Origins of partially reversed alkane δ13C values for biogenic gases in China

Organic Geochemistry 35 (2004) 405–411 www.elsevier.com/locate/orggeochem

Origins of partially reversed alkane d13C values for biogenic gases in China Jinxing Dai*, Xinyu Xia, Shengfei Qin, Jingzhou Zhao Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China

Abstract With increasing molecular weight, d13C values of hydrocarbon gases change in two different manners: the normal order would be d13C1 < d13C2 < d13C3 < d13C4, whereas the reversed order would be d13C1>d13C2>d13C3>d13C4. Partially reversed order is common in gas samples from sedimentary basins in China, which can be attributed to one or several of the following four mixing processes: (a) mixing of biogenic and abiogenic gases; (b) mixing of sapropelic and humic sourced gases; (c) mixing of gases from the same types of source rocks with different maturity; and (d) mixing of gases from the same source rock interval of varying maturity. # 2004 Published by Elsevier Ltd.

1. Introduction The relative d13C values of C1–C4 alkanes in natural gas are important geochemical parameters for the study of natural gas genesis. These can be used to determine if a natural gas pool has undergone alteration, mixing, and migrational fractionation, which is vital for carrying out the study of gas-source correlation. The work presented in this article is based on 2050 d13C1 4 analyses on 1046 gas samples. The gas samples were taken from 16 basins in China, including all of the major gasbearing basins: Sichuan, Ordos, Tarim, Qaidam, Junggar, Turpan-Hami, Bohai Bay, Subei, Qiongdongnan, Yinggehai and East China Sea basins (Fig. 1).

2. Three types of carbon isotopic compositions for natural gas alkanes The d13C values of natural gas alkanes are known to change with increasing molecular weight. In our previous studies (Dai, 1992; Dai et al., 1992, 2000a, b), we have observed two types of carbon isotopic compositions: a normal order (i.e. d13C1 < d13C2 d13C2>d13C3>d13C4). * Corresponding author. 0146-6380/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.orggeochem.2004.01.006

Organic gases with a biogenic origin are commonly characterized by a normal isotopic order. This is true for both gases from petroliferous basins in China (Table 1) and those from elsewhere (Fuex, 1977; Stahl, 1975). Inorganic (abiogenic) hydrocarbon gases show reversed d13C values. The latter is uncommon in petroliferous basins, but recent studies have documented several exceptional cases. These include the Changde and Fangshen-9 gas pools in the Songliao Basin, the Tianwaitian structure in the East China Sea Basin (Guo and Wang, 1994; Dai et al., 2001; Hou and Yang, 2002; Zhang et al., 1991; Table 2), and in the magmatic rock of Russia (Zorikin et al., 1984). We also noted that a considerable number of the gas samples displayed a partially reversed isotopic order, either with d13C1>d13C2 and d13C2 < d13C3 < d13C4 (Table 4), or d13C2 d13C4 (Table 5).

3. Origin of partially reversed d13C orders for natural gas alkanes 3.1. Mixing of biogenic and abiogenic gas The carbon isotope composition of biogenic gas is of normal order (Table 1) whereas that of abiogenic gas is

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of reversed order (Table 2). The mixing of these two types of gases can cause reversed or partially reversed orders. As abiogenic natural gas is uncommon in sedimentary basins, gases with reversed isotope order formed in this way are rare. Gas samples from some of the gas wells in the Shengping Gasfield of the Songliao Basin (e.g. Sheng 61 and 66) display a partially reversed carbon isotopic order. For example, samples taken from the Sheng 61 well have the d13C1 to d13C4 around 28.69, 24.80, 27.08 and 27.42%, respectively. This data, together with the close approximation of the well location to a major basement fault zone, suggests that the gases were formed through the mixing of abiogenic and Mesozoic humic gas. 3.2. Mixing of gases from sapropelic and humic sources Alkanes in natural gases derived from humic organic matter are relatively enriched in 13C compared with those from the sapropelic source rocks at a given maturity (Dai et al., 1992) (Table 3). Thus, partially reversed d13C order often occurs where humic and sapropelic gases were generated from source rocks in close proximity. In the Sichuan Basin, for example, organic matter (OM) in the Lower Permian carbonates

is dominantly sapropelic, whereas that in the Upper Permian coal measures is mostly humic. As shown in Fig. 2, the presence of an unconformity between the two source units has provided ample opportunities for the mixing of the gases derived from the two source rocks with similar maturity to occur, resulting in mixed gases with characteristic carbon isotope compositions (Table 4). Fuex (1977) pointed out that d13C1 > d13C2 is rare in a gas field. While this is generally true elsewhere, the d13C1 values are often greater than the d13C2 values in many gases from the southern Sichuan Basin owing to the mixing of humic and sapropelic gases. Fig. 3 illustrates the mixing curves of end member sapropelic (A) and humic (B) gases, where gas A contains 90% methane with d13C1= 40% and 10% ethane with d13C2= 38%, and gas B has 99.5% methane with d13C1= 31% and 0.5% ethane with d13C2= 28%. For the sake of simplicity, we assume that the C2+ alkanes are not important in quantity in the gas mixture. When the volumetric B/A ratio in the mixture varies from 0.2:1 to 20:1, a reversal in the relative order of d13C1 and d13C2 is observed. For example, a 4:1 humic/sapropelic gas ratio would still display a d13C2 value characteristic of sapropelic gas ( 36.3%), but the resulting d13C1 value would be heavy ( 32.6%) giving a

Fig. 1. Map showing gas field distribution and gas sampling basin locations. Basins: 1. Sichuan; 2. Ordos; 3. Qaidam; 4. Tarim; 5. Junggar; 6. Turpan-Hami; 7. Songliao; 8. Bohai Bay; 9. Subei; 10. Nanyang; 11. Sanshui; 12. East China Sea; 13. Zhujiangkou; 14. Qiongdongnan; 15. Yinggehai; 16. Beibuwan.

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J. Dai et al. / Organic Geochemistry 35 (2004) 405–411 Table 1 Biogenic alkanes with normal carbon isotope orders in Chinese natural gases Basin

Well

Strata

Sichuan

Jiao-37 Zhong-29 Chuan-93 Wo-13 Sai-34 Ren-6 Shan-68 Zhou-1 YH4 LN58-1 Tazhong-1 Zhong-4 Se-21 Y11-6 Hu-2 Wu-15153 Erqu-7518 Wen-1 Hongtai-2 Qiudong-3 Qian32-8 Hong-35 Sheng-81 Ning-3 Wen-63 Ping-4 Wen-23 Su-402 Zhen-98 Dong-60 Ya13-1-2 Ya13-1-4 DF1-1-2 DF15-1-1 LD20-1-1 P3 LS36-1-1

Jt4 T3x2 T3x4 T1j51 T3y3 P1x O1m5 O N1j T O Q1+2 Q1+2 E13 E2z T12 C J2s J2s J2x K1g3 K1gn3 K1q4 Ed3 E2-3s1 E2-3s4 E2-3s4 O E1d E1f2 E3l E3l Nyth Nyth Nyth E2p E21m-E13m

Ordos

Tarim

Qaidam

Junggar

Turpan-Hami

Songliao

Bohai Bay

Subei Qiongdongnan Yinggehai

East China Sea

d13C1(%)

d13C2(%)

d13C3(%)

d13C4(%)

43.13 34.77 34.99 33.13 49.46 35.34 34.0 32.17 32.89 35.92 42.72 67.82 64.90 42.04 37.84 33.89 45.05 39.75 40.45 39.55 49.05 52.83 35.34 52.93 50.42 51.38 27.80 37.73 44.46 50.00 35.60 37.78 33.2 34.64 32.04 36.08 46.13

32.94 24.76 24.38 28.66 37.58 26.38 23.5 25.20 24.68 34.04 40.62 46.52 37.66 28.69 22.96 28.78 39.18 26.73 24.72 27.64 36.77 36.85 32.45 32.44 39.71 32.96 24.31 25.87 28.37 42.97 25.14 25.96 24.8 23.49 24.20 27.44 29.31

30.22 23.70 21.62 25.90 33.65 24.33 21.6 23.87 21.17 31.98 34.26 32.58 23.57 26.31 21.20 28.25 31.63 25.31 24.59 26.12 33.03 34.28 31.91 29.11 34.21 29.94 24.11 24.09 27.34 29.06 24.23 24.51 24.7 20.25 21.34 27.27 27.07

29.34 23.52 20.75 24.21 32.91 23.23 23.12 21.16 28.83 29.15

26.21 21.17 28.15 30.77 24.80 24.30 25.13 32.16 31.09 29.59 28.22 30.19 28.61 23.90 23.92 27.30 28.91 24.13 24.48 23.8 19.02 21.04 27.26 26.93

Table 2 Abiogenic alkanes with reversed carbon isotope orders in China and elsewhere d13C(%)

Sample location State

Basin

Well

China

Songliao

Fangshen-1 Fangshen-2 Fangshen-9 Sheng501 Sishen-1 Tian-1

Russia USA

East China Sea Khibiny Massif Yellow Stone Park

d13C1 18.70 18.90 27.11 27.26 28.0 17 3.2 21.5

Source d13C2 22.40 19.00 30.05 27.69 34.0 22 9.1 26.5

d13C3 24.10 34.10 30.05 28.90 34.1 29 16.2

d13C4 28.20 32.98

Zorikin et al., 1984 Dai et al.,1989

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Table 3 d13C values of humic and sapropelic gases from source rocks of similar maturity Basin

Well

Strata

Gas Type

Ro(%)

d13C PDB (%) d 13C1

Ordos Ordos Qiongdongnan Sichuan Bohai Bay Ordos Junggar

Hua11-32 Se-1 Yang-8 Ya13-1-2 Jiao-2 Su-401 Niu-1 Caican-1

J1y9 P1s T3y2-8 E3l Jt1 O O C2 b

Sapropalic Humic Sapropalic Humic Sapropalic Humic Sapropalic Humic

Mean 1.038 1.04 1.08–1.10 1.09–1.10 Mean 1.045 1.05 1.90 1.90

d

46.41 32.04 47.37 35.60 46.26 36.5 36.71 29.90

13

d 13C3

C2

35.95 25.58 37.20 25.14 32.78 25.6 29.30 22.76

32.30 24.22 33.09 24.23 30.00 23.7 27.31

d 13C4 31.16 23.14 31.68 24.13 29.82

Table 4 Partially reversed d13C order in gas samples from southern Sichuan Basin Gasfield

Well

Depth(m)

d13C, PDB (%)

Reservoir

d13C1 Ziliujing

Zi-3

Baijietan Fujiamiao Laowengchang Naxi

Bai-2 Fu-11 Lao-5 Na-6 Na-33 Na-17 Na-21 Dan-17 Dan-4 Si-23 Si-47 He-4

Danfengchang Miaogaosi Hejiang

2143–2153 2342–2352 2915.5–2982 2266–2326.2 2341–2358 2300–2337.2 2333.5–2355.0 2051–2052.3 2643–2649

2851–2852 3052.7–3085 2891–2897.2

d13C2

d13C3

P1

33.18

35.42

30.53

P31 2A-B

32.21 32.52 33.18 32.25 32.95 32.91 32.09 32.66 32.64 32.72 31.42 30.72

33.47 33.67 33.99 35.17 35.38 35.44 36.14 34.22 34.20 35.17 35.57 34.67

29.85 30.29 29.85 31.89 31.69 31.88 31.94 30.00 27.62 28.34 31.64 31.08

P1 32A P2 P1 P2 P31 P31

2 3

Table 5 Partially reversed d13C order caused by bacterial oxidation Basin

Ordos Junggar

Songliao Bohai Bay

Beibuwan

Well

Sai-18 Hu-401 Mu-3 Mu-4 Siqu-146 Hong-201 5-7 Nu-14 Kou-11 Xing-213 Wu16-1-5

Strata

T3y6 T3y2 J2 J2 K-C1 K1y Ng Es3 P Es4 E2l2

d 13C PDB (%)

Components (%) N2

CO2

CH4

C2H6

C3H8

C4H10

8.79 2.40 5.64 0.65 1.87 1.62 0.13 1.34 3.75 3.71 0.28

0.19 0.84 0.71 14.52 0.00 1.39 1.20 0.58 34.04 0.76 4.01

33.82 91.53 90.10 84.52 95.15 95.32 92.61 83.09 51.95 73.92 52.61

13.07 1.55 0.49 0.12 1.15 0.95 3.86 2.01 2.11 5.96 12.38

27.65 0.60 0.78 0.08 0.70 0.16 1.08 3.93 4.24 5.29 15.54

14.35 2.71 1.43 0.12 0.74 0.29 1.12 4.30 2.75 6.21 10.45

C1 46.95 48.58 44.32 38.80 40.76 50.87 42.83 53.89 40.89 35.36 43.62

C2 38.43 31.30 26.52 26.42 25.96 36.76 26.36 38.89 28.17 25.45 29.76

C3 38.72 29.16 21.97 21.57 26.03 29.82 22.13 28.30 30.19 24.41 29.94

C4 33.12 32.37 24.24 24.76 28.42 30.20 24.30 28.88 31.71 24.84 29.14

J. Dai et al. / Organic Geochemistry 35 (2004) 405–411

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Fig. 2. The Permian stratigraphy and gas source rock characteristics in a representative well of the southern Sichuan Basin.

Fig. 3. Mixing scenario curves of end member gases. End member A representing the oil-type gas of lower maturity has 90% methane and 10% ethane, with d13C1= 40%, and d13C2= 38%. End member B, representing coal-derived gases of high maturity, has 99.5% methane and 0.5% ethane with d13C1= 31% and d13C2= 28%. End member C, representing oil-type gases of high maturity, has 99.5% methane and 0.5% ethane with d13C1= 32% and d13C2= 31%. Mixing lines: 1 & 2. d13C1 & d13C2 for B/A mixture; 3 & 4. d13C1 & d13C2 for C/A mixture.

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reversed d13C order. In our earlier studies (e.g. Dai et al., 2000a), we established some empirical cutoff points using the d13C2 value to distinguish humic and sapropelic gases, i.e. the d13C2 > 28% for humic gases versus the d13C2 < 29% for sapropelic gases. The results presented in Fig. 3 indicate that this empirical rule should be used with caution. In addition, mixing of humic and sapropelic gases from source rocks with different maturity levels could also lead to partially reversed alkane carbon isotope compositions. Dai (1988) presented an example from the Baiyangjing Oilfield, where gases produced from the Bai 10-8 well were believed to be humic gases derived from the Permo-Carboniferous coal measures, mixed with some sapropelic gas from an Upper Triassic lacustrine source rock. Consistent with their general geological setting, the gases display a partially reversed isotope order, with d13C1 to d13C4 values being 35.00, 25.26, 28.57 and 29.16%, respectively. In our large gas database, we have many other examples from the Songliao, Bohai Bay, Ordos, and East China Sea basins. 3.3. Mixture of gases from two source rock intervals of similar kerogen type but of different maturity, or from one source rock unit at varying maturity The partially reversed isotope order for the gases from the eastern Sichuan Basin is believed to have resulted from the mixing of two gas sources of similar kerogen type but of different maturity. For example, gas sample from the Wo-52 well has d13C1 of 32.13%, d 13 C2 of 35.34%, and d 13C3 of 30.48%, with likely source rocks being the Silurian black shale and Lower Permian dark limestone (Tao and Chen, 1984). The kerogens in both source rocks in this part of the Sichuan Basin are sapropelic, but the Silurian source is more mature. When gases from the two source rocks mix in the ratios between 0.2:1 and 100:1, the resulting d13C1 values are greater than the d13C2 (Fig. 3). The partially reversed carbon isotope values observed for gases produced from the Carboniferous reservoirs of the Xiang-18 and Chi-18 wells can be explained similarly (Dai et al., 2000b).

The mixing of low and high maturity gases derived from the same source rock at different maturation stages could also bring about a partial reversal in order of carbon isotopes (Fuex, 1977). 3.4. Bacterial oxidation Bacteria can oxidize alkanes. Different bacteria consume different alkane components. The carbon isotope values of the oxidized light hydrocarbon residues will become heavier as oxidation progresses. An experiment by Lebedew et al. (1969) shows that bacterial oxidation can cause the d13C1 of residual methane to become 2%– 5% heavier. Coleman and Risatti (1981) oxidized methane with two types of bacteria at 11.5 and 26  C, respectively, and showed that d13C1 became heavier with decreasing concentrations of methane. These experiments show that 12C bonds in alkane molecules are more easily oxidized compared with 13C bonds, so the isotopically light molecules are consumed earlier, and the residual gas becomes heavier. This reaction can occur in both gas reservoirs and associated gas reservoirs. Methane produced from bacterial oxidation of larger hydrocarbons is common. The gas seeps from the Menggu area along the Nu River of Baoshan County, Yunnan, SW China contain 90–84% methane, with no C2+ alkanes. These biogenic gases have the d13C1 values ranging from 53.1% to 53.3%, which are greater than that of typical biogenic gas. We attribute the more positive isotope values to bacterial oxidation. The Nu River lies within the Kangtian Shield that separates the Gaoligong and Nu mountains consisting of metamorphic rocks. The Nu River develops along a large rupture that was filled with only tens to hundreds of meters of Quaternary fluvial sediments. The gas samples were taken from a bubbling area of around 2020 m2 along the Nu River. Lebedew et al. (1969) reported another example in the Koshelewska area of Ukraine, where gas samples taken from soils six meters below the surface have a d13C1 value of 70%, typical of biogenic methane. No methane-consuming bacteria were found in this interval. However, a large amount of methaneconsuming bacteria were detected in the shallower

Table 6 Composition and d 13C values of bacterially oxidized natural gas from shallow layers in the Shengli Oilfield, Bohai Bay Basin Well

Gudong-7 Gudongzhong-3-15 Shan-2-1 Shan-2-9

Depth (m)

1288.3–1324 1386–1393 1140–1164.8 1136–1190

Strata

Ng Ng Es1 Es1

Composition (%)

d

N2

CO2

CH4

C2H6

C3H8

C4H10

1.75 0.11 1.57 1.43

0.16 0.59 1.38 1.91

96.98 94.94 95.91 95.34

0.82 1.80 0.81 0.84

0.22 1.66 0.21 0.21

0.07 0.72 0.12 0.17

13

C PDB (%)

C1 42.80 40.54 48.68 48.97

C2 29.17 37.68 30.69 30.27

C3 19.04 19.05 24.27 22.31

C4 23.15 21.80 26.42 25.88

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intervals where the d13C1 values of the gases are much greater (  30% near the surface). Rice and Claypool (1981) also reported the d13C1 values of bacterially oxidized biogenic gases, e.g. around 45% from lacustrine deposits in Africa and Utah, and 47.1% from ooze in California. Ethane and propane can also be oxidized by bacteria and become isotopically heavier. Bacterial oxidization of propane is more common than that of ethane. Table 5 shows that gas samples from the 5-7 well in the Dagang Oilfield of Bohai Bay Basin contains 92.61, 3.86, 1.08 and 1.12% of C1–C4, respectively. The lower proportion of C3H8 compared with that of C4H10, together with the heavier d13C3 ( 22.13%) than d13C4 ( 24.30%), indicates the C3H8 has been partially bacterially oxidized. Such phenomenon also occurred in the Mu-3 and Mu-4 wells in the Junggar Basin. Similar cases were also reported from the Princess gas field in the Alberta Basin (Fuex, 1977). It should be pointed out that bacterial oxidation could lead to heavier d13C3 but not necessarily decrease the relative abundance of C3H8 to C4H10, as indicated by the samples from the Hudong, Gudao, and Chengdong oilfields in the Bohai Bay Basin (Table 6).

4. Conclusions This study demonstrated that partially reversed order for carbon isotope compositions of alkanes in natural gases can result from several alternative mechanisms, involving the mixing of biogenic and abiogenic alkanes, mixing of humic and sapropelic gases, mixing of gases from source rocks of different maturity, and/or bacterial oxidation of alkane components. The reversed order of d13C1 and d13C2 is generally rare, but occurs commonly in the southern Sichuan Basin owing to the mixing of humic gas and sapropelic gas. Therefore, the partially reversed isotope order in natural gas alkanes can serve as an excellent geochemical indicator for mixed gases and secondary gas accumulations.

Acknowledgements The authors wish to thank Dr. Maowen Li for helping us to improve the English writing of the manuscript. Drs. Martin Fowler and Ann-Lise Norman provided excellent review on an earlier version of the manuscript.

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