Geochemistry and accumulation process of natural gas in the Shenmu Gas Field, Ordos Basin, central China

Journal of Petroleum Science and Engineering 180 (2019) 1022–1033

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Geochemistry and accumulation process of natural gas in the Shenmu Gas Field, Ordos Basin, central China

T

Weilong Penga,b,c,∗, Fengtao Guob, Guoyi Huc, Yue Lyuc, Deyu Gongc, Jiayi Liub, Ziqi Fengd,∗∗, Jigang Guoe, Yingchun Guof, Wenxue Hang a

State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, SINOPEC, Beijing, 100083, China Petroleum Exploration and Production Research Institute, SINOPEC, Beijing, 100083, China c Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, 100083, China d Key Laboratory of Deep Oil and Gas (China University of Petroleum (East China)), Qingdao, 266580, China e Strategic Research Center of Oil and Gas Resources, Ministry of Natural Resources, Beijing, 100034, China f Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081, China g College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Shenmu gas field Natural gas Genetic type Coal-type gas Accumulation process Ordos basin

The chemical and stable isotopic compositions of 24 gas samples were investigated to determine the geochemistry and accumulation process of the Shenmu Gas Field (SGF), Ordos Basin, central China. Natural gas of the SGF includes both wet and dry gas with dryness coefficients (C1/C1−5) between 0.907 and 0.958. δ13C1 and δD1 values range from −39.4‰ to −35.6‰ and −191‰ to −201‰, respectively, displaying a positive isotopic series. Geochemical characteristics indicate that gases of the upper Paleozoic reservoirs of the SGF are typical coal-type gas, whereas gases of the lower Paleozoic reservoirs are primarily oil-type gas (occasionally containing a small proportion of coal-type gas). Gas-source correlation demonstrates that gases of the upper Paleozoic reservoirs of the SGF display near-source accumulation, primarily derived from the underlying Carboniferous–Permian humic source rocks, while gases of the lower Paleozoic reservoirs in the SGF are mainly sourced from the carbonate Majiagou Formation (O1m). SGF reservoirs were generally tight prior to gas accumulation. Integrated with the maturation process of source rocks, the accumulation process of gas in the SGF could be separated into four stages: prenatal (before early Jurassic), development (early–late Jurassic), prime (early Cretaceous), and formative (late Cretaceous–present).

1. Introduction The latest data show that the Ordos Basin of China has a natural gas reserve of 3.46 × 1012 m3. With the largest proven reserves and annual production of gas, the Ordos Basin has become one of the most attractive petroliferous basins in China (Dai, 2016). Reservoirs in the Ordos Basin are mostly tight sandstones of Carboniferous-Permian strata, except that the Jingbian Gas Field are mainly carbonates of the lower Paleozoic strata (Dai, 2016). Natural gas exploration in the Ordos Basin is gradually shifting from the center to the edge of the basin (Yang et al., 2012, 2015). Subsequently, a 11.9-m-sandstone gas reservoir was encountered in the upper Paleozoic strata of the Shenmu Area, which produced an economical gas flow of 2.54 × 104 m3/d, marking the discovery of the Shenmu Gas Field (SGF) (Yang et al., 2015). The SGF is situated in northeast of the Ordos Basin, with a north to south trend ∗

(Fig. 1). Proven reserves of natural gas in the SGF are 3.33 × 1011 m3 and the gas-bearing area is 4069 km2 (Yang et al., 2015). By July 2017, the SGF had achieved a cumulative gas production of 3.01 × 109 m3. Geochemistry and accumulation process of several giant gas fields in the Ordos Basin have been thoroughly studied (Hu et al., 2010; Li et al., 2014; Liu et al., 2015a; Huang et al., 2015). However, investigation on the Shenmu Area is mainly focused on structural evolution (Ren et al., 2007), sedimentary characteristics (Lan et al., 2011), reservoir-pressure characteristics (Wang et al., 2013), reservoir diagenesis (Gao et al., 2015), and productivity characteristics (Yang et al., 2015). Thus, few studies about the geochemistry and accumulation process of gas in the SGF has been conducted. Furthermore, the sources of gases in the Shenmu Area is still controversial. Some scholars hold the opinion that gases of the Shenmu Area migrated from the Sulige Area (Wang et al., 1998). Diagenesis investigation in the Ordos Basin

Corresponding author. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing, 100083, China. Corresponding author. E-mail addresses: [email protected], [email protected] (W. Peng), [email protected] (Z. Feng).

∗∗

https://doi.org/10.1016/j.petrol.2019.05.067 Received 6 June 2018; Received in revised form 7 May 2019; Accepted 28 May 2019 Available online 06 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Structural divisions and fields location throughout the Ordos Basin, (b) wells location in the Shenmu Area, and (c) gas reservoirs profile. Modified after Yang et al. (2015) and Dai (2016a). Vitrinite reflectance (Ro) contour line modified from Zhang et al. (2000) and Dai (2016a).

investigation of the geochemical properties and accumulation processes was carried out. This paper presents a systematic discussion on the origin and source of gases in the SGF based on analysis of gas components, isotopes, and geochemical comparison with other giant gas fields in the Paleozoic reservoirs of the Ordos Basin. Along with gas-source correlation and accumulation mechanisms, the accumulation process of natural gas in the SGF, mainly in the upper Paleozoic reservoirs, was investigated. The goal of this study is to determine the geochemistry, origin and accumulation process of gas in the SGF. This paper further provides valuable information on hydrocarbon accumulation process and reservoir assessment of the Ordos Basin.

suggested that gases in the eastern part of the Ordos Basin (i.e., Shenmu Area) migrated from the southwest (Luo, 1996). Contrariwise, according to fluid inclusions in the Shenmu reservoirs, gas in this area migrated from the south (Mi et al., 2003). However, based on investigation in reservoir-forming dynamics and accumulation conditions, natural gas in the Shenmu Gas Field belongs to a near-source accumulation (Meng et al., 2013; Yang et al., 2015). The above controversy arises from different understandings of the geologic conditions and geochemical characteristics of the Shenmu Area. Recently, an economical gas layer has also been drilled in the lower Paleozoic strata of the Shenmu Area by the PetroChina, with different geochemical characteristics from gases of the upper Paleozoic reservoir of Shenmu Area. To delineate the origin and source of gas in the SGF, an 1023

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Fig. 2. Stratigraphic column of the SGF, Ordos Basin. Revised after Liu et al. (2015a) and Feng et al. (2016).

2. Geological setting

Formation (P3s) (Fig. 2). In the Shenmu Area, gas layers in the O1m carbonate rocks have also been drilled in recent years (Yang et al., 2015). Coal and dark mudstones are present in the upper Carboniferous Benxi Formation (C3b), lower Permian Taiyuan Formation (P1t), and Shanxi Formation (P1s). The maximum cumulative thicknesses of dark mudstones and coal seams in the Shenmu Area are more than 80 m and 20 m, respectively. They are dominated by humic organic matter (Dai, 2016). Their total organic carbon (TOC) reach as high as 62.9% and 2.33%, respectively (Meng et al., 2013). These coals have entered the mature stage, with vitrinite reflectance mainly between 0.65% and 0.85% (Zhang et al., 2000). Lithologies of the upper Paleozoic reservoirs are mainly fine to coarse lithic sandstones. These sandstones are tight with low porosity (4–10%; 7%, average) and permeability (0.1–1 mD; 0.6 mD, average) (Dai, 2016). Sandstones are distributed heterogeneously (Yang, 2002). The sandstones have a large cumulative thicknesses, with the maximum of over 200 m (Dai, 2016). The Shangshihezi Formation (P2sh) and overlaying strata have variegated mudstone with a cumulative

The Ordos Basin is the second largest inland sedimentary basin in China, with an area of about 25 × 104 km2 and a thickness of more than 5000 m of sedimentary rocks (Yang and Pei, 1996). The Ordos Basin can be divided into several structural units, namely, Yimeng Uplift, Weibei Uplift, West Margin Thrust Belt, Tianhuan Depression, Yishan Slope, and Jinxi Fault-Fold Belt (Fig. 1) (Yang and Pei, 1996; Dai, 2016). The basement consists of Archean and Proterozoic metamorphic rocks (Dai, 2016). With mostly gentle structures, the fault system is undeveloped in the Yishan Slope. The upper Paleozoic strata are dominated by coal-bearing clastic facies of fluvial and delta environments, while the lower Paleozoic strata mainly consists of marine carbonate rocks (Yang, 2002). The SGF is situated in the Yishan Slope (Fig. 1). The upper Paleozoic strata in the Shenmu Area unconformably overlie the lower Paleozoic Majiagou Formation (O1m) and consist of: upper Carboniferous Benxi Formation (C3b), lower Permian Taiyuan (P1t) and Shanxi formations (P1s), middle Permian Xiashihezi (P2x) and Shangshihezi (P2sh) formations, and upper Permian Shiqianfeng

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coefficients of the upper and lower Paleozoic natural gases are very similar. Gases in the Paleozoic reservoirs contain both dry and wet gas, except the gases in the Jingbian Gas Field, whose dryness coefficients are higher than 0.95 (Fig. 3a). The gases in the SGF contains some nonhydrocarbons, mainly CO2 and N2, with a content of 0.37–3.36% and 0.21–0.63%, respectively. Nonhydrocarbon content in the upper Paleozoic reservoirs of the SGF is generally higher than that of the lower Paleozoic reservoirs (Table 1).

thickness of 110–190 m, which act as the caprock of the Paleozoic reservoirs (Dai, 2016). 3. Materials and analytical methods Gas samples were collected from 24 production wells of the SGF which 22 samples were collected from the upper Paleozoic and 2 samples from the lower Paleozoic reservoirs. Some of the sampling wells are shown on Fig. 1. 22 gas samples come from the upper Paleozoic reservoir and 2 gas samples come from the lower Paleozoic reservoir. Gas samples were collected with steel cylinders under high pressure at the production wellhead. The gas cylinder and steel pipe were rated for pressure to 15 MPa. All sample analyses were carried out in the Key Laboratory of Geochemistry at the Research Institute of PetroChina.

4.2. Carbon isotopic composition δ13C1 (subscript indicates the number of carbon in the alkane gas) of the SGF shows no relation to the dryness coefficient. The δ13C1 of the SGF is generally lighter than that of other Paleozoic fields in the Ordos Basin (Fig. 3b). δ13C1, δ13C2, and δ13C3 isotope values of gas in the upper Paleozoic reservoirs of the SGF range from −36.8‰ to −35.6‰,−25.8‰ to −22.8‰, and −24.3‰ to −21.8‰, respectively. δ13C1, δ13C2, and δ13C3 values in the lower Paleozoic reservoirs of the SGF range from −39.4‰ to −38.6‰,−36.0‰ to −35.7‰, and −30.5‰ to −30.2‰, respectively (Table 1) (Fig. 4a). Alkanes in the SGF display a positive carbon isotopic series (δ13C1 < δ13C2 < δ13C3). Carbon isotopic series of natural gases exhibit different characteristics between the upper Paleozoic reservoirs and the lower Paleozoic reservoirs (Fig. 4a). Alkane carbon isotopic series of the upper Paleozoic reservoirs in the SGF show convex characteristics, while those of the lower Paleozoic reservoirs in the SGF show concave characteristics. The upper Paleozoic gas carbon isotopic values are significantly higher than those of the lower Paleozoic (Fig. 4a).

3.1. Chemical components analysis The components of natural gas were measured by HP7890A gas chromatograph equipped with a flame ionization detector. Components were online separated using a PLOT capillary column (PLOT Al2O3 50 m × 0.53 mm) and Helium as the carrier gas. The chromatograph oven temperature was set at 33 °C for 10 min at the beginning, then increased to 180 °C at the rate of 10 °C/min and finally maintained at the maximum temperature for 20–30 min. Detailed descriptions of analysis method can be referred to Liu et al. (2012). 3.2. Stable carbon and hydrogen isotopes analyses Stable carbon isotope analyses were carried out on a Finnigan Mat Delta S mass spectrometer interfaced with a HP 5890II chromatograph. Gas components were first separated by chromatograph, then transformed into carbon dioxide and injected into mass spectrometer, and each component was separated by chromatographic column. The chromatograph oven temperature was increased from 33 °C to 80 °C at the rate of 8 °C/min, then from 80 °C to 250 °C at the rate of 5 °C/min. The final temperature was maintained for 10 min. Each sample was measured 3 times then averaged with a precision of ± 0.3‰ and was reported relative to the VPDB standard (Dai et al., 2012; Huang et al., 2015). Natural gas hydrogen isotope analyses were conducted on a MAT253 GC/TC/IRMS. Helium was the carrier gas. Separation of natural gas components were conducted with a HP-PLOT Q column (30 m × 0.32 mm × 20 μm). The initial oven temperature was set at 33 °C, then raised to 80 °C at the rate of 8 °C/min, then ramped up to 250 °C at the rate of 5 °C/min. Each sample was measured 3 times and averaged, with a precision of ± 5‰ and was reported relative to the VSMOW standard (Dai et al., 2012; Huang et al., 2015).

4.3. Hydrogen isotopic composition Hydrogen isotopic values distribute widely in the SGF. δD1 (subscript indicates the number of carbon in the alkane gas), δD2, and δD3 values in the upper Paleozoic reservoirs of the SGF range from −201‰ to −193‰, −163‰ to −155‰, and −160‰ to −152‰, respectively. δD1, δD2, and δD3 values in the lower Paleozoic reservoirs of the SGF are from −196‰ to −191‰, −188‰ to −187‰, and −179‰ to −176‰, respectively (Table 1) (Fig. 4b). Alkanes in the SGF display a positive hydrogen isotopic series (δD1 < δD2 < δD3) (Table 1) (Fig. 4b). Hydrogen isotopic series of alkanes in the upper Paleozoic reservoirs of the SGF show convex characteristics, while those in the lower Paleozoic reservoirs show concave features (Fig. 4b). 5. Discussion 5.1. Origin of natural gas in the SGF The molecular components and hydrocarbon isotopic compositions of gases are closely related to their genetic types (Whiticar, 1996; Liu et al., 2018a, 2018b, 2019). Natural gases of different genetic types show different characteristics in their chemical and isotopic compositions (Schoell, 1980; Liu et al., 2009, 2015a; Peng et al., 2017). Alkanes could be categorized into organic-origin and abiogenic gas (Dai et al., 1992, 2004; Horita and Berndt, 1999). The original organic-origin gases generally have positive carbon and hydrogen isotopic series (δ13C1 < δ13C2 < δ13C3, δD1 < δD2 < δD3) (Dai et al., 2004, 2014, 2016). On the contrary, abiogenic gases generally have negative carbon and hydrogen isotopic series (δ13C1 > δ13C2 > δ13C3, δD1 > δD2 > δD3) (Dai et al., 2004, 2014, 2016). Because the isotopic compositions of abiogenic gas are more enriched in 13C, isotopic values of abiogenic gases are much higher than those of organic-origin gases (Sherwood Lollar et al., 1997, 2002; Liu et al., 2019). Alkanes of the SGF are relatively enriched in light isotopes and exhibit positive carbon and hydrogen isotopic series, indicating an organic origin (Table 1; Fig. 4). The organic-origin gas could be divided into bacterial gas and thermogenic gas (Whiticar, 1999; Huang et al.,

4. Results Chemical components, stable carbon and hydrogen isotope values of samples in the SGF are displayed in Table 1. 4.1. Natural gas compositions The gas compositions can be divided into alkanes and nonhydrocarbons. Alkanes includes methane and its homologue, whereas the nonhydrocarbons include CO2, N2, H2S, and a few noble gases—He, Ar, etc (Wakita and Sano, 1983; Wu et al., 2014; Liu et al., 2012, 2016, 2017, 2018a). The SGF has a relatively high content of alkanes, which ranges between 96.2% and 98.75% (Table 1). Alkane contents of the upper Paleozoic reservoirs and the lower Paleozoic reservoirs of the SGF are very similar (Table 1). Alkanes in the SGF are dominated by CH4 ranging from 89.42% to 93.09% (Fig. 3a). Dryness coefficients range from 0.907 to 0.958, and methane contents show a positive correlation with the C1/C1−5 of natural gas (Fig. 3a). The dryness 1025

1026

Sh10-13 Sh10-3 Sh10-5 Sh10-9 Sh11-15C1 Sh11-15C3 Sh11-4C5 Sh19-12 Sh20 Sh21-11 Sh3 Sh5-19 Sh6-18 Sh6-19 Sh7-11 Sh7-12 Sh8-17C3 Sh8-8 Sh9-11 Sh9-11H2 Sh9-12 Sh9-13H2 Sh7-11C1 Sh7-11C3

Z21-24 Z25-38 Z35-28 Y30 Y45 Y69

DK25 DP6 1–74 D1-4-71

Y47-7 Y58 Y44-13 Y44-4 Y45-9 Y46-12

S5 S17 S45 S49 S62 S28

Shenmu Gas Field

Zizhou Gas Field

Daniudi Gas Field

Yulin Gas Field

Jingbian Gas Field

91.55 91.33 92.17 91.62 91.34 91.69 89.42 92.17 91.37 91.61 91.49 92.12 91.77 91.65 93.09 92.58 92.08 91.46 91.44 91.97 92.26 91.34 90.91 93.04

94.22 94.67 94.81 94.10 94.17 94.93

91.93 92.03 91.50 93.48

92.46 92.97 93.19 89.62 92.56 92.56

97.25 93.87 94.92 94.64 96.55 95.96

P1s P1s P1s P1s P1s P1s

P1s P1s P1s P1s

P1s P1s P1s P1s P1s P1s

O1m O1m O1m O1m O1m O1m

CH4

0.49 0.72 0.16 0.31 0.54 0.74

4.42 3.89 4.27 5.66 4.47 4.73

4.11 4.25 4.64 3.22

3.12 2.87 2.97 3.14 3.12 2.85

4.66 4.95 3.78 4.18 4.20 3.81 6.92 3.09 4.40 4.10 4.48 4.00 4.00 4.28 4.09 3.93 4.18 5.53 4.42 3.39 3.57 4.77 4.95 3.34

C2H6

0.06 0.08 0.04 0.03 0.07 0.09

0.80 0.83 0.69 1.67 0.75 0.81

0.91 0.89 0.86 0.61

0.48 0.42 0.44 0.48 0.48 0.40

0.95 1.03 0.75 0.85 0.88 0.76 1.53 0.60 0.80 0.75 0.88 0.79 0.83 0.89 0.86 0.74 0.86 1.16 0.88 0.61 0.68 1.06 1.11 0.63

C3H8

Chemical composition (%)

P2x-P1t P1s- P1t P2x- P1t P2x- P1t P2x- P1t P2x- P1t P2x-P1s P2x- P1t P1s- P1t P2x- P1t P1s- P1t P2x- P1t P2x- P1t P2x- P1t P2x-P1s P2x- P1t P2x- P1t P2x P2x- P1t P1t P2x- P1t P1s O1m O1m

Strata

0.01 0.01 n.d. n.d. 0.01 0.01

0.12 0.16 0.10 0.40 0.11 0.13

0.16 0.15 0.14 0.15

0.08 0.06 0.06 0.07 0.08 0.06

0.22 0.19 0.16 0.19 0.18 0.16 0.25 0.14 0.14 0.14 0.20 0.16 0.17 0.18 0.17 0.16 0.19 0.20 0.21 0.14 0.15 0.25 0.18 0.13

iC4H10

0.01 0.01 n.d. n.d. 0.01 0.01

0.14 0.16 0.11 0.43 0.13 0.13

0.17 0.17 0.14 0.13

0.07 0.07 0.07 0.08 0.08 0.06

0.19 0.20 0.15 0.17 0.17 0.15 0.27 0.11 0.15 0.13 0.17 0.15 0.17 0.18 0.18 0.15 0.18 0.24 0.18 0.11 0.13 0.22 0.30 0.17

nC4H10

n.d. n.d. n.d. n.d. n.d. n.d.

0.04 0.06 0.03 0.19 0.04 0.04

0.13 0.12 0.11 0.13

n.d. n.d. n.d. n.d. n.d. n.d.

0.10 0.09 0.07 0.08 0.08 0.08 0.12 0.06 0.07 0.06 0.09 0.07 0.08 0.09 0.09 0.08 0.09 0.10 0.09 0.06 0.07 0.11 0.12 0.09

iC5H12

n.d. n.d. n.d. n.d. n.d. n.d.

0.02 0.03 0.02 0.09 0.02 0.02

0.08 0.08 0.00 0.00

n.d. n.d. n.d. n.d. n.d. n.d.

0.05 0.04 0.03 0.04 0.04 0.04 0.07 0.03 0.04 0.03 0.04 0.04 0.04 0.04 0.05 0.04 0.05 0.05 0.04 0.03 0.03 0.05 0.09 0.05

nC5H12

3.19 0.62 0.25 0.05 0.64 0.25

/ / / / / /

n.d. n.d. n.d. n.d.

0.32 0.38 0.37 0.38 0.36 0.35

0.41 0.38 0.51 0.44 0.33 0.29 0.57 0.26 0.34 0.36 0.24 0.36 0.29 0.25 0.63 0.34 0.26 0.65 0.26 0.26 0.31 0.30 0.22 0.21

N2

2.71 4.55 4.44 4.52 2.15 2.84

/ / / / / /

2.50 2.30 2.61 2.28

1.58 1.40 1.20 1.62 1.58 1.27

1.62 1.56 2.22 2.28 2.50 2.74 0.57 3.36 2.46 2.66 2.17 2.12 2.46 2.26 0.53 1.78 1.82 0.37 2.22 3.21 2.63 1.61 1.81 2.06

CO2

0.99 0.99 1.00 1.00 0.99 0.99

0.94 0.95 0.95 0.91 0.94 0.94

0.94 0.94 0.94 0.96

0.96 0.97 0.96 0.96 0.96 0.97

0.94 0.93 0.95 0.94 0.94 0.95 0.91 0.96 0.94 0.95 0.94 0.95 0.95 0.94 0.94 0.95 0.94 0.93 0.94 0.95 0.95 0.93 0.93 0.95

C1/C1-5 C2H6 −23.5 −24.8 −23.8 −23.5 −23.3 −23.2 −25.4 −23.2 −25.8 −24.1 −23.2 −23.9 −23.7 −23.8 −24.7 −23.4 −25.6 −25.1 −23.1 −22.8 −23.2 −23.2 −35.7 −36.0 −25.1 −25.7 −25.7 −23.0 −25.2 −26.3 −25.1 −25.5 −23.9 −25.9 −25.1 −25.2 −25.2 −5.0 −25.8 −25.1 −31.3 −30.7 −30.6 −31.8 −33.1 −28.3

CH4 −35.9 −36.5 −36.1 −36.5 −36.4 −36.0 −36.8 −35.9 −36.2 −35.9 −36.7 −36.5 −36.7 −37.0 −36.7 −36.1 −36.6 −37.1 −36.0 −35.6 −36.5 −36.0 −39.4 −38.6 −32.7 −32.6 −32.5 −33.1 −33.2 −32.8 −34.5 −34.4 −33.8 −34.2 −32.0 −31.3 −31.7 −31.4 −32.0 −32.7 −33.8 −33.2 −33.5 −33.4 −32.7 −34.1

δ13C (‰,VPDB)

−27.1 −26.9 −22.9 n.d. −30.0 −27.3

−22.6 −23.6 −22.4 −22.8 −23.5 −22.7

−23.7 −24.3 −23.7 −24.0

−23.2 −23.3 −23.6 −23.4 −23.1 −24.1

−22.3 −23.7 −22.8 −22.3 −22.3 −22.2 −24.2 −21.8 −24.2 −22.9 −22.0 −22.7 −22.4 −22.4 −23.1 −22.2 −23.4 −24.3 −22.0 −22.0 −22.1 −22.1 −30.5 −30.2

C3H8

−172 −169 −168 −166 −171 −173

−182 −180 −185 −185 −187 −186

−186 −187 −192 −182

−183 −185 −181 −183 −183 −179

−198 −196 −195 −199 −200 −200 −198 −193 −193 −195 −199 −197 −198 −201 −197 −199 −197 −198 −199 −198 −198 −198 −196 −191

CH4

n.d. n.d. n.d. n.d. n.d. n.d.

−170 −170 −171 −167 −168 −164

−157 −159 −157 −161

−163 −165 −164 −161 −164 −162

−157 −161 −157 −157 −159 −158 −161 −156 −163 −159 −156 −158 −157 −161 −161 −157 −163 −161 −156 −155 −157 −156 −188 −187

C2H6

δD (‰,VSMOW)

n.d. n.d. n.d. n.d. n.d. n.d.

−166 −166 −160 −160 −160 −156

−155 −156 −156 −155

−155 −154 −157 −154 −155 −151

−152 −159 −156 −154 −156 −158 −155 −155 −160 −157 −152 −157 −156 −159 −155 −155 −156 −158 −152 −157 −156 −154 −176 −179

C3H8

0.23 0.24 0.23 0.24 0.23 0.21

0.19 0.19 0.20 0.19

0.21 0.22 0.22 0.21 0.21 0.21

0.17 0.16 0.16 0.16 0.16 0.16 0.15 0.17 0.16 0.17 0.15 0.16 0.15 0.15 0.15 0.16 0.16 0.15 0.16 0.17 0.16 0.16

Ro (%)*

1.48 1.66 1.55 1.63 1.48 1.32

0.98 1.00 1.10 1.03

1.32 1.34 1.36 1.23 1.21 1.30

0.78 0.71 0.76 0.71 0.72 0.77 0.68 0.78 0.74 0.78 0.69 0.71 0.69 0.65 0.69 0.76 0.70 0.64 0.77 0.82 0.71 0.77

Ro (%)**

Note: Ro (%) (*, **) values were calculated by the δ13C-Ro empirical equations for coal-type gas brought forward by Stahl and Carey (1975) and Dai et al. (1992), respectively. “/" indicates no data."n.d.” means not detected. Data of the ZGF were from Huang et al. (2015); data of the DGF were from Wu et al. (2017b); data of the YGF were from Li et al. (2014); data of the JGF were from Li et al. (2008b).

Well

Field

Table 1 Chemical components, stable isotopic compositions of gas from the SGF (together with other fields in the Ordos Basin).

W. Peng, et al.

Journal of Petroleum Science and Engineering 180 (2019) 1022–1033

Journal of Petroleum Science and Engineering 180 (2019) 1022–1033

W. Peng, et al.

Fig. 3. Plots of (a) dryness coefficient versus methane %, and (b) dryness coefficient versus δ13C1 value of gases from the SGF and other giant fields in the Ordos Basin. Data from Li et al. (2008b), Li et al. (2014), Huang et al. (2015), Wu et al. (2017a) and this study. Through Fig. 3 (a and b), we can compare the drying coefficient, methane content and carbon isotope composition of methane between SGF and adjacent gas fields, and analyze their similarities and differences.

Fig. 4. (a) Carbon isotopic series, and (b) hydrogen isotopic series of alkanes from the SGF. Isotopic series of carbon and hydrogen characteristics are displayed, indicating the similarities and differences of alkanes between gases from the upper Paleozoic reservoirs and gases from the lower Paleozoic reservoirs of SGF.

value versus δ13C2 (Jenden et al., 1988). Gases from the upper Paleozoic reservoirs of the SGF generally follow the trends of gases generated by type III organic matters (Rooney et al., 1995), as do other gases from the upper Paleozoic reservoirs, indicating coal-type gases (Fig. 7a) (Jenden et al., 1988). But gases from the lower Paleozoic reservoirs of the SGF generally follow the trends of gases generated by humic source rock of the Delaware/Val Verde Basin (Rooney et al., 1995), which are similar to gases from the Jingbian Gas Field, indicating oil-type gases (Fig. 7a). Studies have indicated that coal-type gas usually has δ13C2 and δ13C3 values higher than −27.5‰ and −25.5‰, whereas oil-type gas generally has δ13C2 and δ13C3 lower than −27.5‰ and −25.5‰ (Dai et al., 1992; Wu et al., 2017a; Liu et al., 2019), respectively. Gases from the upper Paleozoic reservoirs of the SGF are plotted within the coal-type gas zone, while gases of the lower Paleozoic reservoirs in the SGF are plotted in the oil-type gas zone, similar to the Jingbian Gas Field (Fig. 7b). Hydrogen isotopic composition, which plays a significant role in determining the origin of alkanes, is mainly influenced by organic matter type, maturity of source rock, salinity of the aqueous medium in sedimentary environment (Schoell, 1980, 1983; Wang et al., 2015). Natural gas with different origins exhibits different distribution patterns in a correlation diagram of both δD1 versus δ13C1 and δ13C2. Alkanes of the upper Paleozoic strata from the SGF together with gases from the Daniudi Gas Field, Zizhou Gas Field, and Yulin Gas Feild are all characterized by coal-type gas. Gases of the lower Paleozoic

2017). Bacterial gas is abundant in CH4 with an extremely high dryness coefficient (usually higher than 0.99) and light δ13C1 (usually lighter than −55‰) (Bernard et al., 1976; Whiticar, 1999), whereas thermogenic gas generally exhibits δ13C1 value higher than −50‰ (Berner and Faber, 1988; Huang et al., 2015). Dryness coefficients of the SGF range from 0.907 to 0.958, with the δ13C1 values between −39.4‰ and −35.6‰, suggesting a thermogenic source (Table 1; Fig. 3b). Gases of the SGF as well as of other Paleozoic reservoirs in the Ordos Basin have similar characteristics, showing that C2/iC4 values increase with increasing maturity. Gas maturity of the SGF is significantly lower than that of the nearby Zizhou Gas Field and Yulin Gas Field (Fig. 5). Thermogenic gas could be roughly divided into oil-type and coaltype gases (Galimov, 2006; Dai et al., 2014; Liu et al., 2019). Oil-type gas is generated by the maturation of sapropelic source rocks (including gas generated by secondary cracking of oil) while coal-type gas is generated by the maturation of humic source rocks (Dai et al., 1992, Dai, 2016). It is noteworthy that two samples from the lower Paleozoic reservoir of the SGF may be either oil-type gas or oil-type gas mixed with a little proportion of coal-type gas (Fig. 6). Gases of the upper Paleozoic reservoirs in the Ordos Basin tend to be coal-type gases, except for gases of the Jingbian Gas Field, which shows obvious features of oil-type gas. Coal-type gas generated by humic organic matter has a relatively higher δ13C2 value than oil-type gas but they generally have similar δ13C1 values. As a result, natural gas of different origins shows different evolutionary trends in plot of δ13C1 1027

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Compared with gas from the Jingbian Gas Field, gases from the lower Paleozoic reservoirs of the SGF are closer to oil-type gas. Therefore, gases from the lower Paleozoic reservoirs of the SGF could be considered as oil-type gas (Fig. 8b). Comprehensive study of the gas component (Fig. 6) together with isotopic composition (Figs. 7 and 8) in our study suggests that gases from the upper Paleozoic reservoirs of the SGF have similar characteristics to other fields in the upper Paleozoic reservoirs, indicating that they are all typical coal-type gas. Gases from the lower Paleozoic reservoirs of the SGF have characteristics similar to gases from the Jingbian Gas Field, suggesting oil-type gases (Dai, 2016). 5.2. Source of natural gas in the SGF It is generally accepted that alkanes of the upper Paleozoic reservoirs in the Ordos Basin are sourced from Carboniferous–Permian coal seams (Liu et al., 2015a; Dai, 2016; Wu et al., 2017a, 2017b). A set of layers of humic organic matter in the Paleozoic strata, namely coal seams developed in the Carboniferous-Permian, should be the kitchen of gases within the upper Paleozoic strata of the SGF. However, researchers still have not reached agreements about whether gases of the upper Paleozoic reservoirs of SGF were generated directly by underlying coal seams or migrated from nearby source rocks (Wang et al., 1998; Mi et al., 2003; Li et al., 2009). Average porosity in the upper Paleozoic reservoirs of the SGF is about 7%. The reservoirs are mainly composed of distributary channel sandstones which occur as bands along the northern and southern directions with a poor lateral connectivity (Yang et al., 2015; Qin et al., 2017). The upper Paleozoic reservoirs of the SGF tightened prior to the main charging period of natural gas (Qin et al., 2017). As a result, buoyancy probably had little or no effects on the gas charging process. On the contrary, overpressure formed by hydrocarbon generation and diffusion were more likely to have played an important role in the gas accumulation process (Yang et al., 2015). Therefore, analysis of reservoir characteristics and migration conditions of natural gas indicates that gases of the upper Paleozoic reservoirs of the Shenmu Area did not have the geologic conditions for large-scale lateral migration. Thus, we conclude that gases of the upper Paleozoic reservoirs of the SGF was mainly generated by the underlying humic organic matter. The lower Paleozoic marine carbonates of the Shenmu Area have some hydrocarbon-generating capacity and gypsum layers in the upper O1m could perform as caprocks (Wei et al., 2017a). Therefore, the oil-type gas of the lower Paleozoic reservoirs in the SGF may have been generated by the carbonate rocks in the O1m. Some locations have no gypsum layers in the Shenmu Area (Wei et al., 2017a, 2017b). When the source rocks of the upper Paleozoic reached the maximum hydrocarbon generation intensity, a small amount of coal-type gas might migrate downward to the lower Paleozoic strata, thus mixing it with the oil-type gas of the lower Paleozoic reservoirs and resulting some characteristics of mixed gas (mainly oil-type gas and a small amount of coal-type gas). The thermal maturity of a source rock can be indicated by the δ13C1 value of the natural gas (Schoell, 1980, 1983; Berner and Faber, 1988; Galimov, 2006). Gas-source correlation can be done by combining the analysis of natural gas maturity and organic matter maturity in source rocks (Schoell, 1980, 1983; Berner and Faber, 1988). Galimov (2006) compared the empirical formulas put forward by Stahl and Carey (1975) and Dai et al. (1992), concluded that the former empirical formula was more applicable for instantaneous gas derived in a highly mature stage and the latter empirical formula was more applicable for cumulative gas. Before the late Cretaceous, the Ordos Basin had only one small-scale uplift, and hydrocarbon generation could still be considered as a continuous process. Therefore, from the analysis of geologic conditions, the empirical formula of δ13C1-Ro proposed by Dai et al. (1992) could be more applicable to the SGF. Two sets of Ro values of coal-type gas are calculated by the empirical formula of δ13C1-Ro put forward by both Stahl and Carey (1975)

Fig. 5. Plot between C2/C3 and C2/iC4 values of alkanes in the SGF and other giant fields in the Ordos Basin. Data from Li et al. (2008b), Li et al. (2014), Huang et al. (2015), Wu et al. (2017a) and this study. Fig. 5 shows that the maturities of natural gases in the Ordos Basin are different.

Fig. 6. C1/C2+3 versus δ13C1 of alkane from the SGF and other giant fields in the Ordos Basin. Plot after Bernard et al. (1976); data from Li et al. (2008b), Li et al. (2014), Huang et al. (2015), Wu et al. (2017b) and this study. Natural gases in the Ordos Basin are thermogenic and there are obvious differences between gases in the upper Paleozoic reservoirs and the lower Paleozoic reservoirs.

reservoirs from the SGF, along with gases of the Jingbian Gas Field, fall into the category of mixed-gas or transitional zone (Fig. 8a). In a plot of δD1 versus δ13C2 (Fig. 8b), gases from the upper Paleozoic reservoirs, including those in the SGF, Daniudi Gas Field, Yulin Gas Feild, and Zizhou Gas Field, all display characteristics of coal-type gas. Whereas gases from the lower Paleozoic reservoirs, including in the SGF and Jingbian Gas Field, exhibit mixed-gas characteristics. Gases from the Jingbian Gas Field display oil-type gas characteristics, which is in accordance with previous studies (Liu et al., 2009; Wang et al., 2017). 1028

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Fig. 7. Plot of (a) δ13C2 versus δ13C1, and (b) δ13C3 versus δ13C2 of gas from the SGF and other giant fields in the Ordos Basin. Trend lines revised from Rooney et al. (1995) and Jenden et al. (1988). Data from Li et al. (2008b), Li et al. (2014), Huang et al. (2015), Wu et al. (2017b) and this study. Coal-type gas is mainly developed in the upper Paleozoic reservoirs and oil-type gas is mainly developed in the lower Paleozoic reservoirs.

Fig. 8. Diagrams of (a) δD1 versus δ13C1, and (b) δD1 versus δ13C2 of gas from the SGF and other fields in the Ordos Basin. Diagrams from Wang et al. (2015); data from Li et al. (2008b), Li et al. (2014), Huang et al. (2015), Wu et al. (2017b) and this study. Coal-type gas is mainly developed in the upper Paleozoic reservoirs and mixtures of oil-type gas and coal-type gas (mainly oil-type gas) may be developed in the lower Paleozoic reservoirs.

Fig. 9. Plot of (a) calculated gas maturity Ro versus δ13C2−δ13C1, and (b) calculated gas maturity Ro versus δ13C3−δ13C1 between alkanes in the SGF and other giant fields in the Ordos Basin. Data from Li et al. (2014), Huang et al. (2015), Wu et al. (2017b) and this study. Maturities of natural gases between SGF and adjacent gas fields are different.

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and Dai et al. (1992) (Table 1). The calculation results of empirical formulas, put forward by different scholars, are quite different (Table 1). Ro values calculated through the equation by Dai et al. (1992) range from 0.64% to 0.82%, which is consistent with geologic conditions (Zhang et al., 2000; Dai, 2016). Gases from the SGF, Daniudi Gas Field, Zizhou Gas Field, and Yulin Gas Field have increasing maturities and should come from different sources (Fig. 9). Migration results in lighter carbon isotopic values and higher dryness coefficients for natural gas (Hu et al., 2015; Wu et al., 2017a). However, both δ13C1 values and dryness coefficients of the upper Paleozoic reservoirs in the SGF are lower than those of other gas fields, which further indicates that natural gases in the SGF did not migrate from the Daniudi Gas Field, Zizhou Gas Field, or Yulin Gas Field, but were primarily derived from the underlying humic source rocks. Therefore, analyses from both geological and geochemical characteristics show that natural gases in the SGF are derived from the underlying source rock. 5.3. Accumulation process of natural gas in the SGF The SGF is situated in northeast area of the Yishan Slope (Fig. 1), which is relatively structurally stable and provides favorable conditions for natural gas accumulation (Dai et al., 2017a, 2017b). Burial thermalevolution history of the Shenmu Area indicates that the upper Paleozoic source rocks began to generate gases from the early Jurassic (Yang et al., 2015). Because of greater burial depth, the lower Paleozoic carbonate source rocks had already generated a certain amount of crude oil at that time (Wei et al., 2017a, 2017b). During the middle–late Jurassic, the study area was affected by the Yanshan Movement, which caused a small-scale uplift, but the area soon entered a stable subsidence stage (Ren et al., 2007, 2017; He, 2015). From the late Jurassic to early Cretaceous, the Ordos Basin was generally subjected to rapid subsidence, with no large-scale tectonic uplift (Yang and Pei, 1996; Dai, 2016). It experienced a thermal event from the late Cretaceous, evidenced from ancient geothermal gradient anomalies and the timing of igneous activity (Ren et al., 2007, 2017; He, 2015), which corresponds to a relatively strong tectonic uplift at the early Cretaceous. Because the upper Paleozoic strata were not eroded (only the upper strata of Mesozoic were eroded), the upper Paleozoic gas-reservoir system was not significantly affected (Liu et al., 2015b). Tectonic movement is accompanied by the change of pressure system, which can promote the development of the fractures (Ghosh and Mitra, 2009; Smart et al., 2009). During the Cretaceous uplift in the Ordos Basin (Zhao et al., 2010), a series of structural fractures formed in the upper Paleozoic strata, providing preferential pathways for the short-distance migration of alkane (Li et al., 2011). Subjected to continuous subsidence, the Ordos Basin reached its maximum burial depth in the late early Cretaceous with the source rocks reaching their peak hydrocarbon generation. However, at the beginning of the late Cretaceous, when the Ordos Basin began to uplift, hydrocarbon generation ceased (Fig. 10) (Li et al., 2011). Previous studies of fluid inclusions showed that the upper Paleozoic strata of the Ordos Basin underwent a continuous hydrocarbon charging (Wang et al., 2008), and the main accumulation period was the early Cretaceous (Li et al., 2008a, 2011). The densification time of reservoir was earlier than the large-scale accumulation of natural gas (Fig. 10). The upper Paleozoic reservoirs in the SGF generally were tight sandstones with lower porosity/permeability prior to gas emplacement. Integrating gas-source correlation, the thermal-maturation history of source rock, and the reservoir-densification process, we can divide the gas accumulation process of the SGF into four major stages: prenatal, development, prime, and formative (Fig. 11). Before the early Jurassic, the upper Paleozoic reservoirs have completed its densification process and prepared for large-scale accumulation of gas. Meanwhile, only a small portion of gases were expelled from the source rocks, and some gas was lost in this process. Only a few gas reservoirs were formed in the upper Paleozoic strata. Because the lower Paleozoic

Fig. 10. Thermal-evolution history of major stratigraphic intervals and porosity-evolution process of the upper Paleozoic reservoir in the Shenmu Area. Thermal-evolution history revised from Zhang et al. (2000) and Li et al. (2011); porosity-evolution history of the upper Paleozoic reservoir modified from Qin et al. (2017).

strata have sapropelic organic matter and its maturity was higher than that of the upper Paleozoic (Dai, 2016), a small number of reservoirs were formed in the lower Paleozoic carbonate rocks. At this time, the upper Paleozoic strata were dominated by normal pressure (Yang et al., 2015). This stage was the preparation period of large-area accumulation of tight sandstone gas, i.e. the prenatal stage of the SGF (Fig. 11a). From the early to late Jurassic, the reservoirs were almost all tight, and more source rocks entered the mature stage. The hydrocarbongeneration intensity was relatively low and the scale of the tight sandstone gas reservoirs were still not very large. With the increase of hydrocarbon-generation intensity and area, the scale of gas reservoirs began to gradually increase. At this time, the upper Paleozoic strata were dominated by minor overpressure (Yang et al., 2015). Meanwhile, the lower Paleozoic crude oils were partly cracked and formed coexisting oil and gas, i.e. the development stage of the SGF (Fig. 11b). In the early Cretaceous, the source rocks reached maximum burial depth and hydrocarbon generation, and the upper Paleozoic gas reservoirs also expanded at a large scale. Meanwhile, the lower Paleozoic oils were all cracked into gas (Li et al., 2008a, 2008b; Wei et al., 2017a, 2017b). Because of the hydrocarbon-generating pressurization, it was possible for the upper Paleozoic coal-type gas to make intrusion into the lower Paleozoic O1m in which had no bauxite mudstone. Thus, a portion of the lower Paleozoic gases might display characteristics of oiltype gas mixed with a little portion of coal-type gas (Fig. 8). In this prime stage of the SGF, the scale of the gas reservoir reached its maximum (Fig. 11c), and the upper Paleozoic strata were dominated by abnormal overpressure (Yang et al., 2015). The Ordos Basin was subjected to erosion after the early Cretaceous, and the strata were uplifted. Simultaneously, strata temperature and pressure dropped rapidly, and hydrocarbon generation tended to cease (Yang et al., 2015). The formation of all gas reservoirs involves the accumulation and loss of natural gas (Hunt, 1984) and the amount of lost natural gas was greater than the supply at this time for the SGF. After the strata were uplifted, the area of gas reservoirs diminished. They were characterized by abnormally low pressure. The pressure was released in the process of uplift and fractures occurred in the upper Paleozoic (Li et al., 2011). In addition, some gas reservoirs in the P2sh 1030

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Fig. 11. Reservoir accumulation process of the Shenmu Area: (a) prenatal stage, (b) development stage, (c) prime stage, and (d) formative stage.

gas with the dryness coefficient ranging from 0.907 to 0.958. δ13C1, δ13C2, and δ13C3 values range from −39.4‰ to −35.6‰, −36.0‰ to −22.8‰, and −30.5‰ to −21.8‰, respectively. δD1, δD2, and δD3 values range from−201‰ to −191‰, −188‰ to −155‰, and −179‰ to −152‰, respectively. Carbon and hydrogen isotope values both display positive series (δ13C1 < δ13C2 < δ13C3, δD1 < δD2 < δD3). (2) Geochemical characteristics indicate that gases of the upper Paleozoic reservoirs in the Shenmu Gas Field are coal-type gas, but gases of the lower Paleozoic reservoirs in the Shenmu Gas Field are oil-type gas (perhaps containing a little portion of coal-type gas).

and P3s were subjected to secondary alteration (Fig. 11d). Finally, the four stages of hydrocarbon accumulation led to the formation of the present day SGF gas reservoirs. 6. Conclusions Base on geochemical characterization of 24 gas samples from the SGF, coupled with the geological condition of the Ordos Basin, conclusions are made as follows: (1) The natural gases of the Shenmu Gas Field contain both wet and dry 1031

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Gas-source correlation demonstrates that gases of the upper Paleozoic reservoirs in the Shenmu Gas Field display near-source accumulation and were derived from the underlying Carboniferous–Permian humic source rock. Geses of the lower Paleozoic reservoirs in the Shenmu Gas Field are primarily derived from the carbonate rocks of the O1m. (3) The accumulation process of gas in tight sandstone reservoirs of the Shenmu Gas Field can be divided into four stages: prenatal (before early Jurassic), development (early–late Jurassic), prime (early Cretaceous), and formative (late Cretaceous–present).

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Acknowledgement The authors are grateful to Academician Jinxing Dai from RIPED Petrochina, for his insightful suggestions that have tremendous improved this manuscript. Thanks to Academician Yongsheng Ma and Prof. Quanyou Liu from SINOPEC for his concern and help to the authors. Useful comments and suggestions by the Executive Editor Prof. Tahar Aïfa and anonymous reviewers are also greatly acknowledged. This work is financially supported by the National Key R&D Program of China (Grant No. 2017YFC0603102), the China Postdoctoral Science Foundation (Grant No. 2019M650967), the China National Science & Technology Special Project (Grant No. 2016ZX05007-01), National Natural Science Foundation of China (Grant No. 41802177; 41802161; 41602152), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA14010404). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.05.067. References Bernard, B.B., Brooks, J.M., Sackett, W.M., 1976. Natural gas seepage in the Gulf of Mexico. Earth Planet. Sci. Lett. 31 (1), 48–54. Berner, U., Faber, E., 1988. Maturity related mixing model for methane, ethane and propane, based on carbon isotopes. Org. Geochem. 13 (1−13), 67–72. Dai, J., Pei, X., Qi, H., 1992. Natural Gas Geology in China, vol. 1. Petroleum Industry Press, Beijing, pp. 1–264 (in Chinese). Dai, J., Xia, X., Qin, S., Zhao, J., 2004. Origins of partially reversed alkane δ13C values for biogenic gases in China. Org. Geochem. 35 (4), 405–411. Dai, J.X., Xia, X.Y., Li, Z.S., Coleman, D.D., Dias, R.F., Gao, L., Li, J., Deev, A., Li, J., Dessort, D., Dulerc, D., Li, L.W., Liu, J.Z., Schloemer, S., Zhang, W.L., Ni, Y.Y., Hu, G.Y., Wang, X.B., Tang, Y.C., 2012. Inter-laboratory calibration of natural gas round robins for δ2H and δ13C using off-line and on-line techniques. Chem. Geol. 310−311, 49–55. Dai, J., Gong, D., Ni, Y., Huang, S., Wu, W., 2014. Stable carbon isotopes of coal-derived gases sourced from the Mesozoic coal measures in China. Org. Geochem. 74, 123–142. Dai, J., 2016. Giant Coal-Derived Gas Fields and Their Gas Sources in China. Academic Press, New York, pp. 1–150. Dai, J., Ni, Y., Huang, S., Gong, D., Liu, D., Feng, Z., Peng, W., Han, W., 2016. Secondary origins of negative carbon isotopic series in natural gas. Journal of Natural Gas Geoscience 1, 1–7. Dai, J., Ni, Y., Gong, D., Feng, Z., Liu, D., Peng, W., Han, W., 2017a. Geochemical characteristics of gases from the largest tight sand gas field (Sulige) and shale gas field (Fuling) in China. Mar. Pet. Geol. 79, 426–438. Dai, J., Ni, Y., Qin, S., Huang, S., Gong, D., Liu, D., Feng, Z., Peng, W., Han, W., Fang, C., 2017b. Geochemical characteristics of He and CO2 from the Ordos (cratonic) and Bohaibay (rift) basins in China. Chem. Geol. 469, 192–213. Feng, Z., Liu, D., Huang, S., Gong, D., Peng, W., 2016. Geochemical characteristics and genesis of natural gas in the Yan’an gas field, Ordos Basin, China. Org. Geochem. 102, 67–76. Galimov, E.M., 2006. Isotope organic geochemistry. Org. Geochem. 37 (10), 1200–1262. Gao, H., Wang, Y., Fan, Z., Wen, K., Li, T., 2015. Quantitative classification and characteristics difference of diagenetic facied in Shan 2 sandstone of Shenmu Gas Field, Ordos Basin. Natural Gas Geoscience 26 (6), 1057–1067 (in Chinese with English abstract). Ghosh, K., Mitra, S., 2009. Structural controls of fracture orientations, intensity, and connectivity, Teton anticline, Sawtooth Range, Montana. AAPG (Am. Assoc. Pet. Geol.) Bull. 93 (8), 995–1014. He, L., 2015. Thermal regime of the north China craton: implications for craton destruction. Earth Sci. Rev. 140, 14–26. Horita, J., Berndt, M.E., 1999. Abiogenic methane formation and isotopic fractionation

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