Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

International Journal of Coal Geology 80 (2009) 124–134

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China Jinxing Dai a,b,c,⁎, Caineng Zou a, Jian Li d, Yunyan Ni a, Guoyi Hu d, Xiaobao Zhang e, Quanyou Liu a, Chun Yang a, Anping Hu b a

PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China Department of Earth Science, Zhejiang University, Hangzhou 310027, China Faculty of Natural Resources and Information Technology, University of Petroleum, Beijing, 102200, China d PetroChina Research Institute of Petroleum Exploration and Development-Langfang Branch, Hebei 065007, China e Lanzhou Institute of Geology, CAS, Lanzhou 730000, China b c

a r t i c l e

i n f o

Article history: Received 2 January 2009 Received in revised form 6 August 2009 Accepted 6 August 2009 Available online 20 August 2009 Keywords: Coal-derived alkane gases Middle–Lower Jurassic coal-bearing strata Carbon isotopes Light hydrocarbons China

a b s t r a c t Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ13C values are found in the Tarim and Qaidam basins (δ13C1: − 19.0 to − 29.9‰; δ13C2: − 18.8 to − 27.1‰), and those with lowest δ13C values occur in the Turpan-Harmi and Junggar basins (δ13C1: − 40.1 to − 44.0‰; δ13C2: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C5–8) in the coal-derived gases are lower than those in the oilassociated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coalderived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ13C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Source rock–gas correlation plays an important role in hydrocarbon exploration. Although there are many different correlation methods, no single technique has been regarded absolutely as reliable. Many attempts have been made to deduce the origin of gases from their compositional variations in the past decades. Carbon isotopic composition of methane and its homologues has been found useful for recognition of the origin of a gas. Some coal-derived gases from type III and type II2 organic matters are enriched in 13C in their methane and ethane compared to oil-associated gases from Type I and type II1 organic matters or thermal cracking of oils (Stahl, 1977; Schoell, 1983; ⁎ Corresponding author. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China. Tel.: +86 10 62097084; fax: +86 10 62097046. E-mail address: [email protected] (J. Dai). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.08.007

Dai et al., 1992). 13C in methane increases with maturity (Galimov, 1968). In this work geochemical analysis was performed on natural gas samples from the Middle–Lower Jurassic coal measures in the Tarim, Junggar, Turpan-Harmi and Qaidam basins in the northwestern China, to determine their chemical composition and carbon isotope ratios and hence to identify the origin and migration patterns of gases. In this work, a kind of isotopic correlation method was presented. The aims were to identify the types of natural gas, correlation between source rocks and gases, the distinct carbon isotopic characteristics and the possible migration pattern of the gas. The characteristic carbon isotopes of coal-derived gases may shed light on the future gas exploration in areas with similar geologic settings. Coals are abundant in the northwestern China. According to the 3rd round National Coal Deposit Prediction (1992–1997), total coal reserves are around 55,697.5 × 108 ton within 2000 m depth. Coal reserves in the Xinjiang Autonomous Region account for 34.5% of the

J. Dai et al. / International Journal of Coal Geology 80 (2009) 124–134

total, which makes Xinjiang a region with largest coal reserves in China, and they are mainly accumulated in the Middle–Lower Jurassic coal-bearing strata (China National Administration of Coal Geology, 1998). Middle–Lower Jurassic coal-bearing strata are distributed in more than 80 inland basins of various scales in the northwestern China including the Tarim, Junggar, Turpan-Harmi, Ili, Qaidam, Minhe and Xining basins discussed herein (China National Administration of Coal Geology, 1998). Coal-derived gas and oil (oil sourced from coal measures) fields have been found in the Tarim, Junggar, Turpan-Harmi and Qaidam basins, which have the Middle–Lower Jurassic coal-bearing strata as source rocks (Fig. 1). At present the Tarim, Junggar and Qaidam basins are dominated by coal-derived gas fields whereas the Turpan-Harmi basin is characterized by coal-derived oil fields (Cheng, 1994; Wu and Zhao, 1997). For these basins with discovered coal-derived oil and gas fields, their source rocks are mainly from Early-Middle Jurassic Xishan Formation (J2x), Badaowan Formation (J1b) and of coal measures from equivalent formations (Fig. 2), whereas the coal-derived gas or oil fields are accumulated in the Cretaceous and Tertiary strata except the Turpan-Harmi basin. By the end of 2005, there have been 16, 10 and 17 coal-derived gas fields, oil fields and oil-gas fields, respectively sourced from the Middle–Lower coal-bearing Jurassic strata in the Tarim, Junggar, Turpan-Harmi and Qaidam basins. Among them, Kela 2 gas field was the largest with recoverable reserves of 1999 × 108 m3 and annual gas yield of 111.39 × 108 m3 in 2007.

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2. Geological setting The distribution areas of Middle–Lower Jurassic coal measures are different in Tarim, Junggar, Turpan-Harmi and Qaidam basins. In Junggar basin, coal measures are widespread and nearly reach the whole basin, whereas only half of the Tarim basin was distributed with coal measures, i.e., areas northeast to the Bachu-Qiemo connecting line and in the southwest edge. Coal measures are mainly distributed in the northern Turpan-Harmi basin and north edge of Qaidam basin (Fig. 1). In the Tarim basin, the Middle–Lower Jurassic coal measures are mainly distributed in the Kuqa depression, followed by southwest depression. The thickness of dark mudstone of the Middle–Lower Jurassic source rocks in the Kuqa depression ranges from 300 m to 600 m and can be up to 1035 m. Coal measures are generally 20–30 m thick and can be up to 68 m. The thickness of source rocks gradually thins down toward south (Dai et al., 2000a; Zhao et al., 2002a). In the Kuqa depression, organic matters of source rocks are dominantly humic with kerogen of type III1– III2 (Table 1) and are averagely mature to overmature (Fig. 3a), and plus the high values of TOC, chloroform bitumen A and S1 + S2 (Table 2), the depression is characterized by gas generation. Kuqa depression is an area in northwestern China with most discovered coal-derived gas fields which are correlated with Middle–Lower Jurassic coal measures in China (Fig. 1). Though Middle–Lower Jurassic dark mudstones are generally 100–350 m thick in Yingjisu depression in the east of Tarim Basin, the organic matters are immature to low mature. For example, Ro

Fig. 1. Oil and gas fields from Middle–Lower Jurassic coal-bearing basins in northwest China. Gas fields in Tarim Basin: 1. Ti'ergen, 2. Dina 1, 3. Dina 2, 4. Keziluoke, 5. Yinan, 6. Yaha, 7. Kela 3, 8. Kela 2, 9. Hongqiqu, 10. Dawanqi, 11. Yingmai 7, 12. Yangtake, 13. Quele, 14. Wucan1. Gas fields in Juggar Basin: 1. Dushanzi, 2. Horgos, 3. Pencan2, 4. Hutubi, 5. Gumudi. Gas fields in Turpan-Harmi Basin: 1. Yanbei, 2. Qiuling, 3. Shanshan, 4. Mideng, 5. Qiudong, 6. Wenjisang, 7. Hongtai. Gas field in Qaidam Basin: 1. E'boliang No.I, 2. Lenghu No.4, 3. Lenghu No.5, 4. Nanbaxian, 5. Mahai, 6. Mabei, 7. Hongshan.

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Fig. 2. Jurassic strata correlation in the main northwest Chinese basins.

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Table 1 Maceral composition of Jurassic source rocks from Tarim, Turpan-Harmi and Junggar basins. Basin

Strata

Lithology

Sample number

Vitrinite (%)

Inertinite (%)

Liptinite + sapropel (%)

References

Tarim

J1–2,T3 J1–2

Junggar

J1–2

52 57 77 245 305 142

Qaidam (northern)

J1–2

10.0–90.0 7.1–100 4.8–97.0 4.8–98.4 0–100 15.0–100 0–100 20.7–100

5.0–68.0 0–92.2 0–96.0 0–67.4 0–65.0 0–57.0 0–50 0–73.4

0–80.0 (34.7) 0–25.0 (3.8) 3.0–94.6 (38.9) 0–94.6 (10.7) 0–95.0 (32.0) 0–80.0 (12.4) 0–100 (37.7) 0–58 (11.7)

Dai et al. (2000a)

Turpan-Harmi

Mudstone Coal Mudstone Coal Mudstone Coal Mudstone Coal

(46.1) (50.5) (50.1) (74.0) (59.4) (78.4) (46.0) (60.8)

(18.6) (45.7) (11.0) (15.2) (8.6) (9.2) (16.3) (27.4)

Dang et al. (2003)

Note: Values in bracket are the average contents.

values are 0.48% and 0.45–0.63% for organic matter at depth of 2970– 3100 m of Tienan 2 well and 2940–3450 m of Huayingcan 1 well, respectively, and the average Ro value of 28 samples is 0.53% (Fig. 3a) (Zhao et al., 2002a). Hence no gas fields or oil fields have been found in this area. In the Junggar basin, Jurassic coal measures are distributed in

whole basin except its north edge. The Middle–Lower Jurassic strata are characterized by lake–swamp coal measures with relatively great thickness of 800–1200 m in the center and south edge. Dark mudstones are 300–800 m thick and coal seams are generally 20–80 m thick with thickness more than 200 m in the southern coal accumulating center

Fig. 3. Ro contour of Jurassic source rocks in Tarim (a) and Junggar (b) basins.

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Table 2 TOC, chloroform bitumen A, hydrocarbon generation potential and total hydrocarbon of Jurassic source rocks. Basin

Strata

Lithology

Tarim (Kuqa depression) Junggar

J1–2

Turpan-Harmi

J1–2

Qaidam (northern)

J1–2

Mudstone Coal Mudstone Coal Mudstone Coal Mudstone Coal

J1–2

Sample number

1118 155 437 40

TOC (%)

Chloroform bitumen A (%)

S1 + S2 (mg/g)

HC (%)

References

(2.53) (44.66) 0.97–29.7 (5.7) 43.5–79.0 (65.4) (1.51) 36.55–79.48 (56.95) 1.65–2.74 (2.02) 38.97–71.13 (55.79)

0.108 0.7486 0.0016–0.602 (0.05107) 0.7412–2.896 (1.859)

3.92 107.99

0.0739 0.1302 0.0011–0.3092 (0.030545) 0.1621–0.2357

Dai et al. (2000a)

0.022–0.1617 (0.0672) 0.445–1.0274 (0.6554)

(2.63) 27.85–162.07 (103.42) 0.41–6.29 (4.84) 5.23–112.04 (70.55)

Wang et al. (1998) 0.0043–0.1158 (0.051) 0.1349–0.3534 (0.2072)

Dang et al. (2003)

Note: Values in bracket are the average contents.

(Dai et al., 2000a). Source rocks of this basin are dominated by humic organic matters with kerogen of type III1–III2 (Table 1), and values of TOC, chloroform bitumen A and S1 + S2 are also high (Table 2). Maturity of source rocks is relatively low in the north, generally 0.5–0.8%, but around 1.0–2.0% in the center and south edge, up to 2.0% for the deepest (Fig. 3b). The present discovered gas fields such as Hutubi are all located in the southern depression. In Turpan-Harmi basin, the Middle–Lower Jurassic depositions are characterized by swamp–plain facies and mainly distributed in the north (Figs. 1 and 4a). The Lower Jurassic Badaowan formation and Middle Jurassic Xishanyao formation coal measures are main coal-bearing strata and source rocks as well. Dark mudstones and coal seams are developed in these two formations. The Badaowan formation has dark mudstone generally of 50–200 m thick, TOC of 0.5–3.0% with highest of 3.93%, hydrocarbon generation potential of 0.5–5 mg/g, and coal seams of 40–60 m thick with thickest of 100 m, and it is at low mature to mature stage (Fig. 4a). The Xishanyao formation has dark mudstone generally of 200–400 m thick with thickest of 600 m, TOC of 0.5–3.62%, hydrocarbon generation potential of

1–6 mg/g, coal seams of 40–100 m thick with hydrocarbon generation potential of 100–200 mg/g. The element composition of Middle–Lower Jurassic coal measure is relatively enriched in hydrogen, and its organic matter is dominantly sapropelic–humic, with small amount of humic– sapropelic (Wang et al., 1998) and is at low mature and mature stage. Hence, except for the gas fields of Qiudong and Hongtai, a number of coal-derived oil fields have been found in Turpan-Harmi basin (Qiuling, Shanle, Wenjisang, etc.) (Fig. 1). In the Qaidam basin, the Middle–Lower Jurassic deposition belongs to river–lake–swamp coal measures, and is mainly distributed in the north with an area of 10,000 km2 (Figs. 1 and 4b). Source rocks are dominated by lake–swamp mudstones. Coals and carbonaceous mudstones are not abundant in the Lower Jurassic, and the thickness of mudstone ranges from 100 m to 1200 m with generally of 200–600 m. Coals are relatively developed in the Middle Jurassic source rocks, and the thickness is 100–500 m (generally 100–200 m) for mudstones and 6–51 m (generally 10–30 m) for coal seams. Organic matters are dominantly humic (Table 1). All TOC, Chloroform bitumen A and hydrocarbon generation potential are high (Dang et al., 2003)

Fig. 4. Ro contour of Jurassic source rocks in Turpan-Harmi (a) and Qaidam (b) basins.

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(Table 2). It is regarded as good source rocks. Source rocks subjected to serious segmentation due to tectonic movements and only Nanbaxian and Mahai gas fields are found at present.

3. Analytical methods The molecular composition of gas samples was determined using an Agilent 6890N gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector, at the PetroChina Research Institute of Petroleum Exploration and Development-Langfang (RIPED-Langfang). Individual hydrocarbon gas components (C1–C4) were separated using a capillary column (PLOT Al2O3 50 m × 0.53 mm). GC oven temperature was initially set at 30 °C for 10 min, and then ramped to 180 °C at 10 °C/min. Stable carbon isotope values were determined on a Finnigan Mat Delta S mass spectrometer interfaced with a HP 5890II chromatograph also at RIPED-Langfang. Gas components were separated on a gas chromatograph, converted into CO2 in a combustion interface, and then injected into the mass spectrometer. Individual hydrocarbon gas components (C1–C4) and CO2 were separated using a fused silica capillary column (PLOT Q 30 m × 0.32 mm). The GC oven was ramped from 35 °C to 80 °C at 8 °C/min, then to 260 °C at 5 °C/min, and the oven maintained at the final temperature for 10 min. As gas samples were analyzed in triplicates, and the stable carbon isotopic values are reported in the δ notation in per mil (‰) relative to the VPDB standards. Reproducibility and accuracy are estimated to be ±0.5‰ with respect to VPDB standard.

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4. Carbon isotopes of coal-derived alkane gases Coal-derived gases are sourced from the coal and mudstone in the humic coal measures. The carbon isotopic composition of the coal-derived alkane gases is a good index for the source rock maturity, whereas the carbon isotopes of ethane, propane and butane are significant discriminating criterions for the coal-derived or oil-associated gases (Dai et al., 1992; Wang, 1994; Dai, 1999; Liang et al., 2002). According to analyses of coal-derived gases in China, Dai (1999) proposed that gases with δ13C2 > −27.5‰ and δ13C3 >−25.5‰ are coal-derived gases. Wang (1994) suggested that Jurassic–Sinian natural gases with δ13C2 >−29‰ and δ13C3 >−27‰ in the Sichuan basin are coal-derived gases. Based on studies of the coal-derived gases from the Kuqa depression in the Tarim Basin and compilation of previous studies, Liang et al. (2002) proposed gases with δ13C2, δ13C3, and δ13C4 greater than −28‰, −26‰ and −25‰, respectively, are coal-derived gases. Recently, Dai et al. (2009) found that the coal-derived ethane from Xujiahe formation in the Sichuan basin had highest carbon isotopes, mostly ranging from −24‰ to −28‰, which was greater than that of the oil-associated ethane in the same basin. It is a common phenomenon in China that carbon isotope of coalderived ethane is normally greater than that of the oil-associated ethane. According to these statistical studies, all the gases analyzed herein (Tables 3–6) are coal-derived gases. Except for the carbon isotopic features of C1–C4 alkanes, the high content of methyl cyclohexane (MCC6) of light hydrocarbon also provide a support for the coal-derived origin. Methyl cyclohexane mainly comes from high plant lignin and cellulose (Dai et al., 1992; Peters et al., 2005). Among all C7 hydrocarbons MCC6 accounts for 62.93%, 42.24%, 49.56% and 59.45% for Kela 2 (depth of 3770–

Table 3 Geochemical compositions of Jurassic coal-derived gases in Tarim basin, northwest China. Field

Well

Strata

Depth (m)

Kela 2

Kela 2 Kela 2 Kela 2–8 Kela 2–7 Kela 203 Kela 201 Kela 201 Kela 3 Yi 590 Dina 2 Dian102 YM6 YM9 YM19 YM7-H1 Yaha701 Yaha1 Yaha4 Yaha23,1–18 Yaha23,1–14 Yangtake 1 Yangtake 5-2 Yangtake 5-3 Ti 1 Ti 2 Ti 101 Hongqi1 Hongqi2 Wucan1 Tai1 Dawanqi117-3 Dawanqi109–19 Dawanqi101 Kezi1 Queqin1 Dongqiu1

E K2b E E E K2b K2b E J N1j

3500–3535 3888–3895

Kela 3 Yiqikelike Dina 2 Dian 1 Yingmaili

Yaha

Yangtake

Ti'ergen

Hongqiqu

Dawanqi

Keziluoke Quele Dongqiutake

N1j E E E E K N1j E+K E+K E+K E E1K E N K O K Nj N1–2K N1–2K N K1y K N1j

4050 3630–3640 3770–3795 3105–3199 4597–4876 5768 4420–4426 4683–4801 4702–4704 6000 5600 4997–5001

5234–5332

4837–4840 5298 5400 5918–6010 4847 285–518 456–461 2585–2590 1130–1141 5930 1421–1439

δ13C (‰), VPDB

Main components (%) CH4

C2H6

C3H8

i-C4

n-C4

96.90 98.22 98.41 98.41 97.86

0.31 0.53 0.80 0.8 0.82

0.04 0.05 0.05 0.05

97.70 98.05 65.49

0.59 0.62 16.16

0.50 0.09 9.17

89.47 76.11 86.38 85.05 90.14 86.20 77.65 76.55 86.46 85.89 91.17 83.10 85.97 85.36 80.54 86.65 55.96 77.78 82.11 81.67 88.31 90.04 95.61

7.25 10.12 4.80 7.17 4.62 5.66 7.91 14.91 5.80 6.23 5.32 6.94 6.91 7.03 12.36 6.31 11.55 9.90 11.76 11.06 4.72 5.49 2.97

1.60 4.28 2.01 2.95 1.27 2.24 2.92 4.88 2.17 2.24 1.11 3.67 2.76 2.98 3.44 2.74 12.53 3.83 2.50 4.35 1.53 1.50 0.54

0.55 7.22 1.44 1.51 2.35 0.31 0.30 0.15

0.63

84.38 88.10

6.80 6.95

3.23 1.84

0.74 0.73

0.97

0.03 0.01 0.02 0.03

2.33 0.47 3.48 0.16 1.01 0.30 0.47 2.61 0.76 0.44 0.46 0.62 0.86 0.57 0.66

0.55 0.01

2.66

1.58 0.39 0.61 0.78 0.52 0.55 1.09 0.65 0.80

0.35 0.35

CO2

N2

CH4

C2H6

C3H8

i-C4

1.24 0.50 0.05 0.05 0.66

1.55

− 17.8

0.50 0 0.09

− 19.9 − 19.9 − 19.0 − 19.1 − 19.1

− 21.0 − 21.0 − 19.7 − 19.9 − 19.9

1.24 2.44

− 19.4 − 19.0 − 18.0 − 18.0 − 18.5 − 18.5 − 17.9 − 18.8 − 23.5 − 21.0 − 21.1 − 25.5 − 21.5 − 21.7 − 22.7 − 23.3 − 21.8 − 24.7 − 23.0 − 23.2 − 22.9 − 24.1 − 23.6 − 22.7 − 24.2 − 23.4 − 22.3 − 22.6 − 26.2 − 23.6 − 21.6 − 21.9 − 22.8 − 26.8 − 23.9 − 22.0

− 18.5

0.69 0.69 0.58

− 27.3 − 27.8 − 27.6 − 27.6 − 27.3 − 27.1 − 27.2 − 25.1 − 31.1 − 36.9 − 33.5 − 35.2 − 34.0 − 33.6 − 32.4 − 32.8 − 30.9 − 32.9 − 31.7 − 32.3 − 38.9 − 34.2 − 34.7 − 35.4 − 27.2 − 32.8 − 32.4 − 33.4 − 36.0 − 35.2 − 32.8 − 29.7 − 32.7 − 35.5 − 31.2 − 32.6

− 22.1 − 24.4 − 19.7 − 21.6 − 20.3 − 21.3 − 19.8 − 21.0 − 22.3 − 21.2 − 20.6 − 20.4 − 20.9 − 22.8 − 21.6 − 20.9 − 21.8 − 21.1 − 21.4 − 22.2 − 24.3 − 21.3 − 21.2 − 21.2 − 20.5 − 22.8 − 22.8 − 20.4

− 22.1 − 24.7 − 20.8 − 22.2 − 22.3 − 22.6 − 21.2 − 22.1 − 24.1 − 26.2 − 21.3 − 21.7 − 22.1 − 24.5 − 22.7 − 23.0 − 22.1 − 21.0 − 22.9 − 23.9 − 24.7 − 21.9 − 22.5 − 23.0 − 22.2 − 21.8 − 24.6

0.30 0.12 0.24 0.65 0.12 0.22 1.59 0.82 0.47 0.26 0.12 0.14 0.32 0.28 0.08 0.31 0.20 0.0 0.56 0.38 0 0 0.05

0.89 5.28 5.67 2.58 4.00 3.16 1.12 3.74 3.77 1.84 3.09 2.29 2.10 2.25 2.22 4.96 3.91 2.21

0.17 0.22

2.50 2.05

4.53 2.01 0.67

n-C4

− 20.8

− 20.2

− 20.5 − 21.8 − 24.4 − 20.5 − 21.0 − 23.0 − 24.5 − 20.7 − 20.0 − 22.0 − 23.8

− 21.6 − 22.3 − 19.9 − 23.0

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Table 4 Geochemical compositions of Jurassic coal-derived gases in Junggar Basin, northwest China. Field

Hutubi

Horgos Dushanzi

Gumudi

Well

Hu 2 Hu 001 Hu 002 Hu 005 Huo 3 Du 85 Du 62a Du 68 Du 390 Pencan 2 Mu 3 Mu 4

Strata

E1–2z E1–2E E1–2E E1–2E N1–E3 N2 E3 N2 N2 J1s J2 J2

Depth (m)

δ13C (‰), VPDB

Main components (%)

3651–3875 3584–3590 3536–3572 3530–3579 541–1135 575–597 1285–1926 1331–1427 4721 528.6–532 561.2–569

CH4

C2H6

C3H8

i-C4

n-C4

91.70 89.60 94.00 93.90 84.92 81.46 82.02 81.54 83.62 91.75 90.10 84.52

5.00 5.00 3.70 3.20 5.07 9.90 4.29 9.57 10.90 3.38 0.49 0.12

0.70 1.30 0.60 0.70 0.93 5.34 2.57 4.49 2.74 1.00 0.78 0.08

0.20 0.20 0.10 0.20 0.18 1.77 0.77 0.83 0.38 0.34 0.80 0.07

0.20 0.30 0.10 0.20 0.16

CO2 0.40 0.40

0.61 0.63 0.35

0.40 0.76 0.33 0.58 0.70 0.21

0.63 0.05

0.71 14.52

N2

CH4

C2H6

C3H8

i-C4

n-C4

1.80 2.30 1.20 1.10 7.84 1.20 10.94 1.58 1.73 3.49 5.64 0.65

− 31.0 − 30.5 − 30.6 − 31.0 − 36.5 − 40.4 − 40.7 − 37.5 − 35.8 − 34.7 − 44.3 − 38.8

− 22.0 − 21.6 − 22.0 − 21.6 − 22.5 − 27.5 − 26.5 − 25.6 − 26.3 − 26.8 − 26.5 − 26.4

− 21.1 − 20.8 − 21.3 − 20.9 − 22.3 − 22.7 − 24.6 − 22.7 − 23.8 − 26.4 − 22.0 − 27.6

− 21.1 − 22.1 − 22.3 − 21.8 − 21.6 − 21.0 − 23.4 − 23.2 − 24.4 − 25.5 − 24.3 − 24.8

− 22.2 − 22.3 − 22.1 − 22.6

the light carbon isotope component (12C) leading to an enrichment of 13 C (Coleman and Risatti, 1981; Dai et al., 2000b). Dai et al. (2004a) proposed that the carbon isotopic reversal in well Mu 3 was caused by such oxidation. Secondly, the burial depth of the pay zone of well Mu 3 and well Mu 4 are around 528.6–569 m, and such shallow depth is a favorable environment for oxidizing bacteria (Coleman and Risatti, 1981; Dai et al., 1992). Alkane gases with reservoirs of the Jurassic coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs different from the Jurassic coal measures often have carbon isotopic reversal (Tables 3–6). According to the statistics on the wells with carbon isotopes for C1–C4 n-alkanes (i.e., δ13C1, δ13C2, δ13C3, δ13C4), the carbon isotopic reversal varies in different basins for gases with reservoirs not of coal measures. For example, for gases with reservoirs not of coal measures in the Turpan-Harmi basin, only gases from well Le 10 and well Shengbei 402 have no carbon isotopic reversal (Table 5). For the 9 gas samples with reservoirs not of Jurassic coal measures in the Junggar basin, six have carbon isotopic reversal (Table 4). In the Qaidam basin, 9 of 13 (69.2%) gas samples with reservoirs not of coal measures have carbon isotopic reversal (Table 6). 29 of 33 (87.9%) gas samples with reservoirs not of coal measures in the Tarim basin have carbon isotopic reversal. Why was no carbon isotopic reversal found in the gas samples with reservoirs of coal measures? There are two reasons: Firstly, there are no significant changes of the main geological conditions during the formation and accumulation of alkane gases; Secondly, the migration distance is short and nearly no isotopic fractionation or secondary alteration occurs. A number of studies have demonstrated the alteration of isotope geochemistry of natural gas during migration

3795 m), Kezi 1 (1130–1141 m), Hu 2 (3165–3875 m) and Hu 001 (3584– 3590 m) wells, respectively. 4.1. Alkane gases have no carbon isotopic reversal in the coal measure, whereas the extent of reversal increases with increasing maturity for alkane gases outside the coal measures If carbon isotopic data are available only for methane and ethane, i.e., δ13C1 and δ13C2, it is difficult to differentiate a carbon isotopic reversal. However, if the isotopic data consist of methane, ethane, propane and butane, it is easy to distinguish the presence of a carbon isotopic reversal. Coal-derived gases are of thermogenic origin, and the carbon isotopic composition of the primary (no secondary alteration) alkane gases increases with the molecular mass increasing, i.e., δ13C1 < δ13C2 < δ13C3 < δ13C4 (Dai et al., 1992). As shown in Tables 3–6, natural gases from well Yi 590 (Table 3), wells Pencan 2, Mu 3 and Mu 4 (Table 4) and nearly all wells listed in Table 5 except well Le 10 and well Shengbei 402 are in the coal measure source rocks. Natural gases from all of the above wells except wells Mu 3 and Mu 4 have an orderly carbon isotopic distribution pattern (δ13C1 < δ13C2 < 13 C1 < δ13C2 < δ13C3 < δ13C4) and no reversals. δ13C3 in well Mu 3 and δ13C2 in well Mu 4 become more enriched in 13C leading to a carbon isotopic reversal, which is caused by the bacterial oxidation of some components of the alkane gases making the remaining components more enriched in 13C (Dai et al., 1992). There are two supporting evidences: Firstly, the content of gas component of primary alkane gases often decreases with the increasing of carbon number (Table 4) and nearly all the coal-derived gases have this characteristic except well Mu 3 and well Mu 4. This is because of the bacterial oxidation of

Table 5 Geochemical compositions of Jurassic coal-derived gases in the Turpan-Harmi Basin, northwest China. Field

Well

Strata

Depth (m)

Wengjisang

Wen 1 Wen 1 Wen 8 Wen20 Wen 21 Wenxi 3 Ling 3 Ling 4 Mi 1 Le 1 Le 10 Qiudong 3 Hongtai 2 Shengbei 402

J2s J2x J2s J2s J2x J2s J2s J2s J2s J1b Esh J2x J2s K1

2341–2362 2764–2819 2413–2428

Wenxi Qiuling Mideng Shanshan Qiudong Hongtai Yanbei

2816–2828 2314–2323 2405–2420 2300–2308 2667–2751 2578–2677 663–706 3105–3142 2570–2586 1785–1792

δ13C (‰), VPDB

Main components (%) i-C4

n-C4

CO2

N2

CH4

C2H6

C3H8

C4H10

85.32 83.65

9.01 9.44

3.55 3.54

0.86 0.86

0.64 0.67

0.29 0.23

0.00 1.28

82.98 75.93 66.49 71.94 73.19 66.21 60.99 92.41 89.49 81.16 45.80

8.59 10.33 13.34 12.36 12.93 15.14 15.15 4.46 5.34 7.79 17.92

3.64 5.54 9.20 8.12 8.07 9.06 10.94 1.12 2.65 4.25 18.10

0.88 1.53 3.47 2.56 1.55 2.94 3.79 0.13 0.45 1.06 1.90

0.61 1.25 2.59 2.31 1.20 2.29 3.23 0.57 0.53 1.21 6.04

0.11 0.31 0.34 0.25 0.0 0.31 0.68

2.84 4.37 2.94 0.75 2.55 2.73 3.03

0.43 1.06

2.86 2.68

− 39.8 − 39.4 − 39.5 − 44.0 − 42.1 − 41.5 − 43.0 − 40.1 − 41.3 − 43.1 − 43.1 − 39.6 − 40.5 − 39.0

− 26.7 − 26.9 − 25.9 − 26.0 − 26.3 26.6 − 26.9 − 27.0 − 25.9 − 27.5 − 27.7 − 27.6 − 24.7 − 25.7

− 25.3 − 25.0 − 25.7 − 24.6 − 25.2 − 25.0 − 26.4 − 25.6 − 24.9 − 26.8 − 26.6 − 26.1 − 24.6 − 23.6

− 24.8 − 24.9 − 24.5 − 24.6 − 24.7 − 24.4 − 25.4 − 25.3 − 24.1 − 25.0 − 25.1 − 25.1 − 24.3 − 23.4

CH4

C2H6

C3H8

J. Dai et al. / International Journal of Coal Geology 80 (2009) 124–134

131

Table 6 Geochemical compositions of Jurassic coal-derived gases in Qaidam Basin, NW China. Field

Mabei

Nanbaxian

Mahai Lenghu No.4 Lenghu No.5

Hongshan E'boliang1

Well

Mazhong1 Mabei4 Mabei103 Xian3 Xian11 Xian14 Xian7 Xian9 Xianshi1 Xianshi6 Xianshi7 Xianzhong15 Mazhong1 Shen85 Lengsi581 LengL5-3 Xinjian275 275 9704 Hongshancan2 e12

Strata

E1 E23 E23 N E E31 E13 N12 N12 N12 N12 N12 E3 N1 N N1 N1 N1 N1 K E23

Depth (m)

458–566 1150.8–1152 894–900 1140–1141 2855–2878 2921–3117 2860.65 1249–1251 1033–1048 1480–1488 1311–1315 1261–1329 458–459 700

δ13C (‰), VPDB

Main components (%) CH4

C2H6

C3H8

78.30 90.25 78.25 89.78 88.11

1.17 1.70 15.79 3.47 1.67

0.09

i-C4

n-C4

N2

CH4

C2H6

C3H8

i-C4

n-C4

0.41

17.02 6.62 2.00 4.36 9.92

− 30.6 − 28.4 − 32.89 − 29.4 − 27.8 − 30.3 − 29.3 − 27.9 − 25.3 − 28.2 − 29.7 − 30.0 − 30.6 − 32.3 − 38.0 − 30.9 − 29.4 − 29.0 − 28.2 − 29.9 − 31.4

− 24.6 − 23.2 − 25.2 − 22.8 − 22.0 − 24.6 − 25.2 − 22.7 − 20.9 − 22.8 − 25.4 − 25.1 − 24.6 − 23.3 − 28.8 − 26.6 − 25.0 − 25.7 − 22.7 − 25.2 − 25.1

− 24.0 − 20.9 − 22.5 − 22.0 − 20.8 − 23.7 − 23.6 − 21.2 − 18.7 − 21.3 − 23.5 − 23.4 − 22.9 − 22.2 − 27.4 − 25.3 − 23.4 − 24.0 − 21.7 − 19.9 − 23.1

− 23.7

− 22.9

− 23.7

− 22.6

− 24.1 − 23.1

− 24.6 − 23.4

− 18.3

− 16.6

− 23.6 − 23.2

− 24.3 − 23.7

− 22.1 − 28.6 − 26.3 − 25.6 − 25.0 − 23.8

− 21.3 − 26.6 − 24.7 − 23.5 − 23.9 − 28.0

1.11 0.30

89.27

4.96

5.39

89.58

4.83

5.29

78.30 78.66

1.17 8.24

0.09 3.97

0.71

1.17

0.41

65.28

12.40

5.74

2.28

3.33

422–554

615 2450

CO2

(Pernaton et al., 1996; Prinzhofer and Pernaton, 1997). Short migration distance in coal measures will greatly reduce any isotopic fractionation or the chance to undergo secondary alteration. According to the above discussion, there is no isotopic reversal during the short migration of alkane gases to areas within coal measure (Table 5), whereas the isotopic reversal proportion is 66.7% (Table 4), 69.2% (Table 6) and 87.9% (Table 3) for alkane gases from Junggar, Qaidam and Tarim basins, respectively to migrate to areas outside coal measures. The gradual increase of the extent of carbon isotopic reversal between these basins is related to the maturity level of the source rocks. Previous studies suggested that the carbon isotopic composition of methane became more enriched in 13C with increasing maturity (Galimov, 1968, 2006; Stahl, 1977; Dai et al., 1987; Shen et al., 1988). Based on the δ13C1 data compiled in Tables 3–6, δ13C1 from Turpan-Harmi basin is lowest (Table 5), varying from − 44.0‰ to −39.0‰, with Ro of 0.5%–0.9% (Cheng, 1994; Wu and Zhao, 1997); δ13C1 values from Junggar basin range from −44.3‰ to − 31.0‰ (Table 4), indicating higher maturity levels than that from the TurpanHarmi basin; δ13C1 in the Qaidam basin varies from −38.0‰ to −25.3‰ (Table 6), indicating higher maturity levels than the Junggar basin; The δ13C1 values in the Tarim basin range from −38.9‰ to 25.1‰ (Table 6), demonstrating the highest maturity level in general, e.g., Ro values of organic matters in the Kela 2 and Kela 3 gas fields are as high as 2.5% (Qin et al., 2007). Why does the carbon isotopic reversal proportion of coal-derived gases change systematically with the maturity and δ13C1 values? There are two reasons: Firstly, the migration distance is shorter for low maturity coal-derived gases in the Turpan-Harmi basin while longer for higher maturity coal-derived gases in the other three basins. Gases with short migration distances normally have a lower chance to undergo carbon isotopic fractionation and secondary alteration. For example, based on the depths of the gas reservoirs or source rocks in the Turpan-Harmi basin (Table 5), vertical migration is 600–800 m for well Shengbei 402 and 1600–1800 m for well Le 10 and no isotopic reversal was found. Whereas natural gases with longer migration distances will have greater chance to undergo isotopic fractionation and secondary alteration leading to carbon isotopic reversal. For example, for wells from Kela 2 gas field, gases migrated from the Jurassic coal-bearing strata to present well section and the migration distance was 2000–3500 m. Secondly, higher maturity

17.02

levels may indicate a longer thermal history, which provides opportunities for multiple charging events to occur. For example, the Kela 2 gas field underwent gas charging and alteration twice, i.e., gas charging and destruction during early Himalayan movement and gas charging and alteration during a later Himalayan movement (Zhao et al., 2002b). Multiple gas charging and alteration events lead to the carbon isotopic reversal of the alkane gases from this gas field due to mixing of gases (Dai et al., 2004a). 4.2. Coal-derived alkane gases with high carbon isotopic values The so-called coal-derived alkane gases with high carbon isotopic values indicate that these alkane gases are primary and have not undergone any secondary alteration. The primary thermogenic alkane gases (including coal-derived gases) have an orderly carbon isotopic distribution pattern wherein the C1–C4 n-alkanes become more enriched in 13C with increasing molecular mass. Abiogenic gases are characterized by a carbon isotopic reversal trend wherein the C1–C4 nalkanes become more depleted in 13C with increasing molecular mass (Galimov and Petersilie, 1967; Galimov, 1975; Dai et al., 2004a,b). If the carbon isotopic reversal of alkane gases is due to secondary alteration, it is not suitable to compare their carbon isotopic values. Dai et al. (2000b) demonstrated that alkane gases with δ13C1 > −30‰ from the large-intermediate coal-derived gas fields in China only occur in the Bohai Bay, Songliao and Ordos basins. Based on a recent comparative study of the carbon isotopic values of alkane gases from other coal-derived gas fields in China, we found that other two basins (Qaidam and East Sea) also have coal-derived gases with δ13C1 > −30‰ (Table 7). The δ13C1–4 and gas components of Jurassic coal-derived gases from the four basins in northwestern China are listed in Tables 3–6. Table 7 shows alkane gases with δ13C1 < 13 C1 < δ13C2 < δ13C3 < δ13C4 and δ13C1 > −30‰ from Tables 3–6 and from other basins in China. As shown in Table 7, among the four basins with Jurassic coal-derived natural gases, only the east–west trending Tarim and Qaidam basins (Fig. 1) have δ13C1 > − 30‰. Hence, the Tarim and Qaidam basins have alkane gases with the highest δ13C1 values in China, such as Kela 2 (E) and Kela 3 wells in the Tarim basin and well Xianshi 1 in the Qaidam basin. Alkane gases from well Kela 3 have the highest δ13C values in China, i.e., δ13C1 of −30% and δ13C2 of −18.8‰. In a similar fashion, alkane gases from well Xianshi 1 in the Qaidam basin have δ13C1 of − 25.3‰, δ13C2 of − 20.9‰, δ13C3 of

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Table 7 Geochemical compositions of coal-derived gases with δ13C1 > − 30‰ in China. Basin

Tarim

Qaidam

Songliao

Ordos Bohai Bay East China Sea

Field

Kela 2

Kela 3 Nanbaxian

Mahai Mabei Hongshan Shengping Weixing Wuzhan Taipingzhuang Qilingou Wenliu LS36-1

Well

Strata

Kela2 Kela2 Tai2 Kela3 Xianshi1 Xian7 Xian11 Xian9 Xianshi6 Mazhong3 Mabei4 Hongshancan2 Shengshen1 Sheng58-1 Wei5 Wu2 Wan11 Lingcan1 Wen23 LS36-1-2

E K2b N1j E N12 E31 E N12 N E E23

Depth (m)

δ13C (‰), VPDB

Main components (%)

3500–3535 3888–3895 3105–3199

CH4

C2H6

96.90 98.22 84.79 98.05

0.31 0.53 6.78 0.62

C3H8

i-C4

n-C4

0.04

0.03

0.55

0.09

CO2 1.24 0.50 1.02 0

N2 1.55 4.88 1.24

2861 88.11 89.27 89.58

1.67 4.96 4.83

0.30

90.25

J K1q4 K1sh K1q2 Y P1x Es4

2450 2933 3791 1325 1265 2608–2618 2813–3027 2315–2336

93.57 94.19 87.30 95.99 82.10 91.29 93.61 26.67

9.92 5.39 5.29 6.62

2.98 1.73 3.27 1.51 1.40 5.61 1.81 2.44

−18.7‰, δ13iC4 of − 18.3‰ and δ13nC4 of − 16.6‰ (Table 7). This is due to the uplift and the southward subduction of Tianshan and Qilianshan which led to the deep subsidence (up to 4500 m) of the Jurassic coal-bearing strata and resulted in the high maturity level of the coal measure source rocks. The Upper Carboniferous coal measures in the northwest basin in Germany generated abundant coal-derived gases. Stahl (1979) studied 119 coal-derived alkane gas samples from 36 gas fields (accumulations) from Upper Carboniferous. They found δ13C1 values ranging from −31.8‰ to −20.0‰ (typically from −23‰ to −28‰), δ13C2 values ranging from −22‰ to −25‰ and δ13 C3 values ranging from −21‰ to −23‰. These might be the highest published carbon isotopic values for coal-derived alkane gases occurring worldwide, and probably is due to their high maturity of 1.3%–2.0% (Teichmüller et al., 1979). 4.3. Lowest δ13C values of coal-derived methane in China If only δ13C values of methane are present, it is difficult to tell whether it is primary or secondary. Here the coal-derived methane is confined to the primary alkane gases with δ13C1 <δ13C2 <δ13C3 <δ13C4. Combined this criterion with δ13C1 < −40‰, Table 8 demonstrates a set of coal-derived alkanes gases with the lowest δ13C1 values. As shown in Table 8, coal-derived alkane gases in China with lowest δ13C1 values are only found in the Turpan-Harmi, Junggar and

0.19 0.31 0.11 0.68 0.97 0.35 1.03

0.02 0.04 0.01 0.13 0.13 0.10 0.02

0.05 0.03 0.01 0.4 0.11 0.12 0.04

0 0.32 0 0 0.22 0 0.99 67.61

2.95 3.43 9.06 2.36 14.87 0.59 2.34 1.05

CH4

C2H6

C3H8

i-C4

n-C4

− 27.3 − 27.8 − 29.2 25.1 − 25.3 − 29.3 − 27.8 − 27.9 − 28.2 − 29.3 − 28.4 − 29.9 − 25.8 − 29.8 − 27.2 − 27.4 − 28.3 − 29.2 − 27.8 − 29.0

− 19.4 − 19.0 − 21.4 − 18.8 − 20.9 − 25.2 − 22.0 − 22.7 − 22.8 − 23.4 − 23.2 − 25.2 − 25.3 − 25.8 − 23.3 − 26.7 − 21.5 − 22.4 − 24.3 − 27.1

− 18.5

− 17.8

− 18.2

− 16.1

− 18.7 − 23.6 − 20.8 − 21.2 − 21.3 − 22.2 − 20.9 − 19.9 − 24.2 − 21.6 − 23.1 − 25.2 − 21.2 − 22.2 − 24.1 − 26.5

− 18.3 − 23.1

− 16.6 − 23.4

− 22.6 − 19.9 − 20.9 − 25.4 − 20.8

− 21.3 − 23.1 − 23.4

− 23.9

Sichuan basins and they are abundant in the Turpan-Harmi basin where the lowest δ13C for methane in China is reported. For example, δ13C1 values are as low as − 44.0‰ and − 43.1‰ for well Wen 20 and well Le 10, respectively. Moreover, coal-derived alkane gases from the Ling 3, Ling 16–23 and Le 1 wells all have very low δ13C1 values of − 43.0‰, − 40.1‰ and − 43.1‰, respectively. Such low carbon isotopic compositions of methane in the Turpan-Harmi basin are due to the low maturity level of its Middle–Lower Jurassic coal measure source rocks. Coal-derived gases listed in Table 8 are only distributed in the coal-derived oil-gas fields in the Taibei depression in this basin. Based on a study of coal-derived gas maturity, Cheng (1994) demonstrated that the maturity level (Ro%) of the Middle– Lower Jurassic coal measures in the Taibei depression ranged from 0.53% to 0.77%. Wu and Zhao (1997) presented similar data that Ro varied from 0.5% to 0.9% in the Turpan-Harmi basin. The low maturity level of source rocks is a significant cause for such low carbon isotopic values of methane in the Turpan-Harmi basin based on the statistical relationship between carbon isotope of methane and maturity of potential present source rocks (Stahl, 1977; Dai et al., 1992). As shown in Table 8, Middle–Lower Jurassic coal measures in the Junggar basin also have low δ13C1 values such as the Du 85 and Du 62a wells in the Dushanzi oil-gas field. Patience (2003) proposed the definition of coal-derived gases which are drier than gases formed from the marine source rocks

Table 8 Geochemical compositions of coal-derived gases with δ13C1 < − 40‰ in China. Basin

Field

Well

Turpan-Harmi

Wenjisang

Wen20 Wen21 Wenxi3 Ling3 Ling4 Ling16–23 Mi1 Le1 Le10 Hongtai2 Du85 Du62a Jiao48

Wenxi Qiuling

Mideng Shanshan

Junggar

Hongtai Dushanzi

Sichuan

Bajiaochang

Strata

J2x J2s J2s J2s J2x J2s J2b E5h J2s N2 E3 J3x6

Depth (m)

2816–2828 2314–2323 2405–2420 2300–2308 2741 2667–2751 2578–2677 663–706 2570–2586 575–597 1285–1926 3383–3395

δ13C (‰), VPDB

Main components (%) CH4

C2H6

i-C4

n-C4

CO2

82.98 75.93 66.49 71.94 73.19 85.77 66.21 60.99 92.41 81.16 81.46 80.02 87.53

8.59 10.33 13.34 12.36 12.93

C3H8 3.64 5.54 9.20 8.12 8.07

0.88 1.53 3.47 2.56 1.55

0.61 1.25 2.59 2.31 1.20

0.11 0.31 0.34 0.25

15.14 15.15 4.46 7.79 9.90 4.29 6.26

9.06 10.94 1.12 4.25 5.34 2.57 2.65

2.94 3.79 0.13 1.06 1.77 0.77 0.41

2.29 3.23 0.57 1.21

0.31 0.68

2.84 4.37 2.94 0.75 2.55 0.40 2.73 3.03

1.06 0.36 0.58 0.27

3.68 1.20 10.94 1.96

0.61 0.48

N2

CH4

C2H6

C3H8

i-C4

− 44.0 − 42.1 − 41.5 − 43.0 − 40.1 − 42.9 − 41.3 − 43.1 − 43.1 − 40.5 − 40.4 − 40.7 − 40.6

− 26.0 − 26.3 − 26.6 − 26.9 − 27.0 − 27.9 − 25.9 − 27.5 − 27.7 − 24.7 − 27.5 − 26.5 − 26.4

− 24.6 − 25.2 − 25.0 − 26.4 − 25.6 − 26.7 − 24.9 − 26.8 − 26.6 − 24.6 − 22.7 − 24.6 − 23.6

− 24.6 − 24.7 − 24.4 − 25.4 − 25.3 − 25.1 − 24.1 − 25.0 − 25.1 − 24.3 − 21.0

n-C4

J. Dai et al. / International Journal of Coal Geology 80 (2009) 124–134

(CH4 > 90%) and have carbon isotopes of methane ranging from −22‰ to −38‰. According to our data (Tables 3–6), not all of the gas samples have CH4 > 90%. Most samples have CH4 of 80%–90% or less and some have C2–4 of 10%–20% (Tables 3–5). Moreover, the δ13C1 in China can be lower than − 38‰, and some of them are even lower than − 40‰ (Table 8); whereas the highest one is − 25‰ and is lower than − 22‰.

5. Lower individual carbon specific isotopes of the light hydrocarbon (C5–8) of coal-derived gases compared to that of the oil-sourced gases Comparative study on individual carbon specific isotopes of common light hydrocarbons from coal-derived and oil-associated gases has great significance on the discrimination of these two gases (Dai and Li, 1995). According to the 14 carbon isotopes of light hydrocarbon from the coalderived gases from Tarim (four, i.e., Yingmai7, Yaha23-1-14, Ti101 and Hongqi1), Junggar (two, i.e., Hu2 and Hu001) and Turpan-Harmi (eight, i.e., Ling3, Ling4, Le1, Mi1, Mi3, Wen1 (J2s), Wen1 (J2x) and Hongtai2) basins and six carbon isotopes of light hydrocarbon from the oil-formed gases from Tarim (three, i.e., Tazhong1, Tazhong4 and Tazhong117) and Junggar (three, 546, 5153 and Hong221) basins, and together with the data in Tables 3–5, Fig. 5 and the study of Dai et al. (1995), the δ13C1–4 of the above gas samples have apparent characteristics of coal-derived and oil-associated gases. As shown in Fig. 5, most of the light hydrocarbons from the oil-associated gases have carbon isotope lower than −26.5‰ and can be up to −32‰, whereas most of the light hydrocarbons from the coal-derived gases have carbon isotope greater than −26‰ and can be up to −19.5‰. Thus, the greater carbon isotope values of the C5–8 alkanes of coal-derived gases than those from the oil-formed gases by 1.3‰ is due to the different hydrocarbon-generating materials (Dai et al., 1995). Therefore, comparative study on the carbon isotopes of light hydrocarbon

133

in the natural gas can be used to discriminate the oil-formed and coalderived gases. 6. Conclusion According to the above comparative study of coal-derived gases in northwest China, two conclusions are drawn: 1) Characteristics of carbon isotopes of coal-derived alkane gases: ① No carbon isotopic reversal was found for alkane gases with reservoirs in the coal measures whereas the extent of the carbon isotopic reversal increases with increasing maturity for alkane gases with reservoirs not of the coal measure. Carbon isotopic reversal of alkane gases with reservoirs not of these coal measures is dominantly due to isotopic fractionation during migration and secondary alteration; ② The highest carbon isotope values of coal-derived alkane gases occur in the Tarim and Qaidam basins; ③ The lowest carbon isotope values of coal-derived alkane gases appear in the Turpan-Harmi and Junggar basins. 2) Carbon isotopic values of light hydrocarbon (C5–8) from the coalderived gases are correspondingly greater than those from the oilassociated gases. In China, most giant gas fields are source from coal measures. Detailed studies on the carbon isotopic compositions of coal-derived alkane gases may shed light on future exploration of coalderived gases in the world. The heavy carbon isotope of coal-derived methane and ethane is important in discrimination of gas origin and thus can be used for gas–source rock correlation. Acknowledgements We thank Dr. Changyi Zhao for generously providing the gas composition and carbon isotopes for Le 10 and Shengbei 402 wells. Dr. Paul Hackley is greatly thanked for his helpful and constructive comments.

Fig. 5. Correlation of carbon isotopes of light hydrocarbons (C5–8) between coal-derived gas and oil-associated gas. Mass spectrometry peak numbers respectively represent the following components: 4—2-methyl butane; 5—N-pentane; 9—3-methyl pentane; 10—N-hexane; 17—Cyclohexane; 19—3-methyl hexane; 23—N-heptane; 34—N-octane.

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