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Geochemical characteristics of abiogenic alkane gases

Geochemical characteristics of abiogenic alkane gases celestial bodies. The chemical composition of abiogenic alkane gases varies widely. The content of methane is low and nearly no C is found in the abiogenic alkane gases from fluid inclusions in 2+ volcanic rocks or hot springs in China. In the unsedimented submarine hydrothermal vent system C /C ratios are much greater than those for the thermogenic gases, mostly >800 and in some cases up to 1 2+ 8,000. In the Songliao Basin, China, C /C of some abiogenic gases are often less than 150. Abiogenic 1 2+ alkane gases which have been found in nature often have carbon isotopic reversal among C -C alkanes 1 4 13 13 13 13 (δ C >δ C >δ C >δ C ), whereas both regular and reversed hydrogen isotope distribution pattern 1 2 3 4 among C -C alkanes have been reported. The δ C of abiogenic methane is mainly greater than −30‰ 1 4 though laboratory synthesized methane can have δ C as low as −57‰, and its δD values vary widely and 3 4 overlap with biogenic gases. High He/ He ratios often indicate the addition of mantle-derived helium and 3 4 are related to abiogenic gases. However, some biogenic gases can also have high He/ He ratios up to 8. 3 6 13 The CH / He end-member is often lower than 10 for abiogenic alkane gases while greater than 10 for biogenic gases, and the values between these two end-members often refl ect the mixing of biogenic and abiogenic gases. Abiogenic origin, alkane gas, carbon isotopic reversal trend, geochemical characteristics Key words: information is limited and studies of gas origin and gas- 1 Introduction source correlation are largely dependent on their chemical In nature hydrocarbons are formed largely by the digestion composition and carbon, hydrogen and helium isotopes. of organic compounds by microorganisms (microbial origin) To date, a series of studies have been focused on the (Rice and Claypool, 1981; Schoell, 1988; Whiticar et al, discrimination of biogenic (referred to microbial origin 1986) and the thermal decomposition of organic matters and thermal origin in this text) and abiogenic gases. (thermal origin) (Des Marais et al, 1981; Schoell, 1980; 1988; Jenden et al (1993) suggested three criteria for abiogenic 13 13 13 13 13 Dai et al, 1992). A number of recent studies demonstrated the gases: δ C >− 25‰, δ C >δ C >δ C >δ C , and 1 1 2 3 4 -6 existence of abiogenic alkane gases (Abrajano et al, 1988; R/Ra>0.1(Ra=atmospheric ratio,1.4×10 ). According to Charlou et al, 1996a; Dai, 1988; Dai et al, 2000; 2008a; previous studies, Dai et al (2008a) compiled the carbon Galimov and Petersilie, 1967; Guo and Wang, 1994; Guo et and helium isotopes of biogenic and abiogenic gases in the al, 1997; Jeffrey and Kaplan, 1988; Jenden et al, 1993; Kelley, world and provided four criteria for abiogenic gases: 1) δ C 13 13 13 13 1996; Potter and Konnerup-Madsen, 2003; Sherwood Lollar generally greater than −30‰; 2) δ C >δ C >δ C >δ C 1 2 3 4 et al, 1993; 2002; Wang et al, 2009; Welhan, 1988; Ni et al, and δ C generally greater than −30‰; 3) R/Ra>0.5 and 13 13 3 6 2009), which might result from the mantle-degassing (Craig δ C −δ C >0; and 4) CH / He≤10 . However, due to 1 2 4 and Lupton, 1981; Gold, 1979; Gold and Soter, 1980) or insuffi cient knowledge of abiogenic gases, the discrimination chemical processes such as Fischer-Tropsch synthesis (Berndt of biogenic and abiogenic gases is a hot debate. Recent et al, 1996; Charlou et al, 1998; Craig, 1953; Foustoukos and advances on abiogenic gases from the laboratory have Seyfried, 2004; Fu et al, 2007; Giggenbach, 1980; Horita and deepened our knowledge of the geochemical characteristics Berndt, 1999; Hulston and McCabe, 1962a; 1962b; Kelley of abiogenic gases. For example, the carbon isotope ratio and Früh-Green, 1999; Lancet and Anders, 1970; McCollom of methane formed via Fischer-Tropsch synthesis under and Seewald, 2006; Sherwood Lollar et al, 2002). hydrothermal condition (200°C, 50 MPa) with Fe-Ni as Since natural gases are mainly dominated by methane, catalyst can be as low as −57‰, which is close to the value plus small amount of heavy hydrocarbons (C ) and non- 2+ of biogenic gas (Horita and Berndt, 1999). Other experiments hydrocarbons (CO , N , H S, etc.), the available geochemical 2 2 2 also found that products of the Fischer-Tropsch reactions also have regular distribution pattern of carbon isotopes 13 13 13 13 between C -C n-alkanes, i.e., δ C <δ C <δ C <δ C , 1 4 1 2 3 4 *Corresponding author. email: niyy@petrochina.com.cn which is similar to that of the biogenic gases (Fu et al, 2007; Received March 13, 2009 328 Pet.Sci.(2009)6:327-338 McCollom and Seewald, 2006; Taran et al, 2007). Du et al (b) (2003) found that pyrolysis of lignite under extremely high Oi l and gas reservoi rs pressure and temperature (1-3 GPa, 500-700°C) can also lead to carbon isotopic reversal trend among C -C n-alkanes. 1 4 Recently hydrocarbons have been found on Mars, Enceladus, and Titan (Brown et al, 2008; Filacchione et al, 2007; Krasnopolsky et al, 2004; Niemann et al, 2005; Oze and Sharma, 2005) and discrimination of their origins will have great significance on life origin. In addition, the discrimination of abiogenic alkane gases and discovery of 10 abiogenic alkane gas reservoirs open up a new research field and improve the oil-gas theory. This contribution is concentrated on the geochemical characteristics of abiogenic gases which have been reported in the world in order to 10 30 60 90 120 150 provide some hints for the discrimination of abiogenic gases. C /C 1 2+ 2 Chemical composition of abiogenic gases (c) The chemical compositions of abiogenic gases in nature Oceanic hydrothermal systems vary widely. Dai et al (2000) made a detailed review of the abiogenic alkane gases from the inclusions of volcanic rocks and hot springs in China. They found these abiogenic gases are dominated by CO and/or N . A negative correlation was 2 2 found between CO and N , and the gas could be dominated 2 2 by either of them which could be up to 99%. The amount of alkane gases was low and characterized by methane and 8 virtually no C . However, abiogenic gases found in the 2+ Khibiny and Lovozero massif, Ilímaussaq complex, Canadian shield rocks are dominated by methane. The content of methane is more than 60% in general, and some can be up to 97%. Only a few of them are dominated by H , with some 500 1000 2000 3000 4000 5000 6000 7000 amount of CO and alkanes (Table 1, Fig. 1). The C /C 2 1 2+ C /C ratios are relatively low, normally lower than 100. As shown 1 2+ in Table 1, abiogenic gases from the Oman ophiolite are dominated by H and N , with small amount of methane and 2 2 Fig. 1 Histogram of C /C ratios of abiogenic gases from volcanoes 1 2+ nearly no C . The C /C ratios of abiogenic gases from the (a), oil and gas reservoirs (b) and hydrothermal fl uids (c). Data from 2+ 1 2+ Table 1, Table 2, and Table 3 except the Oman and Philippines data mid-ocean ridges are much greater than those of thermogenic gases (Dai et al, 2005), and most of them are greater than 800, and some can be as high as 8,000 (Table 2, Fig. 1). 3 Isotopes of abiogenic alkane gases The abiogenic gas fields in the Songliao Basin, China are characterized by methane and most of the methane accounts 3.1 Carbon and hydrogen isotopes of methane for more than 90%. Its C /C ratios are normally lower than 1 2+ 150 (Table 3, Fig. 1). Methane is a highly reducing state of carbon. It is also 35 the dominant component of biogenic and abiogenic alkane (a) gases. Its carbon and hydrogen isotopes can refl ect its origin to some extent. For example, gas of microbial origin is Volcanic rocks normally formed at depth less than 1 km, and its carbon and hydrogen isotopes have a range of −110‰<δ C <−55‰, −400‰<δ D <−150‰, respectively; while the carbon and hydrogen isotopes of thermogenic gases vary between −55‰<δ C ≤−20‰ (some as high as −10‰), −275‰<δD <−100‰, respectively (Dai et al, 2008b; Schoell, 1980; 1983; Whiticar, 1999; Dai et al, 1992). The carbon and hydrogen isotopes are −52‰ to −24‰ and −280‰ to −140‰ for coal-related gases, respectively and −58‰ to −30‰ and −300‰ to −130‰ for oil-associated gases, respectively in China (Wang, 1997). The carbon and hydrogen 10 30 60 90 120 150 isotopic compositions are also characteristic for abiogenic methane. Table 4 and Fig. 2 demonstrate some carbon and C /C 1 2+ Number of samples Number of samples Number of samples Pet.Sci.(2009)6:327-338 329 Table 1 Chemical composition of the abiogenic gas from various rocks Composition, vol% Location C /C References 1 2+ CH C H CO CO N He O +Ar 4 2+ 2 2 2 2 2.06-71.96 0-6.54 1.99-30.95 0-71 6.77-51.60 0-16.21 9-42 Petersilie et al, 1961 93.42 1.37 0.91 0.00 2.30 2.20 68 Petersilie, 1962 Petersilie and Sørensen, 38.09-96.75 0-3.77 1.08-61.91 0-0.34 0-1.97 0-0.08 24-53 Khibiny 89.70-90.00 2.15-2.53 1.77-3.45 0.19 1.32-4.13 0-4.59 35-42 Kogarko et al, 1987 24.06-97.64 0.65-8.19 0-13.90 0-2.88 0-70.30 0-0.69 11-42 Voytov, 1992 1.69-95.27 0-4.04 1.48-63.27 0-28.38 2.26-42.57 0-0.19 1-56 Potter, 2000 81.20 6.11 8.34 0.00 4.35 0.00 13 Petersilie, 1962 Petersilie and Sørensen, 79.47-91.36 4.30-8.48 0-1.50 0.12-0.41 0-0.04 10-21 8.52-78.55 0.12-6.25 10.84-65.32 0.15-0.30 4.20-25.71 13-71 Kogarko et al, 1987 Lovozero 15.70-76.10 0.50-5.80 4.80-39.20 3.00-37.50 0.02-1.90 0.80-11.30 11-31 Nivin et al, 1995 62.68-89.43 1.92-6.91 0.04-27.76 2.69-6.48 0-0.05 9-47 Potter, 2000 52.06-89.43 1.92-6.91 4.14-35.24 2.69-6.48 0-0.52 8-47 Potter et al, 2004 Petersilie and Sørensen, 5.30-83.72 0.06-12.07 5.04-93.11 0-0.06 0-6.93 0.16-3.66 7-88 Konnerup-Madsen et al, Ilímaussaq 43.00-80.00 6.50-15.55 3.00-34.00 0.40-4.00 3.60-13.00 3-9 Konnerup-Madsen and 66.30-92.10 2.10-10.65 5.80-18.80 0-1.20 0-6.00 0-0.20 7-44 Rose-Hansen, 1982 Sherwood Lollar et al, Canada 69.30-77.90 6.51-15.10 0.34-12.70 3.88-12.70 1.83-2.45 5-12 2002; 2008 Oman 0.20-4.30 0-0.01 1.00-97.00 1.00-77.00 0-21.00 >80 Neal and Stanger, 1983 1.50-60.60 Philippines 13.00-55.30 0.04-0.22 8.40-42.60 <0.03 0-0.00069 0.1-17.23 251-343 Abrajano et al, 1988 (CO+N ) Table 2 Chemical composition of abiogenic gases from the submarine hydrothermal systems CH C H CO CO N Ar H S 4 2+ 2 2 2 2 Location C /C References 1 2+ mmol/kg nmol/kg mmol/kg nmol/kg mmol/kg mmol/kg umol/kg mmol/kg 1023- Proskurowski 30°N MAR Lost City 0.89-1.98 0-14.38 869-1122 1991 et al, 2008 Charlou et al, 6°14’N MAR Rainbow 2.5 1145 16 5000 16 1.8 1.2 2183 37°17’N MAR Lucky 15.10- 0.30-0.85 84-394 0-0.73 0.50-1.77 11.7-30.2 2.0-3.0 1500-7935 Strike 39.90 Charlou et al, 37°50’N MAR Menez 20.10- 1.35-2.15 285-826 0.02-0.05 0.37-1.82 11-38 1.3-1.8 2603-5263 Gwen 22.60 Charlou et al, 14°45’N MAR Logatchev 0 0 0.3 2.3 0.59 16 0 Jean-Baptiste et 23°N MAR Snakepit 0.06 2.7 al, 1991 Charlou et al, 26°N MAR TAG 0.15-0.16 0.18-0.23 3.31-3.56 0.8-0.95 21.6-24.3 3.2-3.8 1996a 29°N MAR Broken Spur 0.065-0.13 0.43-1.03 6.0-7.1 8.5-11.0 Lein et al, 2000 Welhan and 21°N EPR 0.07 108 1.7   648 Lupton, 1987 Thermogenic 330 Pet.Sci.(2009)6:327-338 Table 3 Chemical composition and helium isotopes of abiogenic gases from the Songliao Basin, China Composition, vol% CH / He C /C R/Ra References 1 2+ CH C H C H C H CO N He H ×10 4 2 6 3 8 4 10 2 2 2 Dai et al, 82.12-95.95 0.74-2.45 0.11-0.64 0.02-0.27 0.1-14.41 0.86-6.35 0.008-0.034 0-1.94 28-105 0.6-1.8 1.2-8.6 2008a Wang et al, 81.59-96.85 0.55-2.37 0.02-1.06 0-0.16 0.09-13.35 1.21-9.05 28-126 hydrogen isotopes of abiogenic methane in the world. The 3.2 Carbon and hydrogen isotopic distribution carbon isotopes of abiogenic methane vary widely, −1‰ to pattern of methane and its homologues (C -C ) 1 4 −44.9‰, however, except the abiogenic methane from the Isotopic analyses found that thermogenic gases (C -C ) 1 4 Canadian and Fennoscandian shield rocks, most methane has become more enriched in C with increasing molecular mass δ C>−30‰. The hydrogen isotopic composition of methane 13 13 13 13 (δ C <δ C <δ C <δ C ) due to kinetic isotopic effects. 1 2 3 4 also varies widely, −102‰ to −419‰, overlapping with that When one alkyl is separated from the original organic matter, of biogenic methane. 12 12 12 13 the C- C bond is weaker than the C- C bond and breaks faster, causing the pyrolytic products to be more depleted in -120 C than the higher molecular weight organic precursor (Des Marais et al, 1981). A typical characteristic of abiogenic -100 alkane gases is the carbon isotopic reversal trend, i.e., the carbon isotope of alkane gases become more depleted in C 13 13 13 13 Bacterial with increasing molecular mass (δ C >δ C >δ C >δ C ) 1 2 3 4 -80 carbonate (Des Marais et al, 1981; Galimov and Petersilie, 1967; reduction Lancet and Anders, 1970; Yuen et al, 1984). The reason is Mix Bacterial -60 & that during the control process of isotope kinetics distillation methyl-type Transition fermentation effect, synthesizing a high molecular homologue from a 12 12 low molecular compound, the C- C bond is weaker and -40 12 13 12 thus breaks faster than C- C, the CH reacts faster than CH to form hydrocarbon chains, that is, in the process of polymerization, C is preferentially incorporated into the -20 long chains formed during polymerization, thus obtaining a reverse carbon isotope trend (Des Marais et al, 1981; Yuen et al, 1984). In a similar fashion, due to the kinetic isotopic -450 -350 -250 -150 -50 effects both thermogenic and abiogenic gases have a regular hydrogen isotopic distribution pattern whereby the C -C CH 4 1 4 alkane gases become more enriched in H with increasing Atmospheric CH 21°N East Pacific Rise molecular mass, i.e., δD <δD <δD <δD . 1 2 3 4 Alkaline rocks 30°N MAR Bacterial oxidation of methane will lead to isotopic Milos New Zealand depletion of C versus C (Coleman and Risatti, 1981), 2 1 Zambales Songliao, China conclusive evidence of isotopic depletion of higher Canadian & Sichuan, China Fennoscandian hydrocarbons with respect to C requires comparison to a larger range of higher hydrocarbons, particularly C and Fig. 2 Compilation of carbon and hydrogen isotope variation in C (Sherwood Lollar et al, 2002). Dai (1990) and Dai et abiogenic methane (data from Table 4) al (2004) proposed that the partial reversal of carbon and Debates arise concerning the lower limit of carbon isotope hydrogen isotopes might be due to secondary alteration of abiogenic methane, namely, >−20‰ (Shen et al, 1991; (e.g., bacterial oxidization) or mixing of gases with different Xu et al, 1993; Zhang, 1991; Chen, 1989), >−25‰ (Jenden origins. At present, there are not too many reports about the et al, 1993), and generally >−30‰ (Dai et al, 2008a; Fuex, carbon and hydrogen isotopic distribution patterns among the 1977; Dai, 1992a; 1992b). At present all these lower limits of abiogenic C -C alkanes (Table 5, Fig. 3, and Fig. 4). Most 1 4 carbon isotopes are mainly a comprehensive generalization of these reports are focused on the abiogenic gases from of the carbon isotopes of various reported occurrences of the mid-ocean ridges, shield rocks, and volcanic rocks, and abiogenic methane in the world, but the field lacks exact the abiogenic alkane gas reservoirs are only reported in the methods. As discussed above, Horita and Berndt (1999) found Songliao Basin, China. Sherwood Lollar et al (1993; 2002) that the carbon isotopes of abiogenic methane formed through and Wang et al (2009) found that the carbon and hydrogen Fischer-Tropsch synthesis in the laboratory can be as low as isotopes of abiogenic gases are negatively correlated while −57‰, which is similar to the value of microbial gases. in contrast Proskurowski et al (2008) found that the carbon CH 4 Pet.Sci.(2009)6:327-338 331 Table 4 Carbon and hydrogen isotopes of abiogenic methane 13 13 3 6 Location δ C , ‰ δD , ‰ δ C , ‰ R/Ra CH / He, ×10 References CH CH CO 4 4 4 2 Zaohetang, Tengchong, China –19.9 –130 –6.3 - –1.9 2.9 Dai, 1993 Tuoba, Ganzi, China –26.6 - –23.8 –213 - –111 –4.8 - –2.4 0.4-0.5 New Zealand –29.5 - –24.4 –197 - –142 –9.1 - –3.2 Lyon and Hulston, 1984 New Zealand –32 - –20.4 –170 - –144 –8.9 - –4.8 4.2-7.5 Giggenbach, 1995 Yellowstone, USA –28.4 - –10.4 –239 Welhan et al, 1983 Milos –17.8 - –9.4 –189 - –104 –1.1 - –0.6 1.0-1.3 Botz et al, 1996 Iceland –39.3 - –22.0 –18.7 - –4.8 2.1-16.8 Sano et al, 1985 Mediterranean Sea –32.3 - –16.4 –4.3 - 0.1 2.5-5.1 Fiebig et al, 2007 Kim et al, 1984; Merlivat et al, 13°N EPR –19.1 - –16.7 –4.4 - –4.1 7.0-7.5 3.1-3.9 Kim et al, 1984; Welhan and 21°N EPR –17.6 - –15.0 –126 - –102 –7.0 7.8 3.5-6.5 Craig, 1983 Charlou et al, 1996b; Jean- 17°-19°S EPR –23.9 - –22.0 6.2 Baptiste et al, 1997 Charlou et al, 1998; Jean- 14°45’N MAR Logatchev –13.6 –4.1 7.2 Baptiste et al, 2004 26°N MAR TAG –9.5 - –8.0 –10.0 - –8.4 7.5-8.2 8.8-52 Charlou et al, 1996a; 2002 Jean-Baptiste et al, 2004; Lein 29°N MAR Broken Spur –19.0 - –18.0 –9.0 8.9 et al, 2000 30°N MAR Lost City –13.6 - –9.4 –147 - –125 Proskurowski et al, 2008 Charlou et al, 2002; Jean- 36°14’N MAR Rainbow –15.8 –3.15 5.7-7.8 Baptiste et al, 2004 37°50’N MAR Menez Gwen –19.6 - –18.8 –9.1 - –6.8 8.7 Charlou et al, 2000; Jean- Baptiste et al, 1998 37°17’N MAR Lucky Strike –13.7 - –12.7 –10.6 - –7.2 8.1 Southwest Indian Ridge T=450-500 °C –33.8 - –9.1 –149 - –99 –28.5 - –8.7 Kelley and Früh-Green, 1999 Southwest Indian Ridge T>900°C –30.1 - –18.8 –244 - –128 –23.9 - –1.9 19°33’S Central Indian Ridge Roger –17.5 1.1-1.6 Kawagucci et al, 2008 Plateau Zambales, Philippines –7.5 - –6.1 –137 - –118 –32 3.98-4.07 Abrajano et al, 1988 –12.8 - –3.2 Galimov, 1975 –28.6 - –12.8 Potter, 2000 –19.3 - –3.2 –8.5 - 10.6 Galimov and Petersilie, 1967 –11.2 - –3.2 –56 - 173 Voytov, 1992 Khibiny –6.5 –66 - 82 Yerokhin, 1978 –14.0 - –7.7 Nivin et al, 2001 –22.4 - –5.4 –16.8 - –13.6 Beeskow et al, 2006 –13.3 - –7.6 –18.6 - –14.6 Potter and Longstaffe, 2007 –15.7 - –7.1 Nivin et al, 2001 Lovozero –11.8 –167 - –132 Nivin et al, 1995 IIímaussaq –7 - –1 –145 - –132 Konnerup-Madsen, 2001 –44.9 - –22.4 –372 - –133 Sherwood Lollar et al, 1993 Canada and Fennoscandia –40.7 - –33.0 –419 - –390 Sherwood Lollar et al, 2002 Dai et al, 2008a; Wang et al, Songliao, China –28.9 - –17.4 –203 - –196 –5.1 - 18.9 0.6-1.8 1.2-8.6 2009 13 C, ‰ 332 Pet.Sci.(2009)6:327-338 3 4 -6 and hydrogen isotopes of abiogenic gases from the 30°N atmospheric helium, He/ He=1.4×10 , i.e., Ra; crustal MAR Lost City are positively correlated (Fig. 4). The exact helium, product of the radioactive decay of U and Th (yielding 4 6 3 3 4 situation is still unclear at the moment. He) and the (n, α) reaction of Li (yielding He), He/ He normally less than 0.04 Ra (Andrews, 1985; Mamyrin and Tolstikhin, 1984); mantle helium, a mixture of radiogenic and 3 4 primordial helium which has He/ He of 100-300 Ra (Craig 0 10 and Lupton, 1981; Mamyrin and Tolstikhin, 1984). Since -5 9 the atmospheric helium in the commercial gas wells can be -10 8 neglected, helium in the natural gas can be approximately regarded as a mixture of crustal and mantle helium. In -15 7 3 4 general, mantle He/ He is far greater than the crustal 3 4 3 4 -20 6 He/ He. For example, the He/ He value is 7-9 Ra for the depleted upper mantle, >30 Ra for the lower mantle and <0.1 -25 5 Ra for the crust (Andrews, 1985; Craig and Lupton, 1981; -30 4 Hiyagon and Kennedy, 1992; Welhan et al, 1983). Hence, the 3 4 high He/ He values of natural gas mean addition of mantle -35 3 3 4 volatiles. Jenden et al (1993) proposed that He/ He>0.1 Ra -40 2 in the natural gas was an indication of the existence of mantle 3 4 -45 1 helium. Dai et al (2008a) suggested that He/ He>0.5 Ra indicates the addition of mantle helium. -50 0 C C C C Jenden et al (1993) compiled the numerous studies of 1 2 3 4 the helium isotopic distribution of different commercial oil Songliao Basin MAR Lost City and gas fi elds in the world and proposed that fi elds overlying Khibiny Kidd Creek Murchison ancient cratonic crust and in foreland basins formed by 3 4 Fig. 3 Carbon isotopic distribution of abiogenic methane and its thrust-plate loading had He/ He ratios close to radiogenic homologues (data from Table 5). Data from the Murchison meteorite are values, while fields located near active plate margins in 3 4 on right y axis and all other data are on left y axis. forearc, backarc, and rift basins might have He/ He ratios approaching mantle values. Xu et al (1998) made a statistical analysis of the distribution of helium isotopes in natural gases in China (Fig. 5) and found that about 30% natural gases were 3 4 characterized by He/ He ratios greater than the atmospheric -50 values (R/Ra>1), indicating the addition of mantle helium. -100 The distribution of the helium isotope ratio (R/Ra) in the hot springs and hydrothermal systems in China are mainly in -150 the range of 0.5-8 (Fig. 6, Table 6), indicating the addition of mantle helium as well. However, due to the helium loss -200 through reservoirs or helium addition from radiogenic 3 4 -250 sources, He/ He ratios may decrease with time, leading to 3 4 the low He/ He ratios in some natural gases of mantle origin -300 (Kamenskiy et al, 1971). On the other hand, biogenic methane 3 4 can also be associated with high He/ He ratios. For example, -350 3 4 Songliao He/ He ratios in the oil and gas fields of the Green Tuff – – MAR Lost City region in Japan are as high as 7.5 Ra (δ C : 62‰ to 33‰) -400 3 4 Kidd Creek (Sakata, 1991); He/ He ratios in the geothermal vents along -450 the East Pacifi c Rise in the Guaymas Basin, Gulf of California C C C C 1 2 3 4 13 – – also approach 8.0 Ra (δ C : 51‰ to 43‰) (Welhan and Fig. 4 Hydrogen isotopic distribution pattern of abiogenic methane and Lupton, 1987). In the Wudalianchi and Changbaishan areas in 3 4 its homologues (data from Table 5) China, the high He/ He ratios are associated with relatively low δ C values of methane (Shangguan and Wu, 2008). According to the recent study on the geochemistry of natural gases from the inactive volcanoes in China by Shangguan and 3 4 3.3 He/ He isotope (R/Ra) 13 Wu (2008), δ C value of methane in the volcanic residual Since the discovery of He anomaly in the East Pacific gas in Changbaishan was as low as 48.0 while the associated 3 4 Rise in the 1960’s (Craig and Lupton, 1981), helium isotopes helium has He/ He ratios of 5.5 Ra (Table 6). The authors have been widely used as traces of mantle volatiles. Due to suggested an origin in the mantle for the hydrocarbons, the different origin of the two stable isotopes of helium, He however, debates exist concerning the mantle contribution to and He, helium can be divided into three different types: the hydrocarbons and further studies are required. D, ‰ C, ‰ Pet.Sci.(2009)6:327-338 333 Table 5 Carbon and hydrogen isotopes of abiogenic methane and its homologues δ C, ‰ δD, ‰ Location Sample References C C C C C C C C 1 2 3 4 1 2 3 4 Inclusion –3.2 –9.1 –16.2 Zorikin et al, 1984          Inclusion –12.8 –24.5 –26.0 Galimov, 1975; Voytov, Inclusion –3.2 –9.1 –25.7          Rischorrite –14.4 –17.1 –17.3 –17.5 Trachytoid foyaite –8.0 –17.5 –19.6 Beeskow et al, 2006 Khibiny Massive foyaite –5.4 –14.3 –13.0 –13.2       Urtite –8.1 –20.6 –19.7 Urtite –88 –154 Potter and Longstaffe, 2007 Urtite –12.4 –23.2 –21.3 –17.4 Urtite –52 –120 Yellowstone, USA –21.5 –26.5   Des Marais et al, 1981 Meteorite Murchison 9.2 3.7 1.2          Yuen et al, 1984 CCS4000 –35 –25.1 –325 Fennoscandian Sherwood Lollar et al, 1993 Juuka 116-254 m –28.6 –25.9    –281      CCS846-2 –35.1 –33.3 –31.2 –237 –215 –185 Sudbury Ontario Sherwood Lollar et al, 1988 CCS844-2 –29.7 –28.0 –27.7   –335 –238    7792 –38.3 –38.2 –37.6 –37.9 –390 –299 –256 –224 Kidd Creek 8558 –32.7 –36.8 –35.3 –35.7 –419 –321 –264 –245 Sherwood Lollar et al, 2002 8282 –35.0 –38.3 –35.9 –36.3 –409 –316 –269 –252 H04-IGT8 –9.5 –15.2 –15.0 –119 –127 –140 Lost City Proskurowski et al, 2008 H05-IGT7 –11.8 –13.7 –13.4 –147 –166 –160 Fs1 –18.6 –23.2          Dai, 1992a Fs1 –18.7 –22.4 –24.1 –28.2 Guo et al, 1997 Fs7 –28.8 –30.4 –32.5 –32.6       Fs9 –27.1 –30.1 –30.5 –33.0       Wang et al, 2004 Xs1 –29.7 –32.9 –34.3 –35.0 Songliao, China Xs6-2 –25.9 –32.4 –33.1 –33.7 Dai et al, 2008a Fs2 –17.4 –22.2 –30.5 –31.4       FS-2 –17.7 –22.0 –30.3 –200 –184 –138 S-2-1 –22.7 –28.0 –32.1 –35.2 –202 –166 –120 Wang et al, 2009 S-502 –29.3 –30.3 –30.9   –197 –189 –188   334 Pet.Sci.(2009)6:327-338 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 0.02 0.05 0.1 0.5 1258 R/Ra R/Ra Fig. 6 Histogram of R/Ra values of natural gases from the Fig. 5 Histogram of the R/Ra values of the natural gases in China (Xu et al, 1998) hydrothermal systems and hot springs in China Table 6 Chemical composition and isotopes of natural gases from the hydrothermal systems and hot springs in China 13 13 3 6 Location N , % CO , % CH , % He, % δ C , ‰ δ C , ‰ R/Ra CH / He, ×10 Reference 2 2 4 1 CO 4 Tuoba, Ganzi, Sichuan 0-2.72 93.00-98.60 1.07-3.64 0-0.008 −26.6 - −24.8 −4.2 - −2.5 0.36-0.50 791.58 Fushoushan, Pingjiang, Hunan 0.17 99.70 0.0038 −5.5 0.29 Rucheng and Ningxiang, 96.88-97.55 0.75-2.92 0-0.38 0-0.24 −25.4 0.07-0.47 16.72 Hunan Fengshun, Guangdong 95.71 3.19 0.07 −11.2 0.83 Xunwu, Jiangxi 99.96 0.04 −27.7 1.36 Chongren, Jiangxi 54.53 29.69 0.0056 −7.3 0.57 Dai et al, 2000 Taishun, Zhejiang 98.86 0.40 0.15 0.44 −10.1 0.24 1.01 Keshiketeng, Inner-Mongolia 97.84 0.19 0.26 0.66 −22.7 −16.1 0.06 4.73 Fujian 84.32-96.31 2.23-13.66 0-0.19 0.049-0.13 0.28-0.48 2.16 Wudalianchi 0.24-2.34 97.41-99.45 0-0.02 0.057-0.067 −5.4 - −4.0 2.98-3.16 0-0.08 Chuxiong, Yunnan 23.00 76.53 1.00 0.006 −6.0 0.82 146.20 Tengchong, Yunnan 0-3.20 96.66-99.92 0.13-1.35 0.005-0.042 −32.7 - −16.2 –5.8 - –1.2 2.86-4.49 0.1-7.6 Other places in Yunnan 86.03-94.83 3.04-11.99 0-6.54 0-0.19 −50.3 - −28.4 –16.3 - –8.9 0.01-0.40 144.76 Sun et al, Hengjing, Jiangxi 0-2.01 96.47-99.84 0.04-1.86 0.002-0.014 −59.3 - −27.7 –5.5 - –4.4 1.36-2.11 1.9-424 Wang et al, Tengchong 14.52-71.23 10.33-80.13 0-1.03 0.0003-0.02 0.48-5.13 0.2-32.6 Changbai Mountain 2.14-10.04 73.55-97.59 0-0.47 0.00002-0.006 –6.6 - –4.2 2.29-5.71 12.5-604.9 Shangguan et al, 1997 Jinjiang 3.86-11.25 79.02-86.53 1.25-2.02 0.01-0.03 −30.4 - −26.2 –7.5 - –7.0 5.36-5.89 5.2-13.3 Shangguan et Tengchong 0.80-32.75 57.49-96.64 0.01-2.73 0.00006-0.04 –5.8 - –0.5 1.01-4.81 0.2-133.4 al, 2000 Wudalianchi 3.09-3.51 Shangguan et Tengchong 4.85 al, 2006 Changbai Mountain 5.62-5.95 Tengchong 2.22-4.42 92.46-94.95 0.01-0.98 <0.000122 –27.3 - –18.8 –6.5 - –3.6 1.62-5.39 Shangguan Changbai Mountain 2.55-9.76 83.45-96.06 0.28-4.15 <0.000207 –48.0 - –26.2 –6.9 - –4.0 4.79-5.51 and Wu, 2008 Wudalianchi 4.90-7.64 90.20-93.41 0.02-0.03 <0.000248 –47.2 - –44.4 –6.6 - –4.5 3.01-3.03 Number of samples Number of samples Pet.Sci.(2009)6:327-338 335 3.4 Methane concentration and He abundance indicative of mixing (Poreda et al, 1986). Similarly, natural gas from the mud volcano in the Copper River Basin in He has proven to be of mantle origin, hence, except Alaska was also explained as mixing origin (Motyka et al, 3 4 3 the He/ He ratios, the correlation between He and the 1989). concentration of other gas composition such as CH can provide some hints for the origin of natural hydrocarbon Crustal gases. Fluids from the East Pacific Rise and Mid-Atlantic 3 6 6 Ridges have CH / He ratios of 10 (1.4-7.0×10 for EPR and 2.4×10 for MAR), which have been considered as mantle origin (Jean-Baptiste et al, 1991; Welhan et al, 1983). 3 10 According to the CH / He ratios of 100 gas wells from the Bohai Bay, North Jiangsu, Songliao, Ordos, Sichuan, Tarim and Turpan-Harim basins in China and the comparison with those from New Zealand, Iceland, and Thailand, etc., Dai et 3 6 al (2008a) proposed that abiogenic gas had CH / He≤10 and Hot springs and 3 11 biogenic gas had CH / He≥10 . Fig. 7 shows the correlation hydrothermal systems in China between He abundance and the methane concentration of Biogenic gas in China 10 Abiogenic gas from Songliao gases from the hot springs, hydrothermal vents and gas fi elds Mantle 21 N East Pacific Rise in China. The CH / He ratios of biogenic alkane gases vary 8 13 3 widely, 10 -10 , overlapped with the CH / He ratios for the abiogenic gases from the Songliao Basin. In general, CH / He 0.001 0.01 0.1 1 10 ratios from the gas fields are greater than those from the R/Ra hot springs and hydrothermal vents, indicating that gases in the gas fi elds are characterized by biogenic gas while gases Plot of CH / He versus R/Ra of various natural gases. Data of Fig. 7 hydrothermal systems and hot springs are from Table 6, the biogenic alkane in the springs and hydrothermal systems are dominated by gas data are from Dai et al (2008b), data of the 21 N East Pacifi c Rise are mantle volatiles. However, if reducing CH -enriched fluids from Welhan and Lupton (1987), and the abiogenic data in the Songliao are degassed from mantle (Gold and Soter, 1980; 1982), Basin are from Table 3 the mantle CH / He ratios can be expected to be as high as 3×10 . The CH -enriched gases from the pyroclastic rock 4 Conclusions reservoirs of the gas fields in the Green Tuff region, Japan 3 9 have CH / He ratios of around 2×10 , regarded as mantle This contribution made a comprehensive study of the origin (Wakita et al, 1990). Whereas, the CH / He ratios of geochemical characteristics of abiogenic alkane gases. The biogenic gases can also be very high, even as high as 10 . For chemical composition of abiogenic alkane gases vary widely. example, gases from the Well 843-2 of the Dagang Oil Field The various volcanic fluid inclusions and hydrothermal 3 8 in the Bohai Bay Basin, China have CH / He ratios of 5.3×10 systems in China have very low amount of methane and and R/Ra ratios of 8.6, while the carbon isotopes of methane, nearly no C . The C /C ratios in the unsedimented 2+ 1 2+ ethane, and propane are –58.1‰, –42.3‰, and –30.7‰, submarine hydrothermal systems are far greater than those respectively; Gases from the Well Qi18 in the Songliao of biogenic alkane gases, mostly greater than 800 and some 3 8 Basin also have CH / He ratios of 5.3×10 and R/Ra ratios of up to 8,000. The C /C ratios of the abiogenic alkane gases 4 1 2+ 2.6, while carbon isotopes of methane, ethane, and propane in the Songliao Basin, China are generally less than 150. The are –54.7‰, –31.9‰, and –24.2‰, respectively (Dai et al, reported abiogenic alkane gases in nature normally have a 2008a). Similarly, the hydrothermal fl uid from the Guaymas reversed distribution pattern of carbon isotope ratios among 3 9 Basin, Gulf of California has CH / He ratios of 3.1×10 , C -C alkanes, while both reversed and regular distribution 4 1 4 but its R/Ra ratios (8.0) are close to those of the 21°N East patterns of hydrogen isotope ratios have been reported. Pacific Rise. The carbon isotope of methane is –15.0‰ to Abiogenic methane generally has δ C> 30‰ and a wide – – –17.6‰ for 21°N East Pacifi c Rise while –43‰ to –51‰ for distribution range of δD, 102‰ to 419‰ which overlaps the Guaymas Basin. The C /(C +C ) ratios of gases from the with the hydrogen isotope ratios of biogenic gases. However, 1 2 3 Guaymas Basin fall in the range of biogenic alkane gases, so δ C of methane synthesized in the laboratory can be as low 3 4 the CH / He ratios in the Guaymas Basin indicate biogenic as 57‰. The He/ He ratios of mantle helium are far greater 3 4 origin of the hydrocarbons, which is a result of the circulation than those of crustal helium, so the high He/ He ratios in the flow of the hydrothermal fluid in the overlying organic natural gases often indicate addition of mantle helium. Hence, 3 4 sediments (Simoneit et al, 1988; Welhan and Craig, 1983; He/ He ratios are often regarded as an auxiliary indicator of Welhan and Lupton, 1987). In fact, the high CH / He ratios abiogenic gases. However, biogenic alkane gases can also 3 4 3 in many locations are explained as a mixture of mantle and have very high He/ He ratios, even up to 8 Ra. The CH / He 3 13 crustal helium. For example, the high CH / He ratios (>10 ) ratios can reflect the gas origin to some extent due to the 3 3 of the gases from volcanic rocks in Japan were interpreted proven mantle origin of He. The CH / He ratios are lower 6 13 that a large contribution of the methane was from crustal than 10 for abiogenic end-members while greater than 10 or subducted sediments (Marty et al, 1989). The variation for the biogenic end-member. It often reflects a mixing of of CH / He ratios in the Sacramento Basin was taken as an biogenic and abiogenic alkane gases. CH / He 4 336 Pet.Sci.(2009)6:327-338 Dai J X. Discrimination of alkane gases. Science in China (Series B). Acknowledgments 1992a. (2): 185-193 (in Chinese) Dai J X. Identifi cation of various genetic natural gases. China Offshore This work is supported by the China Postdoctoral Science Oil and Gas (Geology). 1992b. 6(1): 11-19 (in Chinese) Foundation (20070420393), China Postdoctoral Special Dai J X. 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Geochemical characteristics of abiogenic alkane gases

Petroleum Science , Volume 6 (4) – Nov 26, 2009

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Springer Journals
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Copyright © 2009 by China University of Petroleum (Beijing) and Springer Berlin Heidelberg
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Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
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10.1007/s12182-009-0052-6
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Abstract

celestial bodies. The chemical composition of abiogenic alkane gases varies widely. The content of methane is low and nearly no C is found in the abiogenic alkane gases from fluid inclusions in 2+ volcanic rocks or hot springs in China. In the unsedimented submarine hydrothermal vent system C /C ratios are much greater than those for the thermogenic gases, mostly >800 and in some cases up to 1 2+ 8,000. In the Songliao Basin, China, C /C of some abiogenic gases are often less than 150. Abiogenic 1 2+ alkane gases which have been found in nature often have carbon isotopic reversal among C -C alkanes 1 4 13 13 13 13 (δ C >δ C >δ C >δ C ), whereas both regular and reversed hydrogen isotope distribution pattern 1 2 3 4 among C -C alkanes have been reported. The δ C of abiogenic methane is mainly greater than −30‰ 1 4 though laboratory synthesized methane can have δ C as low as −57‰, and its δD values vary widely and 3 4 overlap with biogenic gases. High He/ He ratios often indicate the addition of mantle-derived helium and 3 4 are related to abiogenic gases. However, some biogenic gases can also have high He/ He ratios up to 8. 3 6 13 The CH / He end-member is often lower than 10 for abiogenic alkane gases while greater than 10 for biogenic gases, and the values between these two end-members often refl ect the mixing of biogenic and abiogenic gases. Abiogenic origin, alkane gas, carbon isotopic reversal trend, geochemical characteristics Key words: information is limited and studies of gas origin and gas- 1 Introduction source correlation are largely dependent on their chemical In nature hydrocarbons are formed largely by the digestion composition and carbon, hydrogen and helium isotopes. of organic compounds by microorganisms (microbial origin) To date, a series of studies have been focused on the (Rice and Claypool, 1981; Schoell, 1988; Whiticar et al, discrimination of biogenic (referred to microbial origin 1986) and the thermal decomposition of organic matters and thermal origin in this text) and abiogenic gases. (thermal origin) (Des Marais et al, 1981; Schoell, 1980; 1988; Jenden et al (1993) suggested three criteria for abiogenic 13 13 13 13 13 Dai et al, 1992). A number of recent studies demonstrated the gases: δ C >− 25‰, δ C >δ C >δ C >δ C , and 1 1 2 3 4 -6 existence of abiogenic alkane gases (Abrajano et al, 1988; R/Ra>0.1(Ra=atmospheric ratio,1.4×10 ). According to Charlou et al, 1996a; Dai, 1988; Dai et al, 2000; 2008a; previous studies, Dai et al (2008a) compiled the carbon Galimov and Petersilie, 1967; Guo and Wang, 1994; Guo et and helium isotopes of biogenic and abiogenic gases in the al, 1997; Jeffrey and Kaplan, 1988; Jenden et al, 1993; Kelley, world and provided four criteria for abiogenic gases: 1) δ C 13 13 13 13 1996; Potter and Konnerup-Madsen, 2003; Sherwood Lollar generally greater than −30‰; 2) δ C >δ C >δ C >δ C 1 2 3 4 et al, 1993; 2002; Wang et al, 2009; Welhan, 1988; Ni et al, and δ C generally greater than −30‰; 3) R/Ra>0.5 and 13 13 3 6 2009), which might result from the mantle-degassing (Craig δ C −δ C >0; and 4) CH / He≤10 . However, due to 1 2 4 and Lupton, 1981; Gold, 1979; Gold and Soter, 1980) or insuffi cient knowledge of abiogenic gases, the discrimination chemical processes such as Fischer-Tropsch synthesis (Berndt of biogenic and abiogenic gases is a hot debate. Recent et al, 1996; Charlou et al, 1998; Craig, 1953; Foustoukos and advances on abiogenic gases from the laboratory have Seyfried, 2004; Fu et al, 2007; Giggenbach, 1980; Horita and deepened our knowledge of the geochemical characteristics Berndt, 1999; Hulston and McCabe, 1962a; 1962b; Kelley of abiogenic gases. For example, the carbon isotope ratio and Früh-Green, 1999; Lancet and Anders, 1970; McCollom of methane formed via Fischer-Tropsch synthesis under and Seewald, 2006; Sherwood Lollar et al, 2002). hydrothermal condition (200°C, 50 MPa) with Fe-Ni as Since natural gases are mainly dominated by methane, catalyst can be as low as −57‰, which is close to the value plus small amount of heavy hydrocarbons (C ) and non- 2+ of biogenic gas (Horita and Berndt, 1999). Other experiments hydrocarbons (CO , N , H S, etc.), the available geochemical 2 2 2 also found that products of the Fischer-Tropsch reactions also have regular distribution pattern of carbon isotopes 13 13 13 13 between C -C n-alkanes, i.e., δ C <δ C <δ C <δ C , 1 4 1 2 3 4 *Corresponding author. email: niyy@petrochina.com.cn which is similar to that of the biogenic gases (Fu et al, 2007; Received March 13, 2009 328 Pet.Sci.(2009)6:327-338 McCollom and Seewald, 2006; Taran et al, 2007). Du et al (b) (2003) found that pyrolysis of lignite under extremely high Oi l and gas reservoi rs pressure and temperature (1-3 GPa, 500-700°C) can also lead to carbon isotopic reversal trend among C -C n-alkanes. 1 4 Recently hydrocarbons have been found on Mars, Enceladus, and Titan (Brown et al, 2008; Filacchione et al, 2007; Krasnopolsky et al, 2004; Niemann et al, 2005; Oze and Sharma, 2005) and discrimination of their origins will have great significance on life origin. In addition, the discrimination of abiogenic alkane gases and discovery of 10 abiogenic alkane gas reservoirs open up a new research field and improve the oil-gas theory. This contribution is concentrated on the geochemical characteristics of abiogenic gases which have been reported in the world in order to 10 30 60 90 120 150 provide some hints for the discrimination of abiogenic gases. C /C 1 2+ 2 Chemical composition of abiogenic gases (c) The chemical compositions of abiogenic gases in nature Oceanic hydrothermal systems vary widely. Dai et al (2000) made a detailed review of the abiogenic alkane gases from the inclusions of volcanic rocks and hot springs in China. They found these abiogenic gases are dominated by CO and/or N . A negative correlation was 2 2 found between CO and N , and the gas could be dominated 2 2 by either of them which could be up to 99%. The amount of alkane gases was low and characterized by methane and 8 virtually no C . However, abiogenic gases found in the 2+ Khibiny and Lovozero massif, Ilímaussaq complex, Canadian shield rocks are dominated by methane. The content of methane is more than 60% in general, and some can be up to 97%. Only a few of them are dominated by H , with some 500 1000 2000 3000 4000 5000 6000 7000 amount of CO and alkanes (Table 1, Fig. 1). The C /C 2 1 2+ C /C ratios are relatively low, normally lower than 100. As shown 1 2+ in Table 1, abiogenic gases from the Oman ophiolite are dominated by H and N , with small amount of methane and 2 2 Fig. 1 Histogram of C /C ratios of abiogenic gases from volcanoes 1 2+ nearly no C . The C /C ratios of abiogenic gases from the (a), oil and gas reservoirs (b) and hydrothermal fl uids (c). Data from 2+ 1 2+ Table 1, Table 2, and Table 3 except the Oman and Philippines data mid-ocean ridges are much greater than those of thermogenic gases (Dai et al, 2005), and most of them are greater than 800, and some can be as high as 8,000 (Table 2, Fig. 1). 3 Isotopes of abiogenic alkane gases The abiogenic gas fields in the Songliao Basin, China are characterized by methane and most of the methane accounts 3.1 Carbon and hydrogen isotopes of methane for more than 90%. Its C /C ratios are normally lower than 1 2+ 150 (Table 3, Fig. 1). Methane is a highly reducing state of carbon. It is also 35 the dominant component of biogenic and abiogenic alkane (a) gases. Its carbon and hydrogen isotopes can refl ect its origin to some extent. For example, gas of microbial origin is Volcanic rocks normally formed at depth less than 1 km, and its carbon and hydrogen isotopes have a range of −110‰<δ C <−55‰, −400‰<δ D <−150‰, respectively; while the carbon and hydrogen isotopes of thermogenic gases vary between −55‰<δ C ≤−20‰ (some as high as −10‰), −275‰<δD <−100‰, respectively (Dai et al, 2008b; Schoell, 1980; 1983; Whiticar, 1999; Dai et al, 1992). The carbon and hydrogen isotopes are −52‰ to −24‰ and −280‰ to −140‰ for coal-related gases, respectively and −58‰ to −30‰ and −300‰ to −130‰ for oil-associated gases, respectively in China (Wang, 1997). The carbon and hydrogen 10 30 60 90 120 150 isotopic compositions are also characteristic for abiogenic methane. Table 4 and Fig. 2 demonstrate some carbon and C /C 1 2+ Number of samples Number of samples Number of samples Pet.Sci.(2009)6:327-338 329 Table 1 Chemical composition of the abiogenic gas from various rocks Composition, vol% Location C /C References 1 2+ CH C H CO CO N He O +Ar 4 2+ 2 2 2 2 2.06-71.96 0-6.54 1.99-30.95 0-71 6.77-51.60 0-16.21 9-42 Petersilie et al, 1961 93.42 1.37 0.91 0.00 2.30 2.20 68 Petersilie, 1962 Petersilie and Sørensen, 38.09-96.75 0-3.77 1.08-61.91 0-0.34 0-1.97 0-0.08 24-53 Khibiny 89.70-90.00 2.15-2.53 1.77-3.45 0.19 1.32-4.13 0-4.59 35-42 Kogarko et al, 1987 24.06-97.64 0.65-8.19 0-13.90 0-2.88 0-70.30 0-0.69 11-42 Voytov, 1992 1.69-95.27 0-4.04 1.48-63.27 0-28.38 2.26-42.57 0-0.19 1-56 Potter, 2000 81.20 6.11 8.34 0.00 4.35 0.00 13 Petersilie, 1962 Petersilie and Sørensen, 79.47-91.36 4.30-8.48 0-1.50 0.12-0.41 0-0.04 10-21 8.52-78.55 0.12-6.25 10.84-65.32 0.15-0.30 4.20-25.71 13-71 Kogarko et al, 1987 Lovozero 15.70-76.10 0.50-5.80 4.80-39.20 3.00-37.50 0.02-1.90 0.80-11.30 11-31 Nivin et al, 1995 62.68-89.43 1.92-6.91 0.04-27.76 2.69-6.48 0-0.05 9-47 Potter, 2000 52.06-89.43 1.92-6.91 4.14-35.24 2.69-6.48 0-0.52 8-47 Potter et al, 2004 Petersilie and Sørensen, 5.30-83.72 0.06-12.07 5.04-93.11 0-0.06 0-6.93 0.16-3.66 7-88 Konnerup-Madsen et al, Ilímaussaq 43.00-80.00 6.50-15.55 3.00-34.00 0.40-4.00 3.60-13.00 3-9 Konnerup-Madsen and 66.30-92.10 2.10-10.65 5.80-18.80 0-1.20 0-6.00 0-0.20 7-44 Rose-Hansen, 1982 Sherwood Lollar et al, Canada 69.30-77.90 6.51-15.10 0.34-12.70 3.88-12.70 1.83-2.45 5-12 2002; 2008 Oman 0.20-4.30 0-0.01 1.00-97.00 1.00-77.00 0-21.00 >80 Neal and Stanger, 1983 1.50-60.60 Philippines 13.00-55.30 0.04-0.22 8.40-42.60 <0.03 0-0.00069 0.1-17.23 251-343 Abrajano et al, 1988 (CO+N ) Table 2 Chemical composition of abiogenic gases from the submarine hydrothermal systems CH C H CO CO N Ar H S 4 2+ 2 2 2 2 Location C /C References 1 2+ mmol/kg nmol/kg mmol/kg nmol/kg mmol/kg mmol/kg umol/kg mmol/kg 1023- Proskurowski 30°N MAR Lost City 0.89-1.98 0-14.38 869-1122 1991 et al, 2008 Charlou et al, 6°14’N MAR Rainbow 2.5 1145 16 5000 16 1.8 1.2 2183 37°17’N MAR Lucky 15.10- 0.30-0.85 84-394 0-0.73 0.50-1.77 11.7-30.2 2.0-3.0 1500-7935 Strike 39.90 Charlou et al, 37°50’N MAR Menez 20.10- 1.35-2.15 285-826 0.02-0.05 0.37-1.82 11-38 1.3-1.8 2603-5263 Gwen 22.60 Charlou et al, 14°45’N MAR Logatchev 0 0 0.3 2.3 0.59 16 0 Jean-Baptiste et 23°N MAR Snakepit 0.06 2.7 al, 1991 Charlou et al, 26°N MAR TAG 0.15-0.16 0.18-0.23 3.31-3.56 0.8-0.95 21.6-24.3 3.2-3.8 1996a 29°N MAR Broken Spur 0.065-0.13 0.43-1.03 6.0-7.1 8.5-11.0 Lein et al, 2000 Welhan and 21°N EPR 0.07 108 1.7   648 Lupton, 1987 Thermogenic 330 Pet.Sci.(2009)6:327-338 Table 3 Chemical composition and helium isotopes of abiogenic gases from the Songliao Basin, China Composition, vol% CH / He C /C R/Ra References 1 2+ CH C H C H C H CO N He H ×10 4 2 6 3 8 4 10 2 2 2 Dai et al, 82.12-95.95 0.74-2.45 0.11-0.64 0.02-0.27 0.1-14.41 0.86-6.35 0.008-0.034 0-1.94 28-105 0.6-1.8 1.2-8.6 2008a Wang et al, 81.59-96.85 0.55-2.37 0.02-1.06 0-0.16 0.09-13.35 1.21-9.05 28-126 hydrogen isotopes of abiogenic methane in the world. The 3.2 Carbon and hydrogen isotopic distribution carbon isotopes of abiogenic methane vary widely, −1‰ to pattern of methane and its homologues (C -C ) 1 4 −44.9‰, however, except the abiogenic methane from the Isotopic analyses found that thermogenic gases (C -C ) 1 4 Canadian and Fennoscandian shield rocks, most methane has become more enriched in C with increasing molecular mass δ C>−30‰. The hydrogen isotopic composition of methane 13 13 13 13 (δ C <δ C <δ C <δ C ) due to kinetic isotopic effects. 1 2 3 4 also varies widely, −102‰ to −419‰, overlapping with that When one alkyl is separated from the original organic matter, of biogenic methane. 12 12 12 13 the C- C bond is weaker than the C- C bond and breaks faster, causing the pyrolytic products to be more depleted in -120 C than the higher molecular weight organic precursor (Des Marais et al, 1981). A typical characteristic of abiogenic -100 alkane gases is the carbon isotopic reversal trend, i.e., the carbon isotope of alkane gases become more depleted in C 13 13 13 13 Bacterial with increasing molecular mass (δ C >δ C >δ C >δ C ) 1 2 3 4 -80 carbonate (Des Marais et al, 1981; Galimov and Petersilie, 1967; reduction Lancet and Anders, 1970; Yuen et al, 1984). The reason is Mix Bacterial -60 & that during the control process of isotope kinetics distillation methyl-type Transition fermentation effect, synthesizing a high molecular homologue from a 12 12 low molecular compound, the C- C bond is weaker and -40 12 13 12 thus breaks faster than C- C, the CH reacts faster than CH to form hydrocarbon chains, that is, in the process of polymerization, C is preferentially incorporated into the -20 long chains formed during polymerization, thus obtaining a reverse carbon isotope trend (Des Marais et al, 1981; Yuen et al, 1984). In a similar fashion, due to the kinetic isotopic -450 -350 -250 -150 -50 effects both thermogenic and abiogenic gases have a regular hydrogen isotopic distribution pattern whereby the C -C CH 4 1 4 alkane gases become more enriched in H with increasing Atmospheric CH 21°N East Pacific Rise molecular mass, i.e., δD <δD <δD <δD . 1 2 3 4 Alkaline rocks 30°N MAR Bacterial oxidation of methane will lead to isotopic Milos New Zealand depletion of C versus C (Coleman and Risatti, 1981), 2 1 Zambales Songliao, China conclusive evidence of isotopic depletion of higher Canadian & Sichuan, China Fennoscandian hydrocarbons with respect to C requires comparison to a larger range of higher hydrocarbons, particularly C and Fig. 2 Compilation of carbon and hydrogen isotope variation in C (Sherwood Lollar et al, 2002). Dai (1990) and Dai et abiogenic methane (data from Table 4) al (2004) proposed that the partial reversal of carbon and Debates arise concerning the lower limit of carbon isotope hydrogen isotopes might be due to secondary alteration of abiogenic methane, namely, >−20‰ (Shen et al, 1991; (e.g., bacterial oxidization) or mixing of gases with different Xu et al, 1993; Zhang, 1991; Chen, 1989), >−25‰ (Jenden origins. At present, there are not too many reports about the et al, 1993), and generally >−30‰ (Dai et al, 2008a; Fuex, carbon and hydrogen isotopic distribution patterns among the 1977; Dai, 1992a; 1992b). At present all these lower limits of abiogenic C -C alkanes (Table 5, Fig. 3, and Fig. 4). Most 1 4 carbon isotopes are mainly a comprehensive generalization of these reports are focused on the abiogenic gases from of the carbon isotopes of various reported occurrences of the mid-ocean ridges, shield rocks, and volcanic rocks, and abiogenic methane in the world, but the field lacks exact the abiogenic alkane gas reservoirs are only reported in the methods. As discussed above, Horita and Berndt (1999) found Songliao Basin, China. Sherwood Lollar et al (1993; 2002) that the carbon isotopes of abiogenic methane formed through and Wang et al (2009) found that the carbon and hydrogen Fischer-Tropsch synthesis in the laboratory can be as low as isotopes of abiogenic gases are negatively correlated while −57‰, which is similar to the value of microbial gases. in contrast Proskurowski et al (2008) found that the carbon CH 4 Pet.Sci.(2009)6:327-338 331 Table 4 Carbon and hydrogen isotopes of abiogenic methane 13 13 3 6 Location δ C , ‰ δD , ‰ δ C , ‰ R/Ra CH / He, ×10 References CH CH CO 4 4 4 2 Zaohetang, Tengchong, China –19.9 –130 –6.3 - –1.9 2.9 Dai, 1993 Tuoba, Ganzi, China –26.6 - –23.8 –213 - –111 –4.8 - –2.4 0.4-0.5 New Zealand –29.5 - –24.4 –197 - –142 –9.1 - –3.2 Lyon and Hulston, 1984 New Zealand –32 - –20.4 –170 - –144 –8.9 - –4.8 4.2-7.5 Giggenbach, 1995 Yellowstone, USA –28.4 - –10.4 –239 Welhan et al, 1983 Milos –17.8 - –9.4 –189 - –104 –1.1 - –0.6 1.0-1.3 Botz et al, 1996 Iceland –39.3 - –22.0 –18.7 - –4.8 2.1-16.8 Sano et al, 1985 Mediterranean Sea –32.3 - –16.4 –4.3 - 0.1 2.5-5.1 Fiebig et al, 2007 Kim et al, 1984; Merlivat et al, 13°N EPR –19.1 - –16.7 –4.4 - –4.1 7.0-7.5 3.1-3.9 Kim et al, 1984; Welhan and 21°N EPR –17.6 - –15.0 –126 - –102 –7.0 7.8 3.5-6.5 Craig, 1983 Charlou et al, 1996b; Jean- 17°-19°S EPR –23.9 - –22.0 6.2 Baptiste et al, 1997 Charlou et al, 1998; Jean- 14°45’N MAR Logatchev –13.6 –4.1 7.2 Baptiste et al, 2004 26°N MAR TAG –9.5 - –8.0 –10.0 - –8.4 7.5-8.2 8.8-52 Charlou et al, 1996a; 2002 Jean-Baptiste et al, 2004; Lein 29°N MAR Broken Spur –19.0 - –18.0 –9.0 8.9 et al, 2000 30°N MAR Lost City –13.6 - –9.4 –147 - –125 Proskurowski et al, 2008 Charlou et al, 2002; Jean- 36°14’N MAR Rainbow –15.8 –3.15 5.7-7.8 Baptiste et al, 2004 37°50’N MAR Menez Gwen –19.6 - –18.8 –9.1 - –6.8 8.7 Charlou et al, 2000; Jean- Baptiste et al, 1998 37°17’N MAR Lucky Strike –13.7 - –12.7 –10.6 - –7.2 8.1 Southwest Indian Ridge T=450-500 °C –33.8 - –9.1 –149 - –99 –28.5 - –8.7 Kelley and Früh-Green, 1999 Southwest Indian Ridge T>900°C –30.1 - –18.8 –244 - –128 –23.9 - –1.9 19°33’S Central Indian Ridge Roger –17.5 1.1-1.6 Kawagucci et al, 2008 Plateau Zambales, Philippines –7.5 - –6.1 –137 - –118 –32 3.98-4.07 Abrajano et al, 1988 –12.8 - –3.2 Galimov, 1975 –28.6 - –12.8 Potter, 2000 –19.3 - –3.2 –8.5 - 10.6 Galimov and Petersilie, 1967 –11.2 - –3.2 –56 - 173 Voytov, 1992 Khibiny –6.5 –66 - 82 Yerokhin, 1978 –14.0 - –7.7 Nivin et al, 2001 –22.4 - –5.4 –16.8 - –13.6 Beeskow et al, 2006 –13.3 - –7.6 –18.6 - –14.6 Potter and Longstaffe, 2007 –15.7 - –7.1 Nivin et al, 2001 Lovozero –11.8 –167 - –132 Nivin et al, 1995 IIímaussaq –7 - –1 –145 - –132 Konnerup-Madsen, 2001 –44.9 - –22.4 –372 - –133 Sherwood Lollar et al, 1993 Canada and Fennoscandia –40.7 - –33.0 –419 - –390 Sherwood Lollar et al, 2002 Dai et al, 2008a; Wang et al, Songliao, China –28.9 - –17.4 –203 - –196 –5.1 - 18.9 0.6-1.8 1.2-8.6 2009 13 C, ‰ 332 Pet.Sci.(2009)6:327-338 3 4 -6 and hydrogen isotopes of abiogenic gases from the 30°N atmospheric helium, He/ He=1.4×10 , i.e., Ra; crustal MAR Lost City are positively correlated (Fig. 4). The exact helium, product of the radioactive decay of U and Th (yielding 4 6 3 3 4 situation is still unclear at the moment. He) and the (n, α) reaction of Li (yielding He), He/ He normally less than 0.04 Ra (Andrews, 1985; Mamyrin and Tolstikhin, 1984); mantle helium, a mixture of radiogenic and 3 4 primordial helium which has He/ He of 100-300 Ra (Craig 0 10 and Lupton, 1981; Mamyrin and Tolstikhin, 1984). Since -5 9 the atmospheric helium in the commercial gas wells can be -10 8 neglected, helium in the natural gas can be approximately regarded as a mixture of crustal and mantle helium. In -15 7 3 4 general, mantle He/ He is far greater than the crustal 3 4 3 4 -20 6 He/ He. For example, the He/ He value is 7-9 Ra for the depleted upper mantle, >30 Ra for the lower mantle and <0.1 -25 5 Ra for the crust (Andrews, 1985; Craig and Lupton, 1981; -30 4 Hiyagon and Kennedy, 1992; Welhan et al, 1983). Hence, the 3 4 high He/ He values of natural gas mean addition of mantle -35 3 3 4 volatiles. Jenden et al (1993) proposed that He/ He>0.1 Ra -40 2 in the natural gas was an indication of the existence of mantle 3 4 -45 1 helium. Dai et al (2008a) suggested that He/ He>0.5 Ra indicates the addition of mantle helium. -50 0 C C C C Jenden et al (1993) compiled the numerous studies of 1 2 3 4 the helium isotopic distribution of different commercial oil Songliao Basin MAR Lost City and gas fi elds in the world and proposed that fi elds overlying Khibiny Kidd Creek Murchison ancient cratonic crust and in foreland basins formed by 3 4 Fig. 3 Carbon isotopic distribution of abiogenic methane and its thrust-plate loading had He/ He ratios close to radiogenic homologues (data from Table 5). Data from the Murchison meteorite are values, while fields located near active plate margins in 3 4 on right y axis and all other data are on left y axis. forearc, backarc, and rift basins might have He/ He ratios approaching mantle values. Xu et al (1998) made a statistical analysis of the distribution of helium isotopes in natural gases in China (Fig. 5) and found that about 30% natural gases were 3 4 characterized by He/ He ratios greater than the atmospheric -50 values (R/Ra>1), indicating the addition of mantle helium. -100 The distribution of the helium isotope ratio (R/Ra) in the hot springs and hydrothermal systems in China are mainly in -150 the range of 0.5-8 (Fig. 6, Table 6), indicating the addition of mantle helium as well. However, due to the helium loss -200 through reservoirs or helium addition from radiogenic 3 4 -250 sources, He/ He ratios may decrease with time, leading to 3 4 the low He/ He ratios in some natural gases of mantle origin -300 (Kamenskiy et al, 1971). On the other hand, biogenic methane 3 4 can also be associated with high He/ He ratios. For example, -350 3 4 Songliao He/ He ratios in the oil and gas fields of the Green Tuff – – MAR Lost City region in Japan are as high as 7.5 Ra (δ C : 62‰ to 33‰) -400 3 4 Kidd Creek (Sakata, 1991); He/ He ratios in the geothermal vents along -450 the East Pacifi c Rise in the Guaymas Basin, Gulf of California C C C C 1 2 3 4 13 – – also approach 8.0 Ra (δ C : 51‰ to 43‰) (Welhan and Fig. 4 Hydrogen isotopic distribution pattern of abiogenic methane and Lupton, 1987). In the Wudalianchi and Changbaishan areas in 3 4 its homologues (data from Table 5) China, the high He/ He ratios are associated with relatively low δ C values of methane (Shangguan and Wu, 2008). According to the recent study on the geochemistry of natural gases from the inactive volcanoes in China by Shangguan and 3 4 3.3 He/ He isotope (R/Ra) 13 Wu (2008), δ C value of methane in the volcanic residual Since the discovery of He anomaly in the East Pacific gas in Changbaishan was as low as 48.0 while the associated 3 4 Rise in the 1960’s (Craig and Lupton, 1981), helium isotopes helium has He/ He ratios of 5.5 Ra (Table 6). The authors have been widely used as traces of mantle volatiles. Due to suggested an origin in the mantle for the hydrocarbons, the different origin of the two stable isotopes of helium, He however, debates exist concerning the mantle contribution to and He, helium can be divided into three different types: the hydrocarbons and further studies are required. D, ‰ C, ‰ Pet.Sci.(2009)6:327-338 333 Table 5 Carbon and hydrogen isotopes of abiogenic methane and its homologues δ C, ‰ δD, ‰ Location Sample References C C C C C C C C 1 2 3 4 1 2 3 4 Inclusion –3.2 –9.1 –16.2 Zorikin et al, 1984          Inclusion –12.8 –24.5 –26.0 Galimov, 1975; Voytov, Inclusion –3.2 –9.1 –25.7          Rischorrite –14.4 –17.1 –17.3 –17.5 Trachytoid foyaite –8.0 –17.5 –19.6 Beeskow et al, 2006 Khibiny Massive foyaite –5.4 –14.3 –13.0 –13.2       Urtite –8.1 –20.6 –19.7 Urtite –88 –154 Potter and Longstaffe, 2007 Urtite –12.4 –23.2 –21.3 –17.4 Urtite –52 –120 Yellowstone, USA –21.5 –26.5   Des Marais et al, 1981 Meteorite Murchison 9.2 3.7 1.2          Yuen et al, 1984 CCS4000 –35 –25.1 –325 Fennoscandian Sherwood Lollar et al, 1993 Juuka 116-254 m –28.6 –25.9    –281      CCS846-2 –35.1 –33.3 –31.2 –237 –215 –185 Sudbury Ontario Sherwood Lollar et al, 1988 CCS844-2 –29.7 –28.0 –27.7   –335 –238    7792 –38.3 –38.2 –37.6 –37.9 –390 –299 –256 –224 Kidd Creek 8558 –32.7 –36.8 –35.3 –35.7 –419 –321 –264 –245 Sherwood Lollar et al, 2002 8282 –35.0 –38.3 –35.9 –36.3 –409 –316 –269 –252 H04-IGT8 –9.5 –15.2 –15.0 –119 –127 –140 Lost City Proskurowski et al, 2008 H05-IGT7 –11.8 –13.7 –13.4 –147 –166 –160 Fs1 –18.6 –23.2          Dai, 1992a Fs1 –18.7 –22.4 –24.1 –28.2 Guo et al, 1997 Fs7 –28.8 –30.4 –32.5 –32.6       Fs9 –27.1 –30.1 –30.5 –33.0       Wang et al, 2004 Xs1 –29.7 –32.9 –34.3 –35.0 Songliao, China Xs6-2 –25.9 –32.4 –33.1 –33.7 Dai et al, 2008a Fs2 –17.4 –22.2 –30.5 –31.4       FS-2 –17.7 –22.0 –30.3 –200 –184 –138 S-2-1 –22.7 –28.0 –32.1 –35.2 –202 –166 –120 Wang et al, 2009 S-502 –29.3 –30.3 –30.9   –197 –189 –188   334 Pet.Sci.(2009)6:327-338 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 0.02 0.05 0.1 0.5 1258 R/Ra R/Ra Fig. 6 Histogram of R/Ra values of natural gases from the Fig. 5 Histogram of the R/Ra values of the natural gases in China (Xu et al, 1998) hydrothermal systems and hot springs in China Table 6 Chemical composition and isotopes of natural gases from the hydrothermal systems and hot springs in China 13 13 3 6 Location N , % CO , % CH , % He, % δ C , ‰ δ C , ‰ R/Ra CH / He, ×10 Reference 2 2 4 1 CO 4 Tuoba, Ganzi, Sichuan 0-2.72 93.00-98.60 1.07-3.64 0-0.008 −26.6 - −24.8 −4.2 - −2.5 0.36-0.50 791.58 Fushoushan, Pingjiang, Hunan 0.17 99.70 0.0038 −5.5 0.29 Rucheng and Ningxiang, 96.88-97.55 0.75-2.92 0-0.38 0-0.24 −25.4 0.07-0.47 16.72 Hunan Fengshun, Guangdong 95.71 3.19 0.07 −11.2 0.83 Xunwu, Jiangxi 99.96 0.04 −27.7 1.36 Chongren, Jiangxi 54.53 29.69 0.0056 −7.3 0.57 Dai et al, 2000 Taishun, Zhejiang 98.86 0.40 0.15 0.44 −10.1 0.24 1.01 Keshiketeng, Inner-Mongolia 97.84 0.19 0.26 0.66 −22.7 −16.1 0.06 4.73 Fujian 84.32-96.31 2.23-13.66 0-0.19 0.049-0.13 0.28-0.48 2.16 Wudalianchi 0.24-2.34 97.41-99.45 0-0.02 0.057-0.067 −5.4 - −4.0 2.98-3.16 0-0.08 Chuxiong, Yunnan 23.00 76.53 1.00 0.006 −6.0 0.82 146.20 Tengchong, Yunnan 0-3.20 96.66-99.92 0.13-1.35 0.005-0.042 −32.7 - −16.2 –5.8 - –1.2 2.86-4.49 0.1-7.6 Other places in Yunnan 86.03-94.83 3.04-11.99 0-6.54 0-0.19 −50.3 - −28.4 –16.3 - –8.9 0.01-0.40 144.76 Sun et al, Hengjing, Jiangxi 0-2.01 96.47-99.84 0.04-1.86 0.002-0.014 −59.3 - −27.7 –5.5 - –4.4 1.36-2.11 1.9-424 Wang et al, Tengchong 14.52-71.23 10.33-80.13 0-1.03 0.0003-0.02 0.48-5.13 0.2-32.6 Changbai Mountain 2.14-10.04 73.55-97.59 0-0.47 0.00002-0.006 –6.6 - –4.2 2.29-5.71 12.5-604.9 Shangguan et al, 1997 Jinjiang 3.86-11.25 79.02-86.53 1.25-2.02 0.01-0.03 −30.4 - −26.2 –7.5 - –7.0 5.36-5.89 5.2-13.3 Shangguan et Tengchong 0.80-32.75 57.49-96.64 0.01-2.73 0.00006-0.04 –5.8 - –0.5 1.01-4.81 0.2-133.4 al, 2000 Wudalianchi 3.09-3.51 Shangguan et Tengchong 4.85 al, 2006 Changbai Mountain 5.62-5.95 Tengchong 2.22-4.42 92.46-94.95 0.01-0.98 <0.000122 –27.3 - –18.8 –6.5 - –3.6 1.62-5.39 Shangguan Changbai Mountain 2.55-9.76 83.45-96.06 0.28-4.15 <0.000207 –48.0 - –26.2 –6.9 - –4.0 4.79-5.51 and Wu, 2008 Wudalianchi 4.90-7.64 90.20-93.41 0.02-0.03 <0.000248 –47.2 - –44.4 –6.6 - –4.5 3.01-3.03 Number of samples Number of samples Pet.Sci.(2009)6:327-338 335 3.4 Methane concentration and He abundance indicative of mixing (Poreda et al, 1986). Similarly, natural gas from the mud volcano in the Copper River Basin in He has proven to be of mantle origin, hence, except Alaska was also explained as mixing origin (Motyka et al, 3 4 3 the He/ He ratios, the correlation between He and the 1989). concentration of other gas composition such as CH can provide some hints for the origin of natural hydrocarbon Crustal gases. Fluids from the East Pacific Rise and Mid-Atlantic 3 6 6 Ridges have CH / He ratios of 10 (1.4-7.0×10 for EPR and 2.4×10 for MAR), which have been considered as mantle origin (Jean-Baptiste et al, 1991; Welhan et al, 1983). 3 10 According to the CH / He ratios of 100 gas wells from the Bohai Bay, North Jiangsu, Songliao, Ordos, Sichuan, Tarim and Turpan-Harim basins in China and the comparison with those from New Zealand, Iceland, and Thailand, etc., Dai et 3 6 al (2008a) proposed that abiogenic gas had CH / He≤10 and Hot springs and 3 11 biogenic gas had CH / He≥10 . Fig. 7 shows the correlation hydrothermal systems in China between He abundance and the methane concentration of Biogenic gas in China 10 Abiogenic gas from Songliao gases from the hot springs, hydrothermal vents and gas fi elds Mantle 21 N East Pacific Rise in China. The CH / He ratios of biogenic alkane gases vary 8 13 3 widely, 10 -10 , overlapped with the CH / He ratios for the abiogenic gases from the Songliao Basin. In general, CH / He 0.001 0.01 0.1 1 10 ratios from the gas fields are greater than those from the R/Ra hot springs and hydrothermal vents, indicating that gases in the gas fi elds are characterized by biogenic gas while gases Plot of CH / He versus R/Ra of various natural gases. Data of Fig. 7 hydrothermal systems and hot springs are from Table 6, the biogenic alkane in the springs and hydrothermal systems are dominated by gas data are from Dai et al (2008b), data of the 21 N East Pacifi c Rise are mantle volatiles. However, if reducing CH -enriched fluids from Welhan and Lupton (1987), and the abiogenic data in the Songliao are degassed from mantle (Gold and Soter, 1980; 1982), Basin are from Table 3 the mantle CH / He ratios can be expected to be as high as 3×10 . The CH -enriched gases from the pyroclastic rock 4 Conclusions reservoirs of the gas fields in the Green Tuff region, Japan 3 9 have CH / He ratios of around 2×10 , regarded as mantle This contribution made a comprehensive study of the origin (Wakita et al, 1990). Whereas, the CH / He ratios of geochemical characteristics of abiogenic alkane gases. The biogenic gases can also be very high, even as high as 10 . For chemical composition of abiogenic alkane gases vary widely. example, gases from the Well 843-2 of the Dagang Oil Field The various volcanic fluid inclusions and hydrothermal 3 8 in the Bohai Bay Basin, China have CH / He ratios of 5.3×10 systems in China have very low amount of methane and and R/Ra ratios of 8.6, while the carbon isotopes of methane, nearly no C . The C /C ratios in the unsedimented 2+ 1 2+ ethane, and propane are –58.1‰, –42.3‰, and –30.7‰, submarine hydrothermal systems are far greater than those respectively; Gases from the Well Qi18 in the Songliao of biogenic alkane gases, mostly greater than 800 and some 3 8 Basin also have CH / He ratios of 5.3×10 and R/Ra ratios of up to 8,000. The C /C ratios of the abiogenic alkane gases 4 1 2+ 2.6, while carbon isotopes of methane, ethane, and propane in the Songliao Basin, China are generally less than 150. The are –54.7‰, –31.9‰, and –24.2‰, respectively (Dai et al, reported abiogenic alkane gases in nature normally have a 2008a). Similarly, the hydrothermal fl uid from the Guaymas reversed distribution pattern of carbon isotope ratios among 3 9 Basin, Gulf of California has CH / He ratios of 3.1×10 , C -C alkanes, while both reversed and regular distribution 4 1 4 but its R/Ra ratios (8.0) are close to those of the 21°N East patterns of hydrogen isotope ratios have been reported. Pacific Rise. The carbon isotope of methane is –15.0‰ to Abiogenic methane generally has δ C> 30‰ and a wide – – –17.6‰ for 21°N East Pacifi c Rise while –43‰ to –51‰ for distribution range of δD, 102‰ to 419‰ which overlaps the Guaymas Basin. The C /(C +C ) ratios of gases from the with the hydrogen isotope ratios of biogenic gases. However, 1 2 3 Guaymas Basin fall in the range of biogenic alkane gases, so δ C of methane synthesized in the laboratory can be as low 3 4 the CH / He ratios in the Guaymas Basin indicate biogenic as 57‰. The He/ He ratios of mantle helium are far greater 3 4 origin of the hydrocarbons, which is a result of the circulation than those of crustal helium, so the high He/ He ratios in the flow of the hydrothermal fluid in the overlying organic natural gases often indicate addition of mantle helium. Hence, 3 4 sediments (Simoneit et al, 1988; Welhan and Craig, 1983; He/ He ratios are often regarded as an auxiliary indicator of Welhan and Lupton, 1987). In fact, the high CH / He ratios abiogenic gases. However, biogenic alkane gases can also 3 4 3 in many locations are explained as a mixture of mantle and have very high He/ He ratios, even up to 8 Ra. The CH / He 3 13 crustal helium. For example, the high CH / He ratios (>10 ) ratios can reflect the gas origin to some extent due to the 3 3 of the gases from volcanic rocks in Japan were interpreted proven mantle origin of He. The CH / He ratios are lower 6 13 that a large contribution of the methane was from crustal than 10 for abiogenic end-members while greater than 10 or subducted sediments (Marty et al, 1989). The variation for the biogenic end-member. It often reflects a mixing of of CH / He ratios in the Sacramento Basin was taken as an biogenic and abiogenic alkane gases. CH / He 4 336 Pet.Sci.(2009)6:327-338 Dai J X. Discrimination of alkane gases. Science in China (Series B). Acknowledgments 1992a. (2): 185-193 (in Chinese) Dai J X. Identifi cation of various genetic natural gases. China Offshore This work is supported by the China Postdoctoral Science Oil and Gas (Geology). 1992b. 6(1): 11-19 (in Chinese) Foundation (20070420393), China Postdoctoral Special Dai J X. 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