 D5 17.45 0.54 444.87 Mudstone Mudstone 4.3 D6 15.14 n 512.13 Silty Mudstone Mudstone 6.12 D7 1.74 0.62 649.36 Silty Mudstone Mudstone 0.56 Heigou H1 3.84 n 338.2 Silty Mudstone Sandstone n H2 3.28 n 407.9 Silty Mudstone Sandstone 0.11 H3 5.45 n 365.25 Silty Mudstone Sandstone n Huoshizui HS1 12.4 n 600.63 Siltstone Siltstone n HS2 4.55 n 590.2 Siltstone Siltstone 1 HS3 2.45 n 455.24 Siltstone Siltstone n Jiangjiahe J1 4.38 n 522.8 Silty Mudstone Mudstone 2.12 J2 8.76 n 510.15 Mudstone Mudstone 0.96 J3 11.05 n 636.8 Silty Mudstone Mudstone n J4 8.95 n 601.35 Mudstone Mudstone n Yadian YD1 17.05 n 783.33 Silty Mudstone Mudstone n Tingnan T1 12.32 n 486.86 Silty Mudstone carbonaceous mudstone 4.78 T2 8.84 n 459.39 Silty Mudstone Silty Mudstone n T3 9 0.62 450.87 Mudstone Muddy Siltstone 0.2 T4 15.19 0.6 439.3 Mudstone Carbonaceous mudstone n T5 15.8 n 400.9 Silty Mudstone Mudstone 3.71 T6 13.05 n 436.82 Silty Mudstone Carbonaceous mudstone 0.4 T7 21.3 n 476.22 Mudstone Carbonaceous mudstone 6.26 Yanjiahe YJH1 7.3 0.61 531.03 Silty Mudstone Muddy Siltstone 0.31 YJH2 7.14 n 508.93 Silty Mudstone Muddy Siltstone n YJH3 4.45 n 467.2 Silty Mudstone Muddy Siltstone 1.07 Yangjiaping YJP1 11.12 n 793.15 Silty Mudstone Muddy Siltstone 0.5 YJP2 4.09 0.68 790.46 Mudstone Siltstone 0.71 YJP3 3.9 n 709.93 Silty Mudstone Muddy Siltstone 0.41 YJP4 4.09 n 648.08 Silty Mudstone Muddy Siltstone 0.98 YJP5 7 0.68 810.05 Silty Mudstone Siltstone 2 YJP6 12.44 n 969.93 Silty Mudstone Siltstone 2.75 Wenjiapo W1 10.71 n 685 Silty Mudstone Silty Mudstone 0.69 W2 4 n 581 Silty Mudstone Silty Mudstone n W3 3.56 0.68 543 Silty Mudstone Silty Mudstone n Shuilian S1 5.45 n 753.17 Mudstone Mudstone 2.58 Xiagou X1 10.7 n 402.66 Mudstone Muddy Siltstone 2.18 Liushicun L1 4.3 n 242.93 Silty Mudstone Sandstone n L2 4.41 0.51 332.7 Silty Mudstone Sandstone n L3 3.75 n 309.58 Silty Mudstone Sandstone n

Refer to Fig. 1 for mine area locations and Fig. 8 for well locations, n=not analyzed.

4.1.2. Physical properties of CBM reservoirs parameter, which directly affects the CBM enrichment and production (Mastalerz et al., 2008; Sing, 1995). Based on the knowledge 4.1.2.1. Pore structure characteristics. The pore structure, including of the nature of coal and its pore system, previous researchers the pore size, pore size distribution, and the geometry and morphology of the interconnecting pore network, is the key reservoir Table 3 Petrographic composition of the Yanan Formation No. 4 coal mine samples studied. Table 2 Water data of the No.4 coal seam in Bingchang area. Sample Coal mine Vitrinite Inertinite Liptinite Mineral Ro,ran no. (vol. %) (vol. %) (vol. %) (vol. %) (%) Well TDS (g/l) Water type Water yield (l/s m) Ting-2 Tingnan 39.3 45 15.2 0.5 0.54

G1 10.334 SO4·Cl–Na 0.00085-0.00381 Ting-4 Tingnan 76.6 14.7 7.6 1.1 0.52 D1 15.82 Cl–Na n Ting-6 Tingnan 46 48.1 5.9 n 0.52 D4 15.79 Cl–Na 0.00007-0.00115 l Huo-1 Huoshizui 8.5 85.6 5.1 0.8 0.54 D5 15.91 Cl–Na n Huo-3 Huoshizui 8.9 81.3 7.2 2.6 0.68

D7 14.12 SO4·Cl–Na n Huo-6 Huoshizui 12.8 83.7 2.3 1.2 0.65

H3 11.16 SO4·Cl–Na n Hei-1 Heigou 16 73.4 10.2 0.4 0.48

YD1 11.58 SO4·Cl–Na n Hei-6 Heigou 38.5 58.5 2.4 0.6 0.46

T2 13.5 SO4·Cl–Na 0.000089–0.003 Hei-8 Heigou 77.7 17.1 3.8 1.4 0.53 T4 13.86 Cl–Na n Xia-1 Xiagou 18 71.8 2 8.2 0.62

YJP5 10.21 SO4·Cl–Na 0.00336 Xia-2 Xiagou 10.4 82.7 1.6 5.3 0.55

W3 12.21 SO4·Cl–Na 0.00007–0.001065 Shui-1 Shuilian 15.1 70.4 11.1 3.4 0.59

HS4 15.45 SO4·Cl–Na 0.000046–0.000079 Yang-1 Yangjiaping 42.9 51.4 5.7 6.7 0.66

L1 10.68 SO4·Cl–Na 0.000243 Ya-1 Yadian 26.1 64.1 1.5 8.3 0.65 Refer to Fig. 12 for well locations, n=not analyzed. Refer to Fig. 1 for sample locations, n=below detection limit. H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11 5

Table 4 macropores are commonly b30% and have b25% mercury intrusion Results of methane isothermal adsorption measurements. saturation. 3 Sample no. Coal mine VL (m /t) PL (MPa) Mad (%) Vad (%) Aad (%) As shown in Table 6, the pore size distribution is dominated by micropores ranging from 32.4% to 59.06%, with a mean of 50.46%. Ting-2 Tingnan 5.205 3.417 7.81 21.05 39.79 Ting-4 Tingnan 10.09 3.92 12.01 32.60 7.03 The macropore distribution ranges from 6.58% to 42.52%, with a Ting −6 Tingnan 7.25 3.47 11.74 32.38 16.02 mean of 23.06%, and the mesopore distribution for the mesopores Huo-1 Huoshizui 10.17 3.57 9.57 29.27 5.21 ranges from 6.49% to 42.54%, with a mean of 21.79%. Huo-3 Huoshizui 7.36 4.94 11.46 24.33 10.13 Based on the macropore classification of Yao and Liu (2007) Huo-6 Huoshizui 5.24 3.22 13.23 23.86 8.78 Hei-1 Heigou 7.36 4.94 15.10 29.33 8.21 for coals from North China, the mercury curves indicate that the Hei-6 Heigou 5.06 3.97 14.73 23.01 22.31 coal samples in the Binchang area can be divided into three types Hei-8 Heigou 6.07 4.56 18.76 26.07 8.54 (Fig. 4): Type A includes two grades of pores, 24.84–42.52% macro- Xia-1 Xiagou 8.72 3.48 3.5 16.84 14.95 pores and 40–80% mercury intrusion saturation, which is a bottleneck Xia-2 Xiagou 12.95 5.59 3.41 20.22 14.61 for CBM production. Type B includes all grades of pores are devel- Shui-1 Shuilian 13.37 2.14 3.84 31.6 12.95 oped, with 11.09–11.76% macropores and 40–50% mercury intrusion Mad =Moisture content (wt.%, air dry basis), Aad =Ash yield (wt.%, air dry basis), saturation, and thus are most favorable for CBM development. V =volatile matter (wt.%, air dry basis). ad Type C has singular macropores less than 10% in volume and has less than 30% mercury intrusion saturation, which is unfavorable for have noted that mercury injection combined with N2 adsorption/ CBM development. In general, samples from the No.4 coal seam desorption could characterize the pore structure. Mercury porosimetry show a Type A mercury curve (Table 5). is a common method for porous structure characterization but is Results of the N2 adsorption/desorption analysis show that the acceptable only as a method to study macropores (>100 nm). Low- Brunauer–Emmett–Teller (BET) (Pickett, 1945) pore surface area of 2 temperature N2 gas adsorption/desorption analysis is used here to the nine coals analyzed ranges from 0.33 to 28.25 m /g, and the study micropores (b10 nm) and mesopores (10–100 nm) (Liu et al., Barrett–Joiner–Halenda (BJH) (Barrett et al., 1951) total pore volume 2009). ranges from 1.2×10− 3 to 19.8×10− 3 ml/g (Table 6). A significant For the coal samples in this study, maximum intrusion saturations negative correlation is apparent between the average pore diameter range from 24.94 to 75.28% (Table 5), and macropore content ranges and the pore surface area as measured by the BET method (Fig. 5). from 6.58% to 42.52% (Table 6). These data indicate that pore connec- This correlation indicates that the mesopores have a smaller pore tivity is poor to medium in both intrusion and extrusion efficiencies surface area than the micropores do. according to the classification standard of Liu et al. (2009): (1) Type In the Binchang area, three types of pore structures were identi- A: all grades of pores are developed, with 40–50% macropores and fied by adsorption /desorption isotherms (Fig. 6). The rapid decrease 60–80% mercury intrusion saturation; (2) Type B: two grades of of the three desorption isotherms represents the capillary evapora- pores are developed, with 30–50% macropores and 40–50% mercury tion of nitrogen taking place at P/P0 =0.5, which corresponds to a intrusion saturation; (3) Type C: singular macropores are b30% in vol- pore diameter of about 3 nm. This decrease suggests that the coals ume and have 25–40% mercury intrusion saturation; and (4) Type D: commonly develop micropores with an ink-bottle shaped (narrow

Table 5 Petrological permeability, porosity and macropore structures of Yanan Formation No. 4 coal.

Sample no. Porosity (%) Permeability (mD) Maximum intrusion saturation (%) Maximum extrusion saturation (%) Extrusion efﬁciency Yao and Liu (2007) type

Ting-2 2.7 0.5 24.94 7.13 71.41 C Ting-4 2.4 0.04 75.28 31.23 58.51 A Ting-6 5 3.26 56.75 35.41 37.6 A Huo-1 7.8 0.04 40.91 24 41.33 B Huo-3 4.8 0.16 29.4 8.95 69.56 C Huo-6 12.4 0.17 47.31 30.1 36.38 B Hei-1 20.1 25.3 64.59 44.3 31.41 A Hei-6 13.1 13 51.3 37.56 26.78 A Hei-8 14.7 45.1 65.91 49.93 24.25 A Xia-1 10.7 0.363 40.90 28.20 31.05 A Xia-2 6.90 14.70 43.28 31.78 26.57 A Shui-1 7.90 0.32 44.25 25.02 43.46 A

Table 6

Pore surface area, pore volume, and pore structure obtained by N2 adsorption analyses of the coal.

2 Sample no. Macropores, >100 nm Mesopores, 100-10 nm Micropores, b10 nm SBET (m /g) BJH (ml/g) Average pore diameter (nm) Yao and Liu (2007) type Ting-2 6.58 32.88 59.06 5.694 0.0132 8.65 C Ting-4 31.23 9.97 56.20 8.074 0.0146 7.1 A Ting-6 34.28 6.49 55.47 15.422 0.0191 5.46 A Huo-1 11.09 34.36 53.05 8.233 0.0173 8.12 B Huo-3 8.8 42.54 47.07 8.222 0.0198 9.36 C Huo-6 11.76 40.50 47.33 6.952 0.0126 7.5 B Hei-1 36.43 4.78 46.09 28.25 0.0266 4.47 A Hei-6 24.84 7.70 57.48 25.41 0.022 4.2 A Hei-8 42.52 16.90 32.40 5.379 0.0091 7.5 A Xia-1 33.28 27.78 38.94 0.331 0.0012 12.68 A Xia-2 31.55 31.45 37.00 0.368 0.0013 11.51 A Shui-1 33.21 28.96 37.83 0.567 0.0023 13.43 A

SBET =pore surface area as measured by the BET method; BJH=total pore volume as measured by the BJH method. 6 H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11

Fig. 3. Vitrinite reﬂectance contour of the Yanan formation No.4 coal seam in the Binchang area. throat and wide body) morphology. In the low-pressure hysteresis, 4.2. CBM adsorption capacity and gas content the adsorption and desorption curves for Types A and B do not coincide at a low relative pressure. Thus, the average pore size of The results of methane adsorption isotherm experiments show the adsorbent molecule has the same width as that of the adsorptive that the coals in the Binchang area have moderate adsorption molecule (Burgess et al., 1989). Type C is similar to Type B in the capacities (Table 4), with dry ash-free (DAF) Langmuir volumes (VL, conﬁguration of the adsorption/desorption curves, but the adsorp- maximum adsorption capacity) ranging from 5.06 to 13.37 m3/t. tion and desorption curves of the former coincide at low relative Considerable research has examined the relationship among pressure. This phenomenon often occurs in the poorly connected, methane adsorption capacity, coal characteristics (e.g., coal lithotype, semi-sealed, and narrow tubular micropores. For the three types rank, organic and inorganic compositions), geologic and hydrologic of isotherms, Type A is the most common in coals from the Binch- properties (e.g., reservoir temperature and pressure, in situ stress, ang area (Table 6). basin history, and hydraulic gradient) (e.g., Bustin and Clarkson, 1999; Clarkson and Bustin, 1996; Gürdal and Yalçın, 2000; Lamberson and Bustin, 1993; Laxminarayana and Crosdale, 1999). 4.1.2.2. Permeability and porosity. The results of mercury porosimetry However, the role of coal maceral compositions and maturity of measurements show that the porosity values of the coals from the low-rank coals in determining the adsorption capacity remains con- Binchang area range from 2.4% to 20.1%, indicating a poor to moder- troversial. Several studies have demonstrated a positive correlation ate coal porosity (Table 5). Results of the permeability measurements between vitrinite content and methane adsorption capacity, which show that the coal reservoir permeability in the Binchang area varies is attributed to the higher micropore volume in vitrinite than that in from 0.04 to 45.1 mD (Table 5), which positively correlates with the intertinite and liptinite groups (Bustin and Clarkson, 1998; Walker mercury porosity. et al., 2001; Zhang et al., 2010). Chalmers and Bustin (2007) suggested that this relationship applies only to high-rank coals. Other studies have found weak or no correlation between maceral compositions and methane adsorption capacity (Laxminarayana and Crosdale, 2002; Mastalerz et al., 2004), or suggested that the inertinite maceral

Fig. 4. Three typical mercury intrusion/extrusion curves for coals in the Binchang area (a. Type A curve, sample Ting-4 from the Tingnan coalfield; b. Type B curve, sample Huo-6 from the Huoshizui coalfield; c. Type C curve, sample Ting-2 from the Tingnan coalfield). Fig. 5. Relationship between SBET and average pore diameter in coal samples. H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11 7

(e.g., fusinite) has a greater sorption than the vitrinite does (Ettinger et al., 1966). The negative relationship is observed between adsorption capacity and moisture in the Binchang area (Fig. 7), consistent with data of other researchers (Faiz et al., 2007; Hildenbrand et al., 2006). This study plotted similar graphs for others factors and found weak correlations among coal lithotype, rank, organic and inorganic compositions; therefore, it appears that this coal type (maceral compositions) did not exert a signiﬁcant inﬂuence on the adsorption capacity of the coals of low rank in the Binchang area.

4.3. CBM enrichment and its controlling factors

Different researchers have different views on the main controlling factors of low-rank coalbed methane accumulation. The study by Beaton et al. (2006) revealed that the interplay of high permeability and thick coals overcame the generally low gas content in the Powder River Basin, while in the Alberta Basin, geological and hydrogeological ﬁ Fig. 6. Three typical pore structures identi ed by N2 adsorption/desorption analyses conditions undoubtedly led to a better understanding of the factors of the coals in the Binchang area (Selected samples for Types A, B, and C are Ting-6, controlling the local CBM enhancement. The relationship among gas Huo-3, and Ting-2, respectively). desorption, gas adsorption, and coal composition is vital in determining which areas in the Surat Basin offer the most economically viable targets for the commercialization of coalbed methane (Scott et al., 2007). Other researchers have emphasized the preservation conditions of CBM, including roof lithology (Zhao et al., 2006), tectonic setting, and hydrological features (Liu et al., 2008). In-place gas content, obtained from core drills, increases from the margin to the center of the Binchang area (Fig. 8), ranging from 0.11 to 6.26 m3/t. In most of the northern and western areas, the in-place gas content is less than 2 m3/t. In the southern synclinal areas, the No. 4 coal seam has a relatively higher gas content (>3 m3/t), and in some areas of the Tingnan and Dafosi mines (Fig. 8), this value can be as high as 6 m3/t. These data showed a positive correlation between gas content with coal thickness (Fig. 9; Table 1).

4.3.1. The lithology and its distribution of roof and floor of the coal The roof and floor lithology and the thickness of the coal seam have a direct influence on CBM preservation. The roof of the No.4 coal seam is dominated by silty mudstone and mudstone (Fig. 10). The mudstone is distributed mainly in the Tingnan and Dafosi areas. However, the floor of the No. 4 coal seam is mainly composed of Fig. 7. Relationship between adsorption capacity and moisture of the No. 4 coal seam. muddy siltstone, mudstone, silty mudstone, sandstone, siltstone,

Fig. 8. In-place gas content contours (in m3/t) of No. 4 coal seam.Refer to Table 1 for well data. 8 H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11

and carbonaceous mudstone. The mechanism of the floor affecting the gas content is similar to that of the roof; the floor with a tight lith- ological characteristic that can prevent the gas dissipation through the bottom of the coal seam. The southern-central region of the study area, where the floor is dominated by mudstone and carbonaceous mudstone, has relatively higher gas content (Fig. 11).

4.3.2. Hydrogeological condition Generally, low-rank coal has a shallower burial depth, and thus the hydrogeological condition of the coal basins/coalfield has an important effect on CBM accumulation (Flores et al., 2008). The gas content, which is strongly dependent upon hydrodynamic factors, reservoir pressure and temperature (Scott and Kaiser, 1996), will change when the existed equilibrium conditions of the reservoir are disrupted. Gas content generally increases where conventional and hydrodynamic trapping of coal gases occurs and may decrease in Fig. 9. Relationship between gas content and No. 4 coal bed thickness in the Binchang fl area. areas of active recharge with downward ow potential or convergent flow where there is no mechanism for entrapment (Scott, 2002).

Fig. 10. Roof lithology of No. 4 coal seam. Refer to Table 1 for well data.

Fig. 11. Floor lithology of No. 4 coal seam. Refer to Table 1 for well data. H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11 9

Fig. 12. Distribution of TDS and types of formation water in the Yanan Formation.

The Binchang area is located in the south limb of a regional depth (400–600 m), favorable structural situation (in the syncline), syncline with conﬁned water. Outcrops of Jurassic coal-bearing strata thick coal seam (>15 m), mudstone roof and ﬂoor, stagnant hydro- rarely occur in the southeast portion of this area. According to the geological conditions (Cl-Na water type), and relatively higher gas drilling data (Table 2), the water yield of the No. 4 coal seam ranges content (>3 m3/t). from 0.000046 to 0.0038 l/s m, indicating no connection of the No.4 coal seam with other aquifers. The water salinity increases from 5. Conclusions northwest to southeast, and the water type gradually shifts from

SO4·Cl–Na to Cl–Na (Fig. 12), indicating that Yanan Formation aquifers are recharged mainly from lateral runoff from the northwestern 1. Coal rank in the Binchang area is mainly sub-bituminous A and area and that a relatively stagnant area exists in the Dafosi Syncline. high-volatile C bituminous (0.46%–0.73% Ro). Most coal pores are Thus, the Binchang area is characterized by a hydraulic monoclinal less than 100 nm in diameter, making them favorable for gas ad- model, and the Dafosi Syncline, with a retained hydrogeological sorption but unfavorable for gas permeability. Pore morphology condition, is favorable for CBM accumulation (Fig. 13). is represented mainly by micro- and mesopores with a well- connected and ink-bottle shaped (narrow throat and wide body) 4.4. Distribution of CBM target area morphology. Permeability is between 0.04 and 25.3 mD, which positively correlates with the mercury porosity. The data analyzed in this study demonstrated that the Binchang 2. The Yanan formation No.4 coal samples in the Binchang area area has a favorable CBM production area from west to east and are characterized by a high adsorption volume of more than from the margin to the center. The most favorable CBM areas are 3.0×10− 3 ml/g, and their maximum adsorption capacity (on a located in the central-southern part of the Binchang area, including dry and ash-free basis) varies from 5.06 to 13.37 m3/t, which the Tingnan and Dafosi mining areas with favorable coal burial have a negative relationship with moisture content.

Fig. 13. Schematic diagram of hydrogeological proﬁle of Binchang area. Refer to Fig. 1 for proﬁle location. 10 H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11

3. The in-place gas content is generally 0.11–6.26 m3/t. Based on a ISO 1171–1997, 1997. Solid mineral fuels—determination of ash content. ISO 11722–1999, 1999. Solid mineral fuels-Hard coal-Determination of moisture in the comprehensive analysis of coal thickness, gas content, hydrogeolo- general analysis test sample by drying in nitrogen. gy conditions, roof, floor, and depth properties, this study indicates ISO 15901.1-2005, 2005. Pore size distribution and porosity of solid materials by that the best prospective target areas for CBM production are fore- mercury porosimetry and gas adsorption—Part 1: mercury porosimetry. ISO 15901.2-2006, 2006. Pore size distribution and porosity of solid materials by mer- casted to be the Tingnan and Dafosi areas, which have favorable cury porosimetry and gas adsorption—Part 2: analysis of mesopores and macro- coal burial depth, favorable structural situation, thick coal seam, pores by gas adsorption. mudstone roof and floor, stagnant hydrogeological conditions ISO 562–1998, 1998. Hard coal and coke-determination of volatile. and relatively higher gas content. ISO 7404.3-1994, 1994. Methods for the petrographic analysis of bituminous coal and anthracite-Part 3: method of determining maceral group composition. ISO 7404.5-1994, 1994. Method for the petrographic analysis of bituminous coal and anthracite-Part 5: method of determining microscopically the reflectance of vitri- Acknowledgements nite. MOD. Lamberson, M.N., Bustin, R.M., 1993. Coalbed methane characteristics of the Gates fi Formation coals, northeastern British Columbia: effect of maceral composition. This work was nancially supported by the National Basic American Association of Petroleum Geologists Bulletin 77, 2062–2076. Research Program of China (973) (2009CB219604), the Key Project Laxminarayana, C., Crosdale, P.J., 1999. Modeling methane adsorption isotherms using of the National Science & Technology (2011ZX05034), and the Funda- pore-filling models: a case study on Indian coals. Proceedings 1999 International – mental Research Funds for the Central Universities (2011YXL052). Coalbed Methane Symposium, Tuscaloosa, Alabama, pp. 117 129. Laxminarayana, C., Crosdale, P.J., 2002. Controls on methane sorption capacity The authors are grateful to the anonymous reviewers for their careful of Indian coals. American Association of Petroleum Geologists Bulletin 86, reviews and detailed comments. The authors also thank Dr. Shifeng 201–212. Dai for his constructive suggestions and valuable assistance in the Li, J.Q., Liu, D.M., Yao, Y.B., Cai, Y.D., Qiu, Y.K., 2011. Evaluation of the reservoir permeability of anthracite coals by geophysical logging data. International Journal of Coal preparation of the manuscript. Geology 87, 121–127. Lin, W.J., Tang, D.Z., Xu, F.Y., Xu, H., Liu, Y.H., 2009. Sequence stratigraphic division of Jurassic Yanan Formation in Binchang Area. Coal Geology of China 21, 5–8 (in Chinese with English abstract). References Liu, H.L., Li, J.M., Wang, H.Y., Zhao, Q.B., 2008. Control of hydrogeological conditions on accumulation of coalbed methane in low-rank coal. Natural Gas Industry 28, 20–22 ASTM D 388–99, 2005. Annual Book of ASTM Standards. Gaseous Fuels; Coal and Coke, (in Chinese with English abstract). vol. 05.06. Standard Classification of Coals by Rank. Liu, D.M., Yao, Y.B., Tang, D.Z., Tang, S.H., Che, Y., Huang, W.H., 2009. Coal reservoir Ayers, W.B., 2002. Coalbed gas systems, resources, and production and a review of characteristics and coalbed methane resource assessment in Huainan and contrasting cases from the San Juan and Powder River Basins. American Associa- Huaibei coalfields, Southern North China. International Journal of Coal Geology tion of Petroleum Geologists Bulletin 86, 1853–1890. 79, 97–112. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. The determination of pore volume and Long, L., Wang, X., 2005. CBM hosting feature and resource utilization. Coal Geology of area distributions in porous substances. Journal of the American Chemical Society China 17, 44–46 (in Chinese with English abstract). 73, 373–380. Mastalerz, M., Gluskoter, H., Rupp, J., 2004. Carbon dioxide and methane sorption in Beaton, A., Langenberg, W., Pana, C.P., 2006. Coalbed methane resources and reservoir high volatile bituminous coals from Indiana, USA. International Journal of Coal characteristics from the Alberta Plains, Canada. International Journal of Coal Geology Geology 60, 43–55. 65, 93–113. Mastalerz, M., Drobniak, A., Strąpoć, D., Acosta, W.S., Rupp, J., 2008. Variations in pore Burgess, C.G.V., Everett, D.H., Nuttall, S., 1989. Adsorption hysteresis in porous characteristics in high volatile bituminous coals: implications for coal bed gas materials. Pure and Applied Chemistry 61, 1845–1852. content. International Journal of Coal Geology 76, 205–216. Bustin, R.M., Clarkson, C.R., 1998. Geological controls on coalbed methane reservoir Pickett, G., 1945. Modification of the Brunauer–Emmett–Teller theory of multimolecu- capacity and gas content. International Journal of Coal Geology 38, 3–26. lar adsorption. Journal of the American Chemical Society 67, 1958–1962. Bustin, R.M., Clarkson, C.R., 1999. Free gas storage in matrix porosity: a potentially Scott, A.R., 2002. Hydrogeologic factors affecting gas content distribution in coal beds. significant coalbeds resource in low rank coals. Proceedings of the International International Journal of Coal Geology 50, 363–387. Coalbed Methane Symposium, Tuscaloosa, Alabama, pp. 197–214. Scott, A.R., Kaiser, W.R., 1996. Factors affecting gas-content distribution in coal beds: a Cai, Y.D., Liu, D.M., Yao, Y.B., Li, J.Q., Qiu, Y.K., 2011. Geological controls on prediction of review (exp. abs.). Expanded abstracts volume, Rocky Mountain Section Meeting: coalbed methane of No. 3 coal seam in Southern Qinshui Basin, North China. Inter- American Association of Petroleum Geologists, pp. 101–106. national Journal of Coal Geology 88, 101–112. Scott, S., Anderson, B., Crosdale, P., Dingwall, J., Leblang, G., 2007. Coal petrology Chalmers, G.R.L., Bustin, R.M., 2007. On the effects of petrographic composition and coal seam gas contents of the Walloon Subgroup-Surat Basin, Queensland, on coalbed methane sorption. International Journal of Coal Geology 69, Australia. International Journal of Coal Geology 70, 209–222. 288–304. Sing, K.S.W., 1995. Physisorption of nitrogen by porous materials. Journal of Porous Clarkson, C.R., Bustin, R.M., 1996. Variation in micropore capacity and size distribution Materials 2, 5–8. with composition of Cretaceous coals of the Western Canadian Sedimentary Basin. Tang, Y.Q., Ji, P., Lai, G.L., Chi, C.Q., Liu, Z.S., Wu, X.L., 2012. Diverse microbial community Fuel 75, 1483–1498. from the coalbeds of the Ordos Basin, China. International Journal of Coal Geology Dai, S., Ren, D., Tang, Y., Shao, L., Li, S., 2002. Distribution, isotopic variation and origin 90–91, 21–33. of sulfur in coals in the Wuda coalfield, Inner Mongolia, China. International Walker, R., Glikson, M., Mastalerz, M., 2001. Relations between coal petrology and gas Journal of Coal Geology 51, 237–250. content in the Upper Newlands Seam, central Queensland, Australia. International Dai, S., Ren, D., Chou, C.-L., Li, S., Jiang, Y., 2006. Mineralogy and geochemistry of the Journal of Coal Geology 46, 83–92. No. 6 Coal (Pennsylvanian) in the Junger Coalfield, Ordos Basin, China. International Wang, S. (Editor in Chief), 1996. Coal Accumulation and Coal Resources Evaluation Journal of Coal Geology 66, 253–270. of Ordos Basin, China. China Coal Industry Publishing House, Beijing, p. 437 (in Ettinger, I., Eremin, I., Zimakov, B., Yanovskaya, M., 1966. Natural factors influencing Chinese with English abstract). coal sorption properties I-Petrography and the sorption properties of coals. Fuel Wang, Y.B., Zhao, S.Y., Liu, H.B., Chen, C.H., Wang, X.H., Wang, S.W., 2004. Discus- 45, 267–275. sion on low rank coalbed methane exploration in China-by taking Sha'erhu Faiz, M., Saghaf, A., Sherwood, N., Wang, N., 2007. The influence of petrological proper- sag as an example. Natural Gas Industry 24, 21–23 (in Chinese with English ties and burial history on coal seam methane reservoir characterization, Sydney abstract). Basin, Australia. International Journal of Coal Geology 70, 193–208. Wang, B., Li, J., Zhang, Y., Wang, H., Liu, H., Li, G., Ma, J., 2009. Geological characteristics Flores, R.M., 1998. Coalbed methane: from hazard to resource. International Journal of of low rank coalbed methane, China. Petroleum Exploration and Development 36, Coal Geology 35, 3–26. 30–34 (in Chinese with English abstract). Flores, R.M., Rice, C.A., Stricker, G.D., Warden, A., Ellis, M.S., 2008. Methanogenic path- Wei, C.T., Qin, Y., Wang, G.X., Fu, X.H., Zhang, Z.Q., 2010. Numerical simulation of ways of coal-bed gas in the Powder River Basin, United States: the geologic factor. coalbed methane generation, dissipation and retention in SE edge of Ordos Basin, International Journal of Coal Geology 76, 52–75. China. International Journal of Coal Geology 82, 147–159. GB/T 19560–2008, 2008. Chinese national standard. Coal, Experimental method of Xu, H., Tang, D.Z., Tang, S.H., Zhang, W.Z., Zhang, S.H., Tao, S., Wang, F., 2010. Coal high-pressure adsorption isothermal to coal-Capacity method. (in Chinese). reservoir characteristics and prospective areas for Jurassic CBM exploration in GB/T 212–2008, 2008. Chinese national standard. Coal, Proximate analysis of coal. (in western Ordos basin. Coal Geology & Exploration 38, 26–28 (in Chinese with Chinese). English abstract). Gürdal, G., Yalçın, M.N., 2000. Gas adsorption capacity of Carboniferous coals Xu, H., Tang, D.Z., Zhang, J.F., Yin, W., Zhang, W.Z., Lin, W.J., 2011. Factors affecting in the Zonguldak basin (NW Turkey) and its controlling factors. Fuel 79, the development of the pressure differential in Upper Paleozoic gas reservoirs in 1913–1924. the Sulige and Yulin areas of the Ordos Basin, China. International Journal of Coal Hildenbrand,A.,Krooss,B.M.,Busch,A.,Gaschnitz,R.,2006.Evolutionofmethane Geology 85, 103–111. sorption capacity of coal seams as a function of burial history—a case study Yao, Y.B., Liu, D.M., 2007. Adsorption characteristic of coal reservoirs in North China from the Campine Basin, NE Belgium. International Journal of Coal Geology 66, and its influencing factors. Journal of China University of Mining & Technology 179–203. 36, 308–314 (in Chinese with an English abstract). H. Xu et al. / International Journal of Coal Geology 95 (2012) 1–11 11

Yao, Y.B., Liu, D.M., Tang, D.Z., Tang, S.H., Che, Y., Huang, W.H., 2009. Preliminary eval- Zhang, S.H., Tang, S.H., Tang, D.Z., Pan, Z.J., Yang, F., 2010. The characteristics of coal uation of the coalbed methane production potential and its geological controls in reservoir pores and coal facies in Liulin district, Hedong coal ﬁeld of China. Interna- the Weibei Coalﬁeld, Southeastern Ordos Basin, China. International Journal of tional Journal of Coal Geology 81, 117–127. Coal Geology 78, 1–15. Zhao, Q., Kang, Y.S., Wang, H.Y., Yang, S., 2006. Analysis of the forming conditions Zhang, L.P., Bai, G.P., Luo, X.R., Ma, X.H., Chen, M.J., Wu, M.H., Yang, W.X., 2009. of shallow low-rank coalbed gas pool. Journal of Xi'an Shiyou University (Natural Diagenetic history of tight sandstones and gas entrapment in the Yulin Gas Science Edition) 21, 20–23 (in Chinese with English abstract). Field in the central area of the Ordos Basin, China. Marine and Petroleum Geology 26, 974–989.