Longitudinal distributions of two formation pathways ...

Science in China Series D: Earth Sciences © 2007

Science in China Press Springer-Verlag

Longitudinal distributions of two formation pathways of biogenic gases in continental deposits: A case study from Sebei 1 gas field in the Qaidam Basin, western China SHUAI YanHua†, ZHANG ShuiChang, ZHAO WenZhi, SU AiGuo & WANG HuiTong Petroleum Geology Research and Central Laboratory, Research Institute of Petroleum Exploration and Development, Beijing 100083, China

The distribution of two formation pathways of biogenic methane, acetate fermentation and reduction of CO2, has been extensively studied. In general, CO2 reduction is the dominate pathway in marine environment where acetate is relatively depleted because of SRB consuming. While in terrestrial freshwater or brackish environment, acetate fermentation is initially significant, but decreases with increasing buried depth. In this paper, character of biogenic gases is profiled in the XS3-4 well of the Sebei 1 gas field in the Sanhu depression, Qaidam Basin. It indicates that those two pathways do not change strictly with increasing buried depth. CO2 reduction is important near the surface (between 50 m and 160 m), and at the mesozone (between 400 and 1650 m). While acetate fermentation is the primary pathway at two zones, from 160 to 400 m and from 1650 to 1700 m. δ 13C of methane generated in those two acetate fermentation zones varies greatly, owing to different sediment circumstances. At the second zone (160－400 m), δ 13C1 ranges from −65‰ to −30‰ (PDB), because the main deposit is mudstone and makes the circumstance confined. At the fourth zone of the well bottom (1650－1700 m), δ 13C1 is lighter than −65‰ (PDB). Because the deposit is mainly composed of siltstone, it well connects with outer fertile groundwater and abundant nutrition has supplied into this open system. The high concentration of acetate is a forceful proof. δ 13C of methane would not turn heavier during fermentation, owing to enough nutrition supply. In spite of multi-occurrence of acetate fermentation, the commercial gas accumulation is dominated by methane of CO2-reduction pathway. A certain content of alkene gases in the biogenic gases suggests that methanogensis is still active at present. biogenic gases, carbon dioxide, carbon isotopes, hydrogen isotopes, Qaidam Basin

Biogenic methane, the product of methanogen in anaerobic environment, plays an important role in geochemical processes in the Earth. Methanogenesis occurs usually in the shallow burial, which induces that biogenic methane is easily released to the atmosphere and contributes increasing part to the present greenhouse warming year and year. However, biogenic methane could be gathered to clean efficient resource at a satisfied condition. As much as 20% of the world’s natural gas resources is estimated to have been generated by

microbes[1]. Biogenic methane could be generated through many pathways, in which acetate fermentation and CO2 reduction are primary. Methane formed by those two pathways should be easily distinguished through the stable

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Received January 12, 2006; accepted April 27, 2006 doi: 10.1007/s11430-007-2012-6 † Corresponding author (email: [email protected]) Supported by the National “973” Project (Grant No. 2001CB209101), the Chinese Postdoctoral Science Foundation (Grant No. 2005037419) and the NFSC (Grant No. 40603015)

hydrogen isotope composition[2,3]. The methane by acetate fermentation is relatively rich with H (−250‰－ −400‰), but that by CO2 reduction rich in D (−150‰ －−250‰). Some researchers proposed hydrogen isotopes of biogenic methane should be affected by the associated water[4,5], but the impact is much weaker than that by the formation pathways. The hydrogen isotope ratio of methane mainly reflects its formation pathway, especially in the natural condition[3]. The formation pathway is deemed to be influenced by some factors, such as sedimentary environment and temperature[5,6]. For example, in marine sedimentary condition, sulfate-reduced bacteria (SRB) are prolific in the shallow sulphate-rich zone, that makes most of the available labile carbon compounds metabolized by SRB and only those non-competitive substrates as methanol, methylated amines, and certain organic sulphur compounds which could not support SRB may make certain types of methanogens survive[7]. But, the generated methane is easily oxided by SRB, only trace concentration level of methane is present at sulphate reduction zones. Once the available sulphate is exhausted, usually at greater buried depths, the SRB become inactive and methanogenesis commences using competitive substrates as H2/CO2. In terrestrial freshwater or brackish environment, acetate fermentation is initially significant. About 70% of biogenic methane is generated by acetate fermentation, and only about 30% by CO2 reduction[3,8,9]. With increasing buried depth, acetate becomes exhausted and the proportion of CO2 reduction pathway increases[3,10]. Finally, CO2 reduction turns to be the only pathway in methane formation. However, the distribution of those two pathways is only descriptive. The factors controlling the pathways of the methane formation are still unclear to us up to now. Sanhu depression, located in the Qaidam Basin, is the biggest biogenic gas producing area in China, with reserves of 3×1012m3. It is a natural laboratory for research of the formation of the biogenic gas in the continental environment. In this paper, character of biogenic gases in the XS3-4 well of Sebei 1 gas field is profiled and the factors controlling the formation pathway of biogenic methane are tried to be found out.

1 Experimental The samples were collected from XS3-4 well in Sebei 1 222

gas field of the Sanhu region at the eastern Qaidam Basin of China. As an exploitative well, XS 3-4 was drilled from July 14 to August 5 in 2004, the depth of which is 1700 m. Because XS3-4 well is located at the top of the Sebei 1 structure, all the five gas-producing layers in the field appear in this well (Figure 1). Canned samples were drilled cutting at 10－20 m intervals. During sampling, the half of the can was filled with cutting sediment, and the other half was left as vacancy. The samples were sterilized with drops of HgCl2 solution (1N). It should be emphasized that the unconsolidated Quaternary sediments could be contaminated by the drilling mud due to weak diagenesis. The samples had been kept in the basement with an ambient temperature around 10－15℃. The desorbed gases were analyzed from November 5 to November 7 in 2004 in the State Key Laboratory of Origanic Geochemistry of Guangzhou Institute of Geochemistry. Two complementary experiments were carried out on 18 to 19 January, and on 10 to 15 September, 2005, respectively. GC analyses were performed on an HP5890A gas chromatography equipped with a Poraplot Q Chromatographic Column (30 m × 0.25 mm × 0.25 μm), and helium as carrier gas. Gas samples were injected at 70℃ and held at 70℃ for 5 min, after which the oven was ramped to 180℃ at 5℃/min and held for 15 min. The analytical error of the system was within 1%. GC-irMS analyses were performed on a VG Isochrom II equipped with Poraplot Q Chromatographic Column and helium as carrier gas. Samples were injected at 50℃ and held at 50℃ for 3 min, the oven was then ramped to 150℃ at 15℃/min and held for 8 min. The analytical error was within 0.3 (‰, PDB). The analyses of the stable hydrogen isotope ratio were performed on a Finningan Delta plus XL Isotope Ratio Mass Spectrometer with Dim Chromatographic Column (18 m × 0.53 mm). Samples were injected at 40℃ and held at 40℃ for 8 min, the oven was then ramped to 110℃ at 3℃/min and held for 2 min. The analytical error was less than 4 (‰, SMOW). The concentrations of volatile organic acid in those canned associated water were measured by ion exclusion chromatography with an HIC-6A ion chromatogram (pure water as carrier flow) in the Center of Testing and Analysis, Tsinghua University.

SHUAI YanHua et al. Sci China Ser D-Earth Sci | February 2007 | vol. 50 | no. 2 | 221-227

Figure 1 The stable carbon isotope ratios of methane were almost stable for three times of analysis at different time. So were the stable hydrogen isotope ratios of methane. By contrast, the lithological column of XS 3-4 well was given in the right.

2 Result and discussion 2.1

Feasibility of canned gas sample

Some experts doubted that the canned gaseous samples were conformable to the natural gases in the sediments. The results analyzed at different time could help to remove the query. Firstly, the data analyzed at different time were consistent. The stable isotopes of methane collected at close buried depths were similar and all of the data were regular and in a good trend with depths (Figure 1). Therefore the dissolved gases were not affected by the preservation and the change by long-time storage could be neglected. It should be noted that methane carbon isotopes at buried depths between 160 m and 410 m scatter to some extent, which likely reflects the sediment properties. The details will be discussed in Sec. 2.3. Secondly, those dissolved gases were in accordance with gases produced from the corresponding producing layers in the field. The difference between them could be neglected, too. For example, δ 13C1/δ D1 from No.0 layer (720 m) in Sebei 1 gas field were −68.2‰ and −233.1‰, respectively. They were slightly different to those at

buried depths of 700 m (δ 13C1/δ D1: −66.89‰, −234.6‰, respectively), but close to those from 750 m (δ 13C1/δ D1: −68.31‰ and –235‰, respectively). So were other samples (Table 1). Therefore, the gases, especially the methane hydrogen and carbon isotope, could be feasible in researching the formation mechanism of biogenic gases. Table 1 The character of natural gas from different layers in Sebei 1 gas field in the Sanhu depression, Qaidam Basin

720

Proved reserve (Tcf) 0.23

Reserve proporation (%) 6.58

990

0.65

18.71

1190

0.90

25.62

Depth (m)

1420

0.76

21.78

1680

0.96

27.30

C2+ a) (%)

δ 13C1a)

δ D1

(‰)

(‰)

99.08

0.07

−68.2

−233.10

98.03

0.12

−65.2

−232.50

95.64

0.1

−64.8

−223.20

98.22

0.1

−65.8

−224.20

C1 a) (%)

a) From Zhang Xiaobao et al.[11].

2.2 Longitudinal distribution of two formation pathways The stable carbon isotope of methane varied with buried depth of the sediment column. So did the stable hydrogen of methane. Based on these data, the column


223

could be classified into four zones: less than 160 m, 160 －400 m, 400－1650 m, and 1650－1700 m. In the upper zone, between 50 and 160 m, only three samples were collected. All of them showed the typical character of methane generated by CO2-reduction pathway. It was a pity that the biogenic methane near the surface could not be available because our first (shallowest) sample was at buried depth of 50 m. In the second zone, 160－400 m, the stable carbon isotopes of methane (δ 13C1) at different buried depths varied greatly, from −65‰ (PDB) to about −30‰. The isotopically anomalous methane (δ 13C1) was difficult to distinguish from the thermal origin. But, the δ D1 was relatively stable, all lighter than −300‰ (SMOW). Those methane rich in 13C could be easily mistaken as a result of bacteria oxidation, because bacteria preferentially consumed −12CH4 and made the remnant methane rich in −13C[12,13]. But, some phenomena could help to deny the possibility of methane oxidation by bacteria. Firstly, the associated δ 13CCO2 was between −18‰ and −20‰ (PDB). Secondly, the carbon isotope ratio of archeaol, the core component of the methanogen membrane, was as heavy as −20‰ (PDB). While on the contrary, bacteria oxidation of methane would make both the CO2 and the lipoid compounds of microbe depleted in −13C[14,15]. Furthermore, bacterial oxidation also makes the remnant methane depleted in H[13], which obviously disagrees with the very light hydrogen isotopes of methane (less than −300‰, SMOW). All those proved that bacteria oxidation had not occurred here and abnormal methane should be in itself controlled by the formation process but not by the oxidation. In fact, microbe could give birth to methane with such a heavy δ 13C1. Previous researches have confirmed that carbon isotope fractionation of acetate fermentation pathway is much lower than that of CO2 reduction pathway and methane generated by acetate fermentation could be heavy up to −40‰[16]. Furthermore, as mentioned above, the stable carbon isotope of archeaol, −20‰, was a little heavier than that of the general terrestrial organic matter. That would indicate that the carbon isotopes of the substrate consumed by the methanogen must be heavy, which usually occur in a relatively confined system. In a confined system, the remnant substrate would become richer and richer in −13C with the increasing conversion of substrate, and the fractionation of carbon isotopes 224

between product methane and parent substrate would be less and less. In the zone, sediment was mainly composed of mudstone, sandy mudstone, and siltstone (Figure 1). The total mudstone was 9 times thicker than the siltstone, which made this zone relatively confined and poor connection among itself. Methanogen consumed variant substrate to produce methane with variant carbon isotope compositions in this zone. In the third zone, 400－1650 m, the stable carbon isotope of methane was about −65‰－−68‰, and the hydrogen isotope of methane was about –230‰, which indicates typical characteristics of methane formed by CO2 reduction pathway. Methanogenesis in the fourth zone, 1650－1700 m, was dominated by acetate fermentation pathway, because the δ 13C1 was among −67‰－−74‰ and δ D1 was about –340‰. 2.3 Possible factors in different pathways As discussed above, this column is unique. Firstly, the phenomena, acetate fermentation substituted by CO2 reduction step by step, are invisible. It is possibly because the switchover of the two pathways usually occurs in the shallow burial, mostly accomplishes at less than 10 m[3]. While our samples started from 50 m. Secondly, the methane formed by acetate fermentation at buried depths of 160－400 m and 1650－1700 m was rather unique in other regions. The presence of acetate fermentation in those deep zones was related to the special sedimentary circumstance. From 150 to 400 m, the strata contained several layers of carbonaceous mudstone with high concentrations of labile organic matter. At the same time, the high salinity of the associated groundwater partially suppressed the degradation rate of the labile organic matter. Consequently, there was still abundant labile organic matter, such as acetate acid, in the associated groundwater (Table 2). Out of question, methanogenesis should have been dominated by acetate fermentation pathway. While at the first zone from 50 to 160 m, deposit was mainly composed of calcium mudstone and sandy mudstone. This zone was relatively short of labile organic matter, otherwise the bacterial degradation of organic matter was much strong due to the low salinity in the associated ground water. Acetate was easily consumed and exhausted, so methanogen had to switch over to H2/CO2 to survive. At the bottom of well, 1650－1700 m, acetate fermentation inevitable


occurred again because of the input of abundant nutrition with the groundwater. The concentration of acetate in associated water was abnormally abundant, ten times higher than those in other zones (Table 2). Table 2 The concentrations of volatile organic acids in associated water from canned cutting of XS 3-4 well Depth (m)

Formate (ppm)

Drill fluid 50 100 200 340 420 700 900 1110 1600 1680 1700

− 288.1 − − − 74.4 187.5 42.8 145.2 − − −

Acetate (ppm) 113.4 1596.3 815.5 2432.9 2125.1 1594.0 1866.5 454.4 1282.8 79.0 13912.3 15822.4

Propionic acid (ppm) − − − − − − − − − − 1021.5 1245.3

Through the same pathway of acetate fermentation, the produced methane in those two zones differs greatly in the carbon isotope components. Here, it possibly contributes to the adequate or not of the nutrition induced by the degree of the system openness. The second zone was confined with mudstone as the primary deposit, and most of nutrition material was mainly self-supplied. Accordingly, carbon isotope fractionation between methane and parent nutrition should become less and less with the nutrition transition increasing. While the fourth zone was relatively open because the deposit was composed of siltstone, argillaceous siltstone and sandy mudstone. The ratio of siltstone to mudstone was 2:1. Because it was impossible to produce so high concentration of acetate by such poverty rocks, the abundant nutrition should be supplied from the outer. Obviously, the remnant nutrition would not be clearly enriched in −13C because the quantity of the substrate consumed by microbe was less than that of the supply by the groundwater. Hence, the bacterial methane would be stable in the stable carbon isotope compositions all along until the system would change. It is concluded that formation pathways may be controlled by sedimentary environment instead only by the temperature or buried depth. Kotelnikova[10] took the content of DOC (soluble organic matter) as the main factor in controlling the formation pathway when he studied the distribution of biogenic methane in the water carrier in the granite rock. In fact, DOC content was not

a definite factor because high DOC did not virtually induce the occurrence of acetate fermentation[17]. On the other hand, methanogen themselves should give some hints about the factors in their metabolization pathways. Among 100 kinds of methanogen found presently, only two kinds feed solely on acetate, and they can survive at a circumstance even with a low concentration of acetate, 20 μm [18]. Seven other kinds of methanogen are multisourced (e.g. Methanosarcina). They could consume acetate, but only with a higher concentration than 50 ppm, otherwise, they turn to CO2/H2 or other methyl compounds such as methanol as their nutrition[18,19]. About 70 kinds of methanogen get energy and carbon solely from H2/CO2. It is easily deduced that a direct factor in formation pathway of biogenic methane should be the acetate content. Acetate fermentation pathway was secondary to CO2 reduction and unimportant to commercial gas accumulation even in this terrestrial sedimentary circumstance. From the column, the second zone was too shallow to reserve those gases. Though the buried depth of the fourth zone was enough to reserve gases, the quantity of gases is strictly confined by the origin and character of groundwater. Therefore, the acetate fermentation pathway is insignificant for natural gas resource. Most gases in this area were still dominated by CO2 reduction which was the favorable pathway to induce potential resource. This is the reason why large commercial biogenic gases formed through CO2 reduction. To increase the efficiency in biogenic gases exploration, the formation mechanism of CO2 reduction should be paid much more attention to. 2.4 Biogenic alkene gases Except methane, trace amount of ethane, propane, propylene and ethane occurred in all the samples. The dryness (C1/C1+) of all those gases was above 0.95 and decreased slightly with increasing buried depth (Table 3). The ratios of C1 to C2+3 ranged from 50 to 103, circuitously decreasing from 103 to 102 (or even lower). Here, the ratios (C1/C2+3) were much less than that of the typical biogenic natural gases (103－105), mainly because of the existence of alkene gaseous components[3]. In the most samples, the percent of ethane was lower than 0.1%. Ethene only occurred at a concentration lower than 0.05% in those two acetate fermentation zones, above 400 m and below 1650 m. The concentration of propylene was much higher than 0.1%, except three


225

Table 3

The relationship between the content of the alkene gaseous components and the sediment lithology

Depth (m)

CH4 (%)

C2H6 (%)

C2H4 (%)

C3H8 (%)

C3H6 (%)

C1/C2+

Lithology

50

99.15689

0.083502

0.034148

0.056934

0.668522

117.609

grey sandy mudstone

100

99.86365

0.080731

0.007177

0.028719

0.01972

732.419

grey sandy mudstone

200

99.90613

0.040794

0.007121

0.005427

0.040528

1064.313

grey sandy mudstone

340

99.98105

0.013348

0.000895

420

99.12118

0.058957

610

98.52298

0.067835

0.01106

1.398122

66.70399

grey mudstone

700

99.38605

0.095709

0.038138

0.480108

161.8785

grey sandy mudstone

810

97.51492

0.015495

0.02113

2.44845

39.24022

grey sandy mudstone

900

98.28584

0.094587

0.022432

1.597138

57.33772

grey mudstone

1000

99.22197

0.084446

0.02639

0.667189

127.5305

grey sandy mudstone light grey sandy mudstone

0.00179

0.00292

5275.28

grey mudy siltstone

0.011485

0.808381

112.7885

grey sandy mudstone

1110

99.71123

0.089303

0.027499

0.171964

345.3013

1200

99.23444

0.121632

0.058541

0.585383

129.6239

grey mudy siltstone

1300

99.5965

0.130071

0.072152

0.201274

246.8333

light grey sandy mudstone

1400

98.45848

0.09551

0.04258

1.403426

63.87123

grey mudstone

1500

98.87298

0.167874

0.096148

0.862997

87.72958

grey sandy mudstone

0.074415

3.711334

24.48674

grey mudstone

0.004335

0.282233

330.8387

light grey siltstone

1600

96.07639

0.13786

1700

99.69865

0.005903

0.00888

samples of 100 m, 200 m and 340 m. The propylene content was remarkably in good relationship with the lithology, decreasing from fine-grained rock to coarsegrained rock, in turn from the grey mudstone, sandy mudstone, siltstone to the mudy siltstone. It is possibly because mudstone and sandy mudstone were the intense gas-producing rocks, while siltstone was inclined to be reservoir rock. Gases should migrate from the mudstone into the siltstone through diffusion and infiltration due to the concentration gradation. During migration, methane leaved source rocks into reservoirs more easily than those heavy gaseous components because the source rocks adsorbed methane more weakly. Therefore much wetter gaseous components should stay in source rocks. This phenomenon exists widely in the thermal gases[20]. By interfusion with the immature thermal gases, it is common that the concentration of C2+3 increased gradually with increasing buried depth[17]. In the Sanhu depression, the increasing content of C2+3 should not be attributed to the thermal gases mixing. The reasons are: (1) Heavy gaseous components were mainly alkenes, other than the saturated heavy gases. By contrary, the natural thermal gases were mainly composed of saturated hydrocarbons. Therefore, the high concentration of C2+3 alkene was not the result of thermal cracking of organic matter, but associated with bacteria degradation of organic matter accompanying biogenic methane formation. Further research is needed to reveal how the alkene gases produced during bacteria activity. (2) Biogenic heavy gaseous components exist in nature. The 226

biogenic ethane with light carbon isotope has been discovered in recent years[21,22]. δ 13C2 in Luliang gas field of China was lower than –60‰. Xu et al.[22] eliminated the possibility of migration fractionation of thermal ethane, and confirmed that trace wet gases should be bacterially produced during the formation of biogenic methane. It was regretful that carbon isotopes of C2+3 were unavailable in all of our samples. Alkene is unstable in the geological condition, and easily transforms into the associated saturated hydrocarbons through gaining hydrogen. Thus, the existence of a high content of propylene in those gases should indicate that methanogenesis is still active at present.

3 Conclusions Through analyzing the dissolved gases from canned cutting in XS 3-4 well of the Qaidam Basin, the following conclusions are obtained: (1) The main factor determining the formation pathway of biogenic methane is the kind of nutrition. The direct factor may be the concentrations of acetate in the associated groundwater. Methane generated by acetate fermentation may become rich in 13C in a confined system. (2) Even in terrestrial environments, the CO2 reduction, other than acetate fermentation, is a possible pathway to form commercial gas accumulation. (3) A certain content of propylene indicates that methanogenesis is active at present.


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