Abnormal carbon and hydrogen isotopes of alkane gases from the Qingshen gas field, Songliao Basin, China, suggesting abiogenic alkanes?

Journal of Asian Earth Sciences 115 (2016) 285–297

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Abnormal carbon and hydrogen isotopes of alkane gases from the Qingshen gas field, Songliao Basin, China, suggesting abiogenic alkanes? Quanyou Liu a,⇑, Jinxing Dai b, Zhijun Jin a, Jian Li c, Xiaoqi Wu a, Qingqiang Meng a, Chun Yang b, Qinghua Zhou d, Zihui Feng e, Dongya Zhu a a

Petroleum Exploration & Production Research Institute, SINOPEC, Beijing, China Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina, Langfang, China d Oil and Gas Company, China Huadian Engineering, Beijing, China e Daqing Oilfield Company, PetroChina, Daqing, China b c

a r t i c l e

i n f o

Article history: Received 14 May 2015 Received in revised form 15 September 2015 Accepted 8 October 2015 Available online 23 October 2015 Keywords: Qingshen gas field Thermogenic gas Abiogenic alkanes CO2 reduction Magmatic activity

a b s t r a c t It is great debate that the alkane gases of abiogenic origin would constitute a major portion of the commercial accumulation of the Qingshen gas field, Songliao Basin, China. In this study, abiogenic gases characterized by heavy d13C1 values, reversal of the usual carbon isotopic trend of C1–C5 alkanes, very narrow variation in d2HC1 values, and low CH4/3He ratios associated with high R/Ra values (>1) were identified. The hydrocarbon gas in the Qingshen gas field is a mixture of thermogenic alkanes derived from Cretaceous mudstone (type I kerogen) or Jurassic coal (type III kerogen) and abiogenic alkanes (mainly CH4) from mantle degassing. A quantitative estimation of abiogenic alkanes contribution to the Qingshen gas field is made based on a d13C1 vs. d13C2 plot: about 30–40% of alkane gases in the Qingshen gas field, along with its helium, are estimated to be derived from the mantle via magmatic activity. Particularly, the abiogenic formation of CH4 generated from the reduction of CO2 by hydrothermal activity may contribute. Our study suggests that abiogenic alkane gases in certain geological settings could be more widespread than previously thought, and may accumulate into economic reservoirs. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction It is generally accepted that alkane gases in commercial oil and gas fields are formed largely by thermal decomposition of organic matter (thermogenic gas) or by microorganisms (microbial gas) (Dai et al., 2005a; Galimov, 1988; Schoell, 1988). These gases from organic matter are dominated by alkane gases. While the occurrence of deep mantle CO2 in the earth’s crust has been documented in various studies (Ballentine et al., 2001; Basu et al., 2006; Dai et al., 2005b; Hopp and Trieloff, 2005; Horita and Berndt, 1999; Sherwood Lollar et al., 1997, 2002; Welhan, 1988; Zhang et al., 2008), there is disagreement about whether or not commercial reservoirs of abiogenic hydrocarbons have yet been found. Therefore, an abiogenic origin for large amounts of hydrocarbon-rich gas has been largely dismissed. Several geochemical parameters have been used to distinguish between natural gases of abiogenic and thermogenic origin (Ballentine et al., 2001; Dai et al., 2005b; Hiyagon and Kennedy, 1992; Horita and Berndt, 1999; Oxburgh ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.jseaes.2015.10.005 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

et al., 1986; Sherwood Lollar et al., 1997, 2002; Xu et al., 1995b), including (a) carbon isotopic compositions of CH4 and CO2, (b) a reversal of the usual order of carbon isotope compositions of n-alkanes to yield d13C1 > d13C2 > d13C3 > d13C4, (c) CH4/3He and CO2/3He ratios, and (d) 3He/4He ratios. The issue of whether natural gases with 13C enriched CH4 and a reversed order of n-alkane isotopic compositions must be abiogenic is still not resolved (Dai et al., 2004, 2005b; Jin et al., 2009; Sherwood Lollar et al., 2002). For example, a few natural gas samples from non-commercial gas wells of the Tarim Basin, NW China, are likely of thermogenic origin (Liu et al., 2008b), because there are no indications of magmatic activity or deep structural fault systems connecting the mantle after the Paleozoic. The unusual geochemical features of gases from the Qingshen field in the Songliao Basin encourage a careful investigation whether or not abiogenic hydrocarbons may contribute significantly to economic reservoirs, since the gas field is located between two abyssal low-angle faults of Xuxi and Xudong (Yin et al., 2002; Zhang et al., 2010b), and the gas is reservoired in volcanic rock. In this paper, we report the chemical composition, the stable carbon and hydrogen isotopic values, and noble gas isotopic values of natural

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gas from the Qingshen gas field. Based on these data we have classified the gases in terms of genetic types. We present recent findings indicating that under some circumstances abiogenic alkanes can comprise a significant portion of the natural gas in gas fields that hitherto were thought to be completely of thermogenic origin and that they could possibly form economic deposits by themselves or when mixed with thermogenic gas in sedimentary basins. 2. Geological setting The Songliao Basin is a large Mesozoic-Cenozoic non-marine sedimentary basin in northeastern China. The basin was formed as a NNE trending asymmetric Neopaleozoic–Mesozoic basin, with a tectonically steep dip in the west and dropoff in the east because of the fold tension between the North China and Siberia plates. In the Mesozoic and Cenozoic, the basin became part of the western Circum-Pacific structural domain. Asthenospheric upwelling and crustal attenuation in the Songliao Basin area have been demonstrated by seismic, electrical conductivity, and gravity measurements (Hu et al., 1998). The thickness of the lithosphere at the center of the basin is only about 80 km (Ma and Wu, 1987); the minimum crustal thickness within the basin is 23 km, some 10–15 km thinner than that of the surrounding Xing’an Mountains (Hsu, 1989). A high geothermal gradient (average = 37 °C/km (Ren, 1999)), and a high surface heat flow (up to 95 mW/m2 (Wu and Xie, 1985) with an average of 81.9 mW/m2 (1.96 HFU) have been measured in the Songliao Basin. Analysis of the distribution of earthquake foci suggests that this area is now under tensional stress (Ma and Wu, 1987). The Qingshen gas field is located in the central part of the Xujiaweizi fault-depression, in the northern Songliao Basin, and covers approximately 5350 km2 (Fig. 1). Proven natural gas reserves at the end of 2013 were about 200
109 m3. A series of gas fields such as Wangjiatun, Songfangtun, Changde, Nong’ancun and Qingshen have been discovered in the Xujiaweizi faultdepression, representative of this structural type in the Songliao Basin. The Xujiaweizi fault-depression is bounded by two lowangle faults, i.e., the Xuxi and Xudong faults (Yin et al., 2002; Zhang et al., 2010b). Three periods of tectonic activity in the Early Cretaceous (Late Shahezi, Early Yingcheng, and DengloukuQuantou) resulted in the development of five structural belts, which consist of the Paleo-Central uplift, the Xuxi depression, the Ande-Shengping uplift, the Xuzhong depression, and the Xudong slope from west to east (Zhang et al., 2010a). The two secondary structural units in the Xuxi depression are the Xuxi and Xunan sags. The Xudong sag and the Fengle low uplift are located in the Xudong depression. The Songzhan low uplift is present in the Xudong slope. Geothermal gradients within the Xujiaweizi fault-depression range from 39 °C/km to 65 °C/km with a mean value of 48 °C/km, a value higher than the average value of 37 °C/km for the entire Songliao Basin. The present geothermal gradients of the Xujiaweizi fault-depression range from 37 to 46 °C/km, with a mean value of 40 °C/km (Zhou et al., 2008). The basement rocks are overlain unconformably by Mesozoic and Cenozoic continental facies. The basin fill consists of Jurassic, Cretaceous, and Cenozoic rocks that are over 11 km thick with just the Cretaceous accounting for a thickness of >7 km (Hu et al., 1998). The deep geology comprises a half-graben with a two-layer structure and igneous extrusive rocks of Middle-Late Jurassic and Early Cretaceous age (Hu et al., 1998). The Middle-Late Jurassic formations are Baicheng (J2b), Taonan (J2t) and Huoshiling (J3h), and the Early Cretaceous formations are the Shahezi (K1sh), Yingcheng (K1yc), Denglouku (K1d), and Quantou (K1q). The basement below the Jurassic strata consists of low-grade metamorphic

Carboniferous-Permian rocks (e.g., slate, phyllite, sandstones). Gas reservoirs occur in the volcanic rocks and sandstones of the Yingcheng (K1yc), Shahezi (K1sh), Denglouke (K1d) and in the Jurassic and Permo-Carboniferous sandstones (Fig. 2). The volcanic rocks act as gas reservoirs in the upper part of the Late Jurassic Huoshiling Fm. (J3h) and the first member of the Early Cretaceous Yingcheng Fm. (K1yc), and to a limited extent in the third member of the Yingcheng Fm. (K1yc). The volcanic rocks in the Huoshiling Fm. are composed of a variety of basic to intermediate and even acidic igneous rocks, including basalt, basaltic andesite, andesite, and rhyolite. The intermediate or acidic igneous rocks, such as rhyolite, andesite and volcanoclastic sediments, occur predominantly in the first member of the Yingcheng Fm., and the basic and intermediate igneous rocks – e.g. basalt and andesitic basalt are developed in the third member of the Yingcheng Fm. Gas wells drilled into these deposits with pressures >25 MPa provide an opportunity to investigate the possibility of abiogenic gas formation and accumulation in sedimentary basins. In this study, 17 gas samples from producing wells in the Qingshen gas field were collected and analyzed with respect to their chemical, stable carbon and hydrogen isotopic, and He isotopic composition. The origin of the hydrocarbon gases in the Qingshen gas field is discussed in the context of the local geology, tectonic evolution, and magmatic activity. Our study makes a case for the accumulation of commercial abiogenic alkanes within sedimentary settings.

3. Natural gas samples and analytical methods All samples analyzed in this study were pure gases collected directly from the wellheads in commercial hydrocarbon production fields after first flushing the lines for 15–20 min to remove air contamination. We used a 25 cm diameter stainless steel cylinder (about 10 l) equipped with two shut-off valves with a maximum pressure of 22.5 MPa to collect the gas samples. The pressure inside the container was kept generally higher than 5.0 MPa. After collecting samples, the cylinder was inserted into water for a leak check. In the laboratory, the gas container was connected directly to the inlet part of a purification line using an O-ring high-vacuum connector. An aliquot (0.5–2 cm3 STP) of gas was admitted to the purification line. Noble gases (i.e. He, Ne) were isolated from other major components such as hydrocarbons, carbon dioxide, and nitrogen using two getters with hot Ti foils (800 °C). After purification, the noble gas fraction was analyzed with a VG 5400 sector mass spectrometer (Fisons Instruments). Air standards were measured repeatedly between sample analyses to correct for mass discrimination of the mass spectrometer. A blank prepared with the same procedure as the sample was measured before, after, and during sample analyses (Xu et al., 1995b, 1998). Abundances of major components (including CH4, C2H6, C3H8, He, Ar) were determined using a Finnigan MAT-271 instrument in accordance with standard procedures for mass spectrometry (State Standard of China GB/T 6041-2002 and GB/T10628-89). The concentrations were calculated by the comparison with synthetic reference gas standards. The analytic conditions were as follows: ion source: EI; electronic energy: 86 eV; mass range: 1–350 amu; resolution: 3000; acceleration voltage: 8 kV; emission: 0.200 mA; vacuum: <1.0
107 Pa. Stable carbon isotope compositions of the gases were measured with a Finnigan MAT-252 instrument. The analytical conditions were as follows: gas chromatographic column: Porapak Q (30 m), column ID: 19091P-Q04, film thickness: 20 lm; oven temperature from 40 to 160 °C at a heating rate of 15 °C/min; pure helium carrier gas, flow rate: 1.2 ml/min. The analytical error in the d13C values was <0.3‰. Each sample was measured three times, and the results of the three measurements were averaged.

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Fig. 1. Map showing the tectonic units, and the location of major gas wells in the Xujiaweizi fault-depression (left) and west-east section AB of the Xujiaweizi faultdepression (bottom right), Songliao Basin, China. A location map is shown at top right.

Stable hydrogen isotope analysis was performed on a Deltaplus XP mass spectrometer (GC-TC-IRMS) manufactured by ThermoFinnigan. The analytical conditions for gas chromatography (GC) were as follows: ATC-2000 type column (30 m
0.32 mm ID, 2.5 lm film thickness) with an initial flow rate of 1.5 ml/min and an initial hold at 30 °C for 5 min. Temperature was first increased to 80 °C at a heating rate of 8 °C/min and then to 260 °C at a heating rate of 4 °C/min with a final hold time of 10 min. The settings of mass spectrometer were: electron ionization voltage (EI) at 124 eV, emission current of 1.0 mA, acceleration voltage of 3 kV, mass range of 70. The measurement precision is 3‰ for d2HVSMOW. Each gas sample was measured three times, and the results of the three measurements were averaged. 4. Results and discussion 4.1. Gas chemical compositions and carbon isotopic compositions Two groups of natural gases occur in our sample set from the Qingshen gas field. One is CH4-rich hydrocarbon gas with relatively low CO2, N2 and He content, and the other comprises CO2-rich gas with CO2 content >83%. The occurrence of the CO2-rich gases is restricted to the western part of the field. These are considered to originate from mantle degassing based on a d13CCO2 range of 5.9‰ to 6.2‰, which covers the typical carbon isotope composition of CO2 from mantle degassing or magmatic origin

(Javoy et al., 1986; Martel et al., 1989; Marty and Tolstikhin, 1998). The origin of CO2 in basinal fluids cannot be resolved by their d13C values alone because the ranges for magmatic and crustal gases overlap. These values may be further perturbed through interaction with host rocks and connate waters. Further evidence from helium isotopes for the origin of the CO2 from mantle degassing will be discussed in Section 4.3. CH4-rich natural gases are widely distributed in the Qingshen gas field (Table 1). This type of natural gas in the Qingshen gas field is produced economically from igneous and sandstone reservoirs, and commonly is accompanied by alkaline water. The C2+ alkane concentrations in the gases are generally >2% (Table 1), suggesting a thermogenic origin (Chung et al., 1988; Dai et al., 2005a). The good linear correlation between d13Cn-C4 and d13Cn-C5 shown in Fig. 3, suggests that these heavier wet gases originate from the thermal decomposition of organic matter since the difference in carbon isotope values between gaseous hydrocarbons and source rocks would be smaller with increasing molecular weight (Chung et al., 1988). However, the entire reversal of the carbon isotope trend of gaseous alkanes, i.e. d13C1 > d13C2 > d13C3 > d13C4, observed in most of the natural gases we studied from the Qingshen gas field does not support an exclusive origin from thermal decomposition of organic sources (Table 1, Fig. 4). A reversal of carbon isotope compositions for n-alkane gases (C1–C5) from the Tarim Basin results from the deficiency of gas input and diffusion of gas through cap rocks. The experimental studies confirm that the

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Fig. 2. Stratigraphic column and petroleum system in the Qingshen gas field, Songliao Basin, China.

diffusion of gas through cap rocks can result in d13C values depleted by 4.4‰ (Krooss and Leythaeuser, 1997; Zhang and Krooss, 2001). Other trends of 13C depletion in C2–C4 with respect to C1 (e.g. d13C2 > d13C3, or d13C3 > d13C4) from the Ordos and Tarim basins mainly resulted from the mixing of sapropelic and humic gases and/or mixing of gases from two source rock intervals of similar kerogen but different maturity (i.e., from one source rock of varying maturity) (Dai et al., 2004). In this study, C1/(C2 + C3) ratios ranging from 6.86 to 51.75 suggest a thermogenic origin (Chung et al., 1988). Nonetheless, we cannot rule out the presence of a significant abiogenic source of alkane gases in this economic gas field.

The 2 other samples from the gas reservoirs of SS1 and W 9-12, have similar d13C2 and d13C1 values, both depleted in 12C, consistent with mixing of an abiogenic methane end-member if we use the criterion for identifying methane as abiogenic origin with d13C1 values higher than 25‰ (Horita and Berndt, 1999) (Table 1). Because hydrocarbon gas generation from organic matter under subsurface conditions is a kinetically controlled process, thermogenic hydrocarbons are generally characterized by a normal isotope distribution pattern, in which the d13C values of gaseous alkanes gradually become enriched in 13C with increasing molecular weight or carbon number (Dai et al., 2005a; Des Marais et al.,

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Table 1 Chemical and isotopic composition of natural gas from the Qingshen gas field, Songliao Basin, China (n.d., not detected; n.a., not analyzed; 3He/4He, n
106). R/Ra values were calculated from the 3He/4He ratios (R) of measured gas samples and an atmospheric ratio (Ra) of 1.4
106. For the carbon isotopes, the overall analytical error is ±0.3‰. Gas concentrations are measured with reference to the State Standard of China GB/T 6041-2002 and GB/T10628-89. Well

FS2

FS9

FS701

WS1

SSG2

SS2-1

Depth (m) Strata Lithology CH4 (%) C2H6 (%) C3H8 (%) C+4 (%) CO2 (%) N2 (%) He (%) Ar (%) d13CCO2 (‰, PDB) d13C1 (‰, PDB) d13C2 (‰, PDB) d13C3 (‰, PDB) d13n-C4 (‰, PDB) d13n-C5 (‰, PDB) d2HC1 (‰, VSMOW) d2HC2 (‰, VSMOW) d2HC3 (‰, VSMOW) 3 He/4He Error R/Ra 4 He/20Ne CH4/3He CO2/3He

3099 K1d Sandstone 91.92 2.19 0.12 0.04 2.69 3.04 0.034 0.011 16.5 17.4 22.2 30.5 32.5 31.5 202 247 237 8.18 ±0.23 5.84 3097 3.3E+08 4.51E+04

3632 K1yc Igneous 15.96 0.3 0.02 0 82.49 1.23 0.013 0.012 6.2 27.5 30.9 31.9 33.5 33.3 204 180 159 3.45 ±0.10 2.46 2365 3.6E+08 1.71E+05

3840 K1yc Igneous 5.08 0.57 0.17 0.06 93.99 0.12 0.002 0.003 5.9 26.5 29.9 31.8 30.7 29.8 204 180 150 2.74 ±0.08 1.96 621 1.2E+09 1.71E+06

3528 K1yc Igneous 91.25 2.27 0.25 0.17 1.35 4.71 0.046 0.022 13.5 25.9 28.6 33.2 34.2 34 197 160 163 2.5 ±0.09 1.79 1474 7.9E+08 4.45E+04

2965 K1yc Igneous 92.08 2.17 0.14 0.04 2.46 3.11 0.03 0.013 14.8 27.2 28.1 32.7 36 36 202 172 129 2.48 ±0.07 1.77 3482 1.2E+09 8.97E+04

2997 K1yc Igneous 91.9 2.16 0.12 0.04 2.69 3.09 0.02 0.011 14.5 26.8 29.1 33.5 37.8 38.2 202 174 126 2.46 ±0.07 1.76 2448 1.9E+09 9.71E+04

SS2-25

FS6

FS5

XS603

XS1-1

XS1

3021 K1yc Igneous 92.05 2.2 0.13 0.04 2.51 3.07 0.024 0.011 13.2 26.6 28.8 32.6 36.5 36.2 203 174 135 2.44 ±0.07 1.74 4140 1.6E+09 8.81E+04

3409 K1yc Igneous 81.49 2.17 0.29 0.07 14.32 1.67 0.016 0.008 7.5 28.3 30.4 32.6 32.6 32.9 205 195 177 2.2 ±0.06 1.57 1455 2.3E+09 1.10E+05

3210 K1d Sandstone 94.46 2.39 0.2 0.05 0.41 2.49 0.024 0.01 16 27.1 28.5 30.8 30.9 31.8 201 187 162 2.13 ±0.06 1.52 2565 1.8E+09 7.66E+04

3521 K1yc Igneous 94.98 3.08 0.27 0.19 0.42 1.06 0.012 0.007 12.3 27 30.4 32.3 35.8 35.1 202 195 143 1.69 ±0.05 1.21 1505 4.7E+09 2.36E+05

3960 K1sh Igneous 93.64 3.08 0.37 0.19 1.44 1.28 0.01 0.012 5.5 28.9 32.6 33.3 34.3 34.4 203 211 168 1.54 ±0.05 1.10 1959 6.1E+09 1.60E+05

4548 K1yc Igneous 93.17 3.18 0.51 0.25 1.81 1.08 0.011 0.006 5.9 29.7 32.9 34.3 35.5 34.1 201 215 185 1.54 ±0.04 1.10 1922 5.5E+09 1.78E+05

Depth (m) Strata Lithology CH4 (%) C2H6 (%) C3H8 (%) C+4 (%) CO2 (%) N2 (%) He (%) Ar (%) d13CCO2 (‰, PDB) d13C1 (‰, PDB) d13C2 (‰, PDB) d13C3 (‰, PDB) d13nC4 (‰, PDB) d13nC5 (‰, PDB) d2HC1 (‰, VSMOW) d2HC2 (‰, VSMOW) d2HC3 (‰, VSMOW) 3 He/4He Error R/Ra 4 He/20Ne CH4/3He CO2/3He

Depth (m) Strata Lithology CH4 (%) C2H6 (%) C3H8 (%) C4+ (%) CO2 (%) N2 (%) He (%) Ar (%) d13CCO2 (‰, PDB) d13C1 (‰, PDB) d13C2 (‰, PDB) d13C3 (‰, PDB) d13nC4 (‰, PDB)

XS6

SS1

WS5

FS8

W9-12

4060 K1sh Igneous 94.52 3.12 0.48 0.26 0.43 1.21 0.008 0.004 13 28.3 33.2 34.3 35.3

2933 J Sandstone 93.57 2.98 0.37 0.13 0 2.95 0.024 0.011 10.5 25.8 25.3 24.2 21.3

3791 K1sh Sandstone 87.3 3.27 0.31 0.08 0 9.06 0.024 0.053 n.d 27.2 23.3 23.1 23.1

4201 C-P Sandstone 94.94 3.24 0.45 0.16 n.d 1.22 0.013 0.005 9.7 27.8 30.9 30.9 30.4

2958 C-P Sandstone 96.15 1.58 0.28 0.09 0.8 1.71 n.a n.a n.d 23.9 23.4 25 24.9 (continued on next page)

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Table 1 (continued)

d13nC5 (‰, PDB) d2HC1 (‰, VSMOW) d2HC2 (‰, VSMOW) d2HC3 (‰, VSMOW) 3 He/4He Error R/Ra 4 He/20Ne CH4/3He CO2/3He

XS6

SS1

WS5

FS8

W9-12

35.5 204 212 179 1.46 ±0.04 1.04 2753 8.5E+09 1.63E+05

20.8 200 178 150 1.39 ±0.04 0.99 3615 2.8E+09 4.81E+04

23.2 203 178 140 1.15 ±0.03 0.82 1960 3.2E+09 1.82E+04

31.2 204 218 183 1.08 ±0.03 0.77 1608 6.8E+09 1.36E+05

n.d n.a n.a n.a n.a n.a n.a n.a 6.8E+09

-15

-25

y=0.9998x+0.1036 R2=0.9821

FS9

FS701

WS1

SSG2

SS2-1

SS2-25

FS6

FS5

XS603

XS1-1

XS1

XS6

SS1

WS5"

FS8

13

δ C5 (‰)

-20

FS2

-30

-35

-40 -40

-35

-30

-25

13 δ C4 (‰)

-20

-15

Fig. 3. Plot of d13C4 versus d13C5 values for n-alkane gases. Linear regression of the data (R2 = 0.981) indicates a thermogenic origin, derived from organic matter, because the carbon isotope values of these heavier hydrocarbons are very close to those of their source organic matter.

1981; James, 1983; Schoell, 1980; Tang et al., 2005, 2000), i.e. d13C1 < d13C2 < d13C3 < d13C4. The full reversal of carbon isotopes of alkanes (i.e. d13C1 > d13C2 > d13C3 > d13C4) for all the gases listed in Table 1 (except W 9-12) cannot be interpreted by the three known possibilities for the partial reversal of carbon isotopes of C2, C3, and C4 alkanes observed in most natural gases (Dai et al., 2004). These are: (i) mixing of sapropelic and humic gases, (ii) mixing of gases from two source rock intervals of similar kerogen but of different maturity (i.e., from the same source rock of varying maturity), or (iii) microbial oxidation (James and Burns, 1984; Schoell, 1983). The pyrolysis results for the shale and coal collected from the Xujiaweizi fault-depression and crude oil from Well FS 2 suggest that a partial carbon isotopic reversal of heavier gaseous hydrocarbons would occur at high temperatures (Table 2). It has been observed that the onset of the reversal for propane and butane occurs at pyrolysis temperatures of 500 °C (equivalent vitrinite reflectance is about 2.3–2.4%), and the reversal for propane and ethane takes place when the temperature is >550 °C (equivalent Ro > 2.4%) (Yang et al., 2008). However, these trends do not extend to carbon isotopic reversal for CH4 and C2H6, which often results from the mixing of thermogenic gases from different source rocks and/or different thermal maturity. Additionally, the reversal of the carbon isotope trend of gaseous alkanes (C1–4) was observed during experimental pyrolysis of lignite at high pressures (1–3 GPa) and high temperatures (500– 700 °C) (Du et al., 2003). Galimov (2006) inferred that pressure does not significantly affect the carbon isotope effect, since a small carbon isotope effect was observed at pressures exceeding 40 GPa.

It is unclear from our present understanding of the geological history that such extreme pressures would be reached, hence influencing the carbon isotopic composition of the natural gases. We can rule out a microbial oxidation process, because this will always result in an absolute enrichment of 12C in CH4 (Schoell, 1988; Tang et al., 2005). Although the entire reversal of the carbon isotope trend of gaseous alkanes, i.e. d13C1 > d13C2 > d13C3 occurs in some shale gases (Hao and Zou, 2013; Zumberge et al., 2012), shale gases often have a very high maturity, and there is no continued isotopic reversal in the heavier alkane gases (>C4). The plot of d13C2 vs. d13C1 suggests that a possible interpretation to the reversal of d13C1 > d13C2 (Fig. 5) in our gas samples is the mixing of abiogenic gas and thermogenic gas. As shown in Fig. 5, the natural gases of the Songliao Basin can be divided into two groups. One family plotting below the diagonal (d13C1 < d13C2) and with d13C1 < 25‰ is considered to be derived largely from thermal decomposition of organic matter. The W 9-12 gas and the typical thermogenic gas present in the Chaoyanggou area belong to this group, and they are characterized by normal trends of carbon isotopes of gas alkanes (Table 3). The other group of samples plots above the diagonal (d13C1 > d13C2) with a linear correlation of d13C1 and d13C2 (except for the Well FS 2). These gases are all in the Qingshen gas field, the gas of which is characterized by extreme 13C enrichment for C1 and C2. The variation in d13C2 values of gas in the Qingshen field likely results from the thermal maturation of the gas source, which commonly produces methane and ethane increasingly enriched in 13C (Schoell, 1988). The regression line of d13C1 and d13C2 for the Qingshen gases is almost parallel to that for the Chaoyanggou thermogenic gases originating from the

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Cretaceous mudstone with type I kerogen. However, the presence of thermogenic gas alone cannot explain the observation of d13C1 > d13C2 in the Qingshen gas field; additional gas characterized by 13C enriched methane must have contributed to gases of the Qingshen gas field. Under special conditions of pressure and temperature, abiogenic hydrocarbon gases can be formed by the reduction of carbon dioxide in hydrothermal systems as a result of water–rock interactions, Fischer–Tropsch reactions and the serpentinization of ultramafic rocks (Berndt et al., 1996; Charlou et al., 2000, 2002; Fiebig et al., 2007; Jean-Baptiste et al., 1998; Keir et al., 2006; Lancet and Anders, 1970; Sherwood Lollar et al., 1997; Wakita and Sano, 1983; Wang et al., 1997). These abiogenic hydrocarbon gases consist, however, almost exclusively of CH4 and have only extremely low amounts of C2H6 and C3H8 (commonly below the detection limit) (Berndt et al., 1996; Horita, 2001; Janecky and Seyfried, 1986; McCollom and Seewald, 2001). Thus, abiogenic gas is composed dominantly of methane and characterized by d13C1 > d13C2 and d13C1 > 25‰ (Horita and Berndt, 1999). Previous studies have been conducted on natural gas from the Well FS 2 such as Dai et al. (2005a,b) and references therein, and the reported d13C1 and d13C2 values are in the ranges of 18.9‰  16.7‰ and 19.9‰  19.2‰, respectively. Since the natural gas from the Well FS 2 in this study is characterized by d13C1 = 17.4‰ and d13C2 = 22.2‰, which are essentially consistent with previous studies, we consider that abiogenic gas is dominant in natural gas for this part of the Qingshen gas field. 4.2. Hydrogen isotopic compositions

Fig. 4. Plot of d13C values of individual n-alkanes vs. carbon number for gas samples from the Qingshen gas field, Songliao Basin, and for thermogenic gases from the Ordos and Tarim basins. (a) Gas samples (36) from the economic gas fields of Qingshen, Ordos (data from Dai et al., 2005a,b) and Tarim (data from Liu et al., 2008a,b), Fourteen (14) from the Qingshen and 2 from the Tarim Basin show a trend of 13C depletion in n-alkane gases. (b) For 14 of the 17 gas samples from the Qingshen gas field, a reversal trend of carbon isotope compositions for n-alkane gases (C1–C5) is observed.

Table 2 Carbon isotope compositions of alkane gases from possible source rocks and crude oils subjected to closed system pyrolysis. The possible source rocks were collected from the well cores in the Xujiaweizi fault-depression; the oil was collected from the Denglouke Formation (K1d) of the Well FS 2 at the depth of 2720.3–3038.4 m. Sample

Temperature (°C)

Ro (%)

d13C1 (‰)

d13C2 (‰)

d13C3 (‰)

d13C4 (‰)

Coal from Well Zhaoshen6

250 300 350 400 450 500 550

/ / / / / / 2.41

23.6 28.3 32.8 31.5 23.4 19.6 21.1

29.5 26.6 26.9 23.5 14.8 16.7 16.5

27.3 24.9 25.4 20.7 17.6 20.6 22.4

27.1 24.9 24.7 19.2 22.1 21.4 23.3

Mudstone from Well Du13

250 300 350 400 450 500 550

/ / / / / / 3.19

32.5 33.6 38.9 33.8 24.8 22.4 25.7

27.9 31.4 31.3 25.6 17.5 15.4 14.1

21.7 30.7 28.8 23.1 21.8 23.7 22.8

/ 29.6 27.5 22.7 26.8 27.6 /

Oil + sandstone + water

450 500 550

1.84 2.33 2.80

48.5 37.0 39.6

35.0 29.6 34.8

31.4 26.6 25.7

30.9 28.0 27.4

Oil + volcanic rock + water

400 450 500 550 600

1.35 1.84 2.33 2.80 3.40

50.9 48.5 39.3 24.0 24.3

41.4 38.3 30.9 / 19.4

37.9 34.5 29.1 / 25.8

36.4 33.4 32.2 / 27.3

In order to further investigate the gas origins in the Qingshen gas field, the hydrogen isotopes of C1, C2 and C3 were measured. The results are listed in Table 1. Generally, the gaseous alkanes thermally derived from organic matter display a large variation in methane d2H values and a relatively narrow range of d2H values of C2 and C3, since the increase of methane, ethane and propane d13C and d2H values with carbon number is likely to be result of Rayleigh fractionation effects during formation (Chung et al., 1988; Schoell, 1980; Tang et al., 2005). The d2H values of alkane gases originating from kerogens are affected by the depositional paleo-environments of the source rock as well (Liu et al., 2008a; Schoell, 1980). Generally, the hydrogen isotope values of thermally degraded methane become enriched in deuterium from fresh to weakly saline lacustrine to saline lacustrine-marine environmental conditions. In the Qingshen gas field, a small range of methane d2H values of 205‰ to 197‰ is observed, and a reversal of hydrogen isotopes in the order of C1, C2, C3 (i.e. d2HC1 > d2HC2 > d2HC3) occurs for some samples (e.g. FS 2, FS 8, XS 1, XS 1-1, and XS 6) (Fig. 6). The results also indicate the complexity of the origins of hydrocarbon gas. The narrow variation of d2Hmethane and a reversal of hydrogen isotopes of methane and ethane are observed in TSR-altered gas (Liu et al., 2014), but thermochemical sulfate reduction can be excluded in the case of these gases, because no H2S is present. It is unclear whether the narrow range of methane d2H values indicates a fresh water depositional paleo-environment (Schoell, 1980) or is due to a contribution of deep abiogenic methane with more negative d2Hmethane values (Jin et al., 2009; Sherwood Lollar et al., 2002). According to Whiticar’s carbon and hydrogen isotope scheme for the identification of the origin of methane (Whiticar, 1999; Whiticar et al., 1986), the isotope values for the Qingshen gas field indicate methane formed under geothermal, hydrothermal and crystalline conditions (Fig. 7), indicating the methane and heavy hydrocarbon gases under the hydrothermal system were generated from over-mature organic matter interaction with exotic hydrogen (Schimmelmann et al., 2001; Whiticar and Suess, 1990). With increasing thermal maturity of organic sources,

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Fig. 5. Plot of d13C1 versus d13C2 for gaseous alkanes from the Songliao Basin. Grey open circles and small size open triangles = gases derived from type-I kerogen source and type-III kerogen in the Chaoyanggou area, respectively. These natural gases can be discriminated based on their d13C1 and d13C2 values. Gases below the diagonal (d13C1 < d13C2) and with d13C1 < 25‰ are considered biogenic, and those plotting above the diagonal (d13C1 < d13C2) with d13C1 < 25‰ are mixtures of biogenic and abiogenic gases. The heavy biogenic gases can be readily identified by their ethane carbon isotopes (d13C2 = 28‰ or greater); the oil-type gases derived from the Cretaceous mudstone generally have d13C2 values less than 28‰, whereas the coal-type gases derived from the Jurassic coal have d13C2 values heavier than 28‰.

Table 3 Carbon isotopic composition of natural gas from the Chaoyanggou gas pool, Songliao Basin, China (n.d., not detected; n.a., not analyzed; 3He/4He, n
106). The observed normal trend of carbon isotope compositions for the n-alkane gases (C1–C4) supports the interpretation that they are derived from the thermal decomposition of organic matter; R/Ra ratios less than 1.0 suggest that the contribution of mantle-sourced gas is limited in the Chaoyanggou gas pool. # Data cited from Wang et al. (2006). Well

S18 Sh81 Sh63 San3 Zhuang5-2 Sh69 Shuang17 Wu106 San203 Wushen1 Zhuangshen1 Songshen4 Wu101 San202 San4-1 Shengshen6 Wu102 Wan11

Strata

K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1 K1

Depth (m)

1872.8 1358.4 1847.4 595 2236.4 707.3 813.2 693.3 713.1 743.2 2523 818.2 804.3 953 3163.4 812.3 1265.3

Lithology

Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Sandstone Conglomerate Sandstone Sandstone Sandstone Conglomerate Sandstone Sandstone

however, the d2HC2 and d2HC3 values of thermogenic gases in the Tarim Basin increase and a positive relationship among them is observed (Liu et al., 2008a). A similar tendency between d2HC2 and d2HC3 values in the Qingshen gas field is also observed (Fig. 8), supporting our view that the heavy alkane gases were thermogenic being derived from organic matter. Even though the CH4 sources are uncertain, the slightly positive correlation (except for the Well FS 2) between d2HC2 and d13C1 (Fig. 9) suggests that a significant portion of CH4 in the Qingshen gas field could be from thermal decomposition of organic matter. 4.3. Helium isotopic compositions The R/Ra in natural gas from the Qingshen gas field range from 0.77 to 5.84 (Table 1). The 4He/20Ne ratios, ranging from 621 to 4140, are much higher than the value for air (air ratio = 0.288) and thus indicate that the gas sampling and analytical procedures did not introduce air contamination (Xu et al., 1995b, 1998). Therefore, the wide variation in helium isotopic compositions of

Carbon isotope composition (‰, PDB)

R/Ra

d13C1

d13C2

d13C3

d13C4

d13C5

33.8 35.3 33.0 32.6 32.7 31.2 35.0 32.2 31.8 31.1 32.8 31.7 30.6 33.3 32.7 32.2 30.3 28.3

32.6 32.5 31.4 30.5 30.1 29.2 26.0 25.7 25.1 24.6 24.3 24.1 23.8 23.7 23.6 23.3 22.7 21.5

32.6 31.9 30.0 32.7 27.7 28.7 26.1 26.9 26.2 26.7 25.5 23.2 26.5 28.4 28.7 23.1 24.9 21.2

30.8 29.6 28.8 31.2 26.9 27.4 27.0 n.d n.d 27.2 26.4 22.4 27.2 28.9 29.2 21.6 n.d 20.8

n.d n.d n.d 31.6 27.0 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d

n.a n.a n.a 0.084 0.219 n.a n.a n.a n.a n.a 0.186 n.a n.a n.a n.a n.a n.a n.a

the natural gas must be attributed mainly to the mixing of the crust-derived noble gases enriched in 4He and the mantlederived noble gases enriched in 3He. Since CO2 is likely the major volatile carrier phase for mantlederived noble gases in the crust (Dai et al., 1996; Zhang et al., 2008), the greatest potential for unambiguous identification of mantle-derived carbon would be in the gas reservoirs with the highest CO2 contents (Staudacher, 1987). Unlike the inert noble gas tracers, CO2 is a reactive species and is subject to loss to a wide variety of potential crustal sinks and to mixing and dilution by many sources of crust-derived carbon (Sherwood Lollar et al., 1997). Since mantle-derived helium is enriched in 3He by several orders of magnitude compared to crust-derived 4He, this noble gas tracer has been widely used to quantify the flux of mantlederived volatiles in continental regions (Ballentine et al., 2001; Jean-Baptiste et al., 1998; Jin et al., 2009; Keir et al., 2006; Lupton et al., 1999; Martel et al., 1989; Sano and Pillinger, 1990). The d13CCO2 values of our CO2-rich gases range from 5.9‰ to 6.2‰, which overlaps the range of CO2 from mantle degassing

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-140

-160

Thermogenic gas

-200

2

δ HC1 (‰)

-180

-220

-240

-260 -260

-240

-220

-200 2

δ HC2 (‰)

-180

-160

-140

FS2 FS9 FS701 WS1 SSG2 SS2-1 SS2-25 FS6 FS5 XS603 XS1-1 XS1 XS6 SS1 WS5 FS8 Tarim Basin

Fig. 6. Plot of d2HC1 versus d2HC2 for gaseous alkanes. The narrow distribution of d2HC1 values ranging from 197‰ to 205‰ and a wide range of d2HC2 values, as well as the reversal of hydrogen isotopes of hydrocarbon gases for some samples (e.g. FS 2, FS 8, XS 1, XS 1-1, and XS 6) suggest that the isotopically heavy gaseous alkanes were derived from organic matter and that the CH4 is multiply sourced. The area of thermogenic gas is bounded by data from the Tarim Basin (Liu et al., 2008a).

4.4. Estimation of the contribution of abiogenic methane to the Qingshen gas field 4.4.1. CH4/3He parameter Estimation of the contribution of abiogenic CH4 from mantle degassing to the Qingshen gas field is difficult because it will always be mixed with considerable amounts of thermogenic gas from organic sources (Horita and Berndt, 1999; Wakita and Sano, 1983). As discussed above, any atmosphere-derived He contamination in natural gases can be regarded as negligible (Sherwood Lollar et al., 1997) due to much higher 4He/20Ne ratios in natural gases than the atmosphere (0.288). Therefore, the variation of the 3 He/4He ratios in this study can simply be attributed to mixing of mantle-derived He with crust-radiogenic He. Here, we can estimate the proportion of the mantle-derived He and crust-radiogenic He in the natural gas assuming the He isotopic compositions of the two end-members (Ballentine and O’Nions, 1992) using the equation:

He ð%Þ ¼ ½ðR=RaÞ  ðR=RaÞc
100=½ðR=RaÞm  ðR=RaÞc

Fig. 7. Origin of natural gas indicated by carbon and hydrogen isotopes of methane component (modified from Whiticar, 1999).

and magma (Exley et al., 1986; Marty and Tolstikhin, 1998; Zhang et al., 2008). These gases exhibit 3He/4He ratios >1.0 Ra, indicating that mantle-derived helium enriched in 3He made a significant contribution. As shown in Fig. 10, the relationship of R/Ra versus CO2/3He and CH4/3He was used to interpret the possible crustal and magmatic sources for the gases by constraining the end-member of typical crustal origin (Dai et al., 2005b; Xu et al., 1995a) and typical upper mantle origin (Poreda and Craig, 1989; Wakita and Sano, 1983; Welhan, 1988). R/Ra and CO2/3He values are plotted with respect to mixing lines between crust- and mantle-derived CO2 endmembers (grey area). In fact, while CO2-rich gases fall into the mixture area close to the end-member of mantle origin, all CH4-rich gases plot outside the mixing area and appear to have undergone a significant loss of CO2 relative to 3He (Fig. 10a). Although there are many conceivable processes resulting in the loss of CO2, including precipitation as carbonate minerals or reduction to graphite (Sherwood Lollar et al., 1997), the reduction of CO2 to CH4 under certain pressure and temperature have been suggested in this instance (Horita and Berndt, 1999; Wakita and Sano, 1983).

where R/Ra, (R/Ra)c, and (R/Ra)m represent air-normalized He isotopic compositions of natural gas sample, crust radiogenic and mantle-derived components, respectively. The isotopic composition of the present mantle-derived He is well known to be R/Ra = 8 (Oxburgh et al., 1986; Poreda and Craig, 1989; Sherwood Lollar et al., 1997; Xu et al., 1995b), while the isotopic composition of the crust radiogenic He is gotten as 0.32 according to empirical statistics of 58 natural gas samples from Ordos Basin as a typical crust basin (Dai et al., 2005b). The contributions of crust radiogenic and mantle-derived He can be estimated quantitatively according to the method proposed by Ballentine and O’Nions (1992). Based on this approach, the contribution of mantle-derived He in the Qingshen gas field was estimated to range widely from 8.9% to 71.9% (Table 4). The negative correlation between R/Ra and CH4/3He (Fig. 10b) may also provide a way to estimate the relative fraction of thermogenic and abiogenic methane present in our gases (Poreda and Craig, 1989; Wakita and Sano, 1983; Xu et al., 1995b). The empirical CH4/3He range for thermogenic gases lies between 109 and 1012 with R/Ra < 0.32 (Dai et al., 2005b), and that of the typical mantlederived gases from the hot springs and hydrothermal vents of the East Pacific ridge ranges from 105 to 107 with R/Ra > 1 (Dai et al., 2001; Welhan, 1988). As shown in the Fig. 10b, most of gas samples from the Qingshen gas field are above the two-component mixing

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-100 -120

-160 -180

2

δ HC3 (‰)

-140

-200 -220 -240

Thermal maturity increasing

-260 -260

-240

-220

-200

-180

-160

-140

FS2 FS9 FS701 WS1 SSG2 SS2-1 SS2-25 FS6 FS5 XS603 XS1-1 XS1 XS6 SS1 WS5 FS8

δ2HC2 (‰) Fig. 8. Plot of d2HC2 versus d2HC3 for gaseous alkanes.

-15

Mantle source -20

δ13C1 (‰)

Mixing of mantle and crust sources -25

-30

-35

Thermal maturity increasing Crust source

-40 -260

-240

-220

-200

-180

-160

δ2HC2 (‰)

-140

-120

-100

FS2 FS9 FS701 WS1 SSG2 SS2-1 SS2-25 FS6 FS5 XS603 XS1-1 XS1 XS6 SS1 WS5 FS8 Tarim Basin

Fig. 9. Plot of d13C1 versus d2HC2 for natural gases from Qingshen gas field. The slightly positive correlation between d13C1 and d2HC2 suggests methane was mainly derived from organic matter with the exception of the FS 2 gas at top left. The area of crust-sourced gas is bounded by data from the Tarim Basin (Liu et al., 2008a).

area between crustal and mantle end-members, suggesting that additional methane input has resulted in high CH4/3He values in samples associated with high R/Ra ratios. Three possible sources could make a contribution to abnormally high methane content: biogenic methane, thermogenic methane, and/or hydrothermal methane from CO2 reduction. The d13C1 values in the Qingshen gas field range from 17.4‰ to 29.7‰, and are much heavier than those of typical biogenic methane (<60‰). Therefore, the addition of microbial methane cannot explain the observed high CH4/3He values. CO2 reduction by H2 under hydrothermal conditions in the deep earth was presumed to be the path for abiogenic methane formation, and CH4 formation from CO2 reduction is favorable under elevated pressure and low temperature (Horita and Berndt, 1999; Wakita and Sano, 1983). For example, in the oil and gas fields in the Sea of Japan, the higher CH4/3He values ranging from 1011 to 1014 are considered to result from the abiogenic CH4 formation through CO2 reduction (Wakita and Sano, 1983). In the process of CO2 reduction by H2 under hydrothermal conditions, 12C in CO2 will react preferentially with H2 to form CH4 and result in the residual CO2 enriched in 13C, because the energy required to break the chemical bonds of

12

C–12C is lower than that of 13C–12C and 13C–13C (Chung et al., 1988; Tang et al., 2005). Although methane formed by CO2 reduction is relatively enriched in 12C, it contains relatively more 13C than thermogenic methane which is depleted in 13C. Therefore, mixing of abiogenic methane formed by mantle sourced carbon and hydrogen with thermogenic alkane gases led to the narrow variation of carbon and hydrogen isotopes of methane in gases in this study. Even though it is hard to quantitatively evaluate the proportion of abiogenic CH4 to the Qingshen gas field formation based on the simple mixing of crust and mantle end members (Fig. 11), our results suggest that portions of the CH4 in the Qingshen gas field are abiogenic and originate from mantle degassing, which is consistent with the observed distributions of carbon isotopes of alkane gases (Fig. 3), the CO2/3He and the CH4/3He ratios (Fig. 10). Furthermore, high 3He/4He ratios associated with high CH4/3He value of 3.3
108, i.e. much higher than those from hot springs and hydrothermal vents of the East Pacific ridge (Dai et al., 2005b; Welhan, 1988), provide additional evidence for the generation of abiogenic CH4 from the reduction of carbon dioxide.

Q. Liu et al. / Journal of Asian Earth Sciences 115 (2016) 285–297

Fig. 10. Plot of R/Ra ratios versus CO2/3He and CH4/3He with respect to mixing lines between crustal and mantle end-members. (a) Two component mixing of CO2 between crustal and mantle end-members, with the parameters for the crustal endmember taken as 0.32Ra for 3He/4He ratios and a range of 108–1010 for CO2/3He ratios, according to empirical data from the Ordos basin (Dai et al., 2005a; Xu et al., 1995a). The mantle member is at 4.0Ra for 3He/4He ratios, and in the range of 1010– 1013 for CO2/3He ratios (Wakita and Sano, 1983; Xu et al., 1995b). Higher R/Ra ratios in combination with lower CO2/3He values suggest that CO2 has been partly reduced to CH4. Figure (b) shows two-component mixing of CH4 between crustal and mantle end-members. When choosing CH4/3He parameters of crustal and mantle-derived end-members according to empirical data (Dai et al., 2005b; Welhan, 1988; Xu et al., 1995b), the contributions of abiogenic CH4 are estimated to be slightly higher than those resulting from simple mixing of crust- and mantle-derived CH4. This difference also indicates the formation of CH4 by carbon dioxide reduction.

4.4.2. d13C1 vs d13C2 plot As discussed above, the natural gas in the Qingshen field is the mixing product of abiogenic and thermogenic gas. If it is possible to determine d13C1 values for each end-member, a quantitative estimate of their contribution to Qingshen gas formation can be achieved. The d13C1 values of abiogenic methane range from 3.2‰ to 36.2‰ (Dai et al., 2005b). Although there is no typical value that can be applied, d13C values of abiogenic methane are generally considered to be heavier than 25‰ (Horita and Berndt, 1999). For our purpose, we will assume that the gas from Well Fengshen 2 is a typical abiogenic gas, characterized by d13C1 = 17.4‰, d13C1 > d13C2 > d13C3 > d13C4 and R/Ra as high as 5.84. It is therefore assumed that the abiogenic end-member methane in the Qingshen gas field has a homogeneous d13C1 value equivalent to the FS 2 gas – i.e., 17.4‰. The next step is to determine the d13C1 values of the other end-member, that of the thermogenic gas from organic sources. The carbon isotope

295

compositions of methane and its gaseous homologues are highly affected by organic source types and thermal maturity. According to the evaluation of the hydrocarbon source rock potential, the Cretaceous mudstones with type I or II1 kerogen and the Jurassic coal with type III kerogen are the only known and effective (i.e., thermally mature) sources in the area (Hu et al., 1998). They have been proved as gas source in the Chaoyanggou gas pool (Wang et al., 2006), which is located to the southeast of the Qingshen gas field (Fig. 1). Thermogenic gases from humic and sapropelic source can be readily identified by the stable carbon isotope ratios of ethane and propane. The oil-type gases derived from sapropelic source rocks generally display d13C2 and d13C3 values lower than 28‰ and 25‰, respectively (Dai et al., 2005a). Oil-type gases from the Cretaceous mudstone and coal-type gas from the Jurassic coal are characterized by d13C1 < d13C2 < d13C3 < d13C4, typically for an organic origin, as expected (Xu et al., 1995a). As shown in Fig. 5, oil-and coal-type gases in the Chaoyanggou gas pool all plot below the d13C1 = d13C2 diagonal. The linear increase between d13C1 and d13C2 results from thermal maturation of organic matter, i.e., an increase of both d13C1 and d13C2 with increasing maturity. As stated above, the regression trend line for gases from the Qingshen gas field is almost parallel to that for gases in the Chaoyanggou gas pool. This strongly suggests that the gas in the Qingshen gas field represents a combination of thermogenic gas from Cretaceous mudstone and Jurassic coal. Therefore, we assume that d13C1 and d13C2 of thermogenic gases from the Cretaceous mudstone and Jurassic coal in the Qingshen field at different thermal maturities follow regression line I for oil-type gas and regression line II for coal-type gas in Fig. 5. Therefore, when the d13C2 value is less than 28‰, we use the regression line I for oil-type gas, while the regression line II is used for coal-type gas with d13C2 value over 28‰. Having determined the d13C values for abiogenic methane and two sources of thermogenic methane, we can easily use d13C1 vs. d13C2 plot to calculate the contribution of each component to the natural gas in a specific gas well. Thus, in Well FS 6, the measurement yielded d13C1 and d13C2 of 28.3‰ and 30.4‰, respectively. Herein ethane was mainly from thermogenic decomposition of organic matter of the Cretaceous mudstone. According to the regression line I (y(d13C1) = 0.8957
x(d13C2)  5.3104, r2 = 0.8082), the calculated d13C1 is about 32.5‰ when d13C2 is 30.4‰. The natural gas in Well FS 6 with d13C1 of 28.3‰ is a mixture of 72% thermogenic gas (d13C1 = 33.0‰) and 28% abiogenic gas (d13C1 = 17.4‰). Following the same calculation procedure, the contribution percentage for the rest of wells can be estimated from equations I and II for oil- and coal-type gas, respectively (Fig. 5). Based on our calculation, abiogenic gas accounts for 25.1–53.0% for most gas producing wells in the Qingshen gas field (Table 4). Our results are consistent with the geologic history of this area. The periods of volcanic activity that occurred in the study area during the Late Cretaceous and Middle Jurassic (Li et al., 2005) provided appropriate geological conditions for abiogenic gas formation and migration. The extensional stress field in the region from Jurassic to Cenozoic (Hu et al., 1998) resulted in the development of large-scale NW–SE trending grabens, which were parallel to the present volcanic front (Dai et al., 2005b). The extensional regime opened fractures and created high permeability in the area. Volcaniclastic rocks were deposited throughout the depressions mostly in lacustrine environment. By the processes of diagenesis and strong hydrothermal alteration over geological time, gas originally contained in volcaniclastic rocks would be released and mixed with gases from the thermal decomposition of organic matter. Subsequently, hydrocarbons and other gases (e.g. CO2, N2, He) may have migrated upwards or laterally through faults (Li et al., 2005) to accumulate into gas reservoirs.

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Table 4 Estimation of abiogenic gases in the Qingshen gas field. The contribution of mantle helium was calculated according to the method of Ballentine and O’Nions (1992) assuming typical He isotopic values of R/Ra = 8 for mantle and 0.32 for crustal (radiogenic) sources. The quantitative estimation of abiogenic methane to the Qingshen gas field was performed assuming mixing of abiogenic gas and thermogenic gas from the Cretaceous mudstones and Jurassic coals. Well

Strata

Lithology

Helium (%)

Thermogenic gas (%)

Abiogenic gas (%)

d13C1-cal (‰, PDB)

Organic source

FS2 FS9 FS701 WS1 SSG2 SS2-1 SS2-25 FS6 FS5 XS603 XS1-1 XS1 XS6 FS8

K1d K1yc K1yc K1yc K1yc K1yc K1yc K1yc K1d K1yc K1sh K1yc K1sh C-P

Sandstone Igneous Igneous Igneous Igneous Igneous Igneous Igneous Sandstone Igneous Igneous Igneous Igneous Sandstone

71.91 27.92 21.32 19.08 18.90 18.71 18.53 16.29 15.64 11.55 10.16 10.16 9.41 5.88

0.0 64.8 61.9 62.8 74.9 67.3 67.1 72.0 72.2 63.4 67.2 70.8 61.8 66.7

100.0 35.2 38.1 37.2 25.1 32.7 32.9 28.0 27.8 36.6 32.8 29.2 38.2 33.3

17.4 33.0 32.1 30.9 30.5 31.4 31.1 32.5 30.8 32.5 34.5 34.8 35.0 33.0

Cretaceous mudstone

W9-12 SS1 WS5

C-P J K1sh

Sandstone Sandstone Sandstone

47.0 53.6 71.4

53.0 46.4 28.6

31.2 33.1 31.1

Jurassic coal

8.76 6.53

The presence of both highly porous media of volcaniclastics and impermeable cap rocks has been favorable for the formation of natural gas fields. Our results indicate that in certain geological settings where these processes are operative, abiogenic hydrocarbons could play an important role in the creation of economically valuable reservoirs. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgement

Fig. 11. Plot of the percentage of mantle He versus CH4/3He. The grey-shaded area is mainly representative of the simple mixing of CH4 from crustal and mantle endmembers. Data located above the grey-shaded area are likely indicate a contribution of abiogenic CH4 from carbon dioxide reduction to the gas reservoir. However, this contribution is difficult to quantify.

5. Conclusions Natural gases in the Qingshen gas field, Songliao Basin are thermogenic and abiogenic in origin. The thermogenic gas can be divided into oil-type gas derived from the Cretaceous mudstone (type I kerogen) and coal-type gas derived from Jurassic coal (type III kerogen). In contrast, abiogenic gas came from the mantle degassing with He and gaseous alkanes (dominantly methane) which were formed through the reduction of carbon dioxide in hydrothermal systems. The mantle degassed CO2 with depleted 13 C was altered into abiogenic CH4 with enriched 13C by H2 under hydrothermal conditions, and narrow variations of carbon and hydrogen isotopes of methane (d13C1, d2HC1) were observed when this abiogenic methane was mixed with thermogenic gases. According to the Ballentine and O’Nions method (1992), about 9–72% of the He in the Qingshen gas field was mantle-derived. About 30–40% of the hydrocarbon gas, in particular methane in the Qingshen gas field, was derived from mantle degassing via the reduction of carbon dioxide in hydrothermal systems.

We would like to express our gratitude to Daqing Oil Field Company for technical support and gas sampling. We would also like to acknowledge the Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese Academy of Sciences and the Laboratory of Organic Geochemistry, Natural Gas Institute, Langfang Branch of Research Institute of Petroleum Exploration and Development (RIPED), PetroChina for measurement of chemical and isotopic compositions of natural gas. We appreciate improvement of the English and constructive comments from Dr. Bernhard M Krooss, Prof. Lloyd Snowdon, and Dr. Tongwei Zhang. This research was financially supported by the Chinese National Natural Science Foundation (Grant Nos: 41322016, 41302118 & 41230312) and the National Key Foundational Research and Development Project (Grant No: 2012CB214800). References Ballentine, C.J., O’Nions, R.K., 1992. The nature of mantle neon contributions to Vienna Basin hydrocarbon reservoirs. Earth Planet. Sci. Lett. 113, 553–567. Ballentine, C.J., Schoell, M., Coleman, D., Cain, B.A., 2001. 300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin. Nature 409, 327– 331. Basu, S., Stuart, F.M., Klemm, V., Korschinek, G., Knie, K., Hein, J.R., 2006. Helium isotopes in ferromanganese crusts from the central Pacific Ocean. Geochim. Cosmochim. Acta 70, 3996–4006. Berndt, M.E., Allen, D.E., Seyfried, W.E., 1996. Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar. Geology 24, 351–354. Charlou, J.L., Donval, J.P., Douville, E., Jean-Baptiste, P., Radford-Knoery, J., Fouquet, Y., Dapoigny, A., Stievenard, M., 2000. Compared geochemical signatures and the evolution of Menez Gwen (37°500 N) and Lucky Strike (37°170 N) hydrothermal fluids, south of the Azores Triple Junction on the Mid-Atlantic Ridge. Chem. Geol. 171, 49–75. Charlou, J.L., Donval, J.P., Fouquet, Y., Jean-Baptiste, P., Holm, N., 2002. Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°140 N, MAR). Chem. Geol. 191, 345–359.

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