Microbial alteration of natural gas in Xinglongtai field of the Bohai Bay Basin, China

Chin J Geochem. (2012)31: 055-063DOI:10.1007/11631-012-05493Microbial alteration of natural gas in Xinglongtaifield of the Bohai Bay Basin, ChinaYANG Weiwei", LIU Guangdi, GONG Yaojin, and FENG Yuancting, China University of Petroleum, Beijing 102249, ChinaResearch institute of Exploration and Development, Petro China Liaohe Oilfield Company, Panjin 124010, ChinaPetro China Changqing Oilfield Company, Xi'an 710021, China*CorrespondingauthorE-mail:yangweiwei_850101@yahoo.com.cnReceived January 17, 2011; accepted February 20, 2011o Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2012Abstract Xinglongtai field has been an important petroleum-producing area of Liaohe Depression for 30 years. Oilexploration and production were the focus of this field, but the gas was ignored. This study examined twenty five gsamples with the purpose of determining the gas genetic types and their geochemical characteristics. Molecularcomponents, stable carbon isotopic compositions and light hydrocarbons were also measured, and they proved thatmicrobial activity has attacked some of the gas components which resulted in unusual carbon isotopic distributionsPropane seems to be selectively attacked during the initial stage of microbial alteration, with abnormally lower con-tent compared to that of butane as well as anomalously heavy carbon isotope. As a consequence, the carbon isotopicdistribution among the gas components is partially reverse, as 8C<8c2<8c>>8C4. Besides, n-alkanes of C3+gas components are preferentially attacked during the process of microbial alteration. This is manifested thatn-alkanes are more enriched in C than corresponding iso-alkanes. As a result, the concentrations of n-alkanes becarbony low, which may be misleading in indentifying the gas genetic types. As to four gas samples, light hydro-carbon compositions display evidence for microbial alteration. The sequence of hexane isomers varies obviouslywith high content of 2, 3-DMC4, which indicates that they have been in the fourth level of extensively bacterial al-teration. So the geochemical characteristics can be affected by microbial alteration, and recognition of microbialalteration in gas accumulations is very important for interpreting the natural gas genetic typesKey words Liaohe Depression; Xinglongtai field; microbial alteration; carbon isotope; light hydrocarbon1 Introductioneasily affected compared with C bonds. So in theprocess of microbial alteration, molecules, composedMicrobial alteration of oils has been well docu- of light isotope C, are preferentially oxidized, asmented up to now(Baily et al., 1973: Milner et al., result the carbon isotope composition of the oxidized1977; Rowland et al., 1986; Blance and Connan, 1992, component residues becomes heavier and the contentGeorge et al., 2002; Roeling et al., 2003; Larter et al., reduces. It can be summarized from the relevant lit2003: Larter and diprinio, 2005; Ma Anlai et aleratures and case studies as follows2005). And there have been several studies concerningFirstly, bacterial alteration of methane has beenmicrobial alteration of gas accumulations, especially suggested in several documents(Lebedew et al., 1969;in recent years(James and Burns, 1984; Zhang Linye Zehender and Brock, 1979, 1980; Coleman et aland Li Xuetian, 1990; Pallasser, 2000 Katz et al., 1981; Rice et al., 1981; Zyakun et al., 1985; Ward et2002; Dai Jinxing et al., 2004; Wang Wanchun et al, al., 1987; Blair and Aller, 1995; Kotelnikova, 20022005, 2006, 2007, 2008; Kinnaman et al., 2007: Sassen et al., 2004; Dai Jinxing et al., 2004, 2005). InKniemeyer et al., 2007). Microbial alteration of gases the Koshelewska area of Ukraine, the methane stablehas been found in a variety of geologic settings, both carbon isotope of the recent sediments becomes heavonshore and offshore, in China, offshore Australia, ier with the decreasing depth and it gets to-30%0western Canada, Ukraine and the USA ( Lebedew et al., the Earths1969).Riced1969; Fuex, 1977; Dai Jinxing, 1990; Katz et al, 2002; Claypool (1中国煤化工gases InWang Wanchun et al., 2005: Kinnaman et al., 2007; near-surfaceCN MH Gand CalifoGao Gang et al., 2008). Coleman et al.(1981)pro- were isotopically heavy methane. Also, it was reportedposed that C bonds in alkane molecules were more that the methane stable carbon isotope of the gas seepwww.gyig.ac.cnwww.springerlink.com2 SpringerChin.Geochem. (2012)31: 055-063in Boshan of Yunnan Province in China ranged from duction in 1971. The main hydrocarbon reservoirs of-53. 1%o to -53. 3%o, which was a little heavier than that this region are the Paleogene Dongying Formationof typical biogas(Dai Jinxing et al., 2004). All of the (Ed), Shahejie Formation(Es)and buried hill reservoir.above phenomena are attributed to microbial oxida- And the gases are mainly produced from the Paleotion of methane whose original carbon isotope is gene Dongying Formation(Ed) and Sha 1 Memberlighter. Besides, the gas may become wet due to the(Esi) of the Paleogene Shahejie Formation with thedecrease of methane(Coleman et al., 1981)depth lower than 1800 m. However, the shallow gasSecondly, it was proposed that microbial altera- was ignored and unexploited during approximately 40tion can extend into the range of Ci-C4 gaseous hy- years of hydrocarbon exploration by Liaohe Oilfielddrocarbons, preferentially attacking propane during Company because of the limited technology and thethe initial stages, resulting in unusual carbon isotopic lack of well logging. As a consequence, the fields gascompositions(James and Burns, 1984). Through the has been largely understudied. Geochemical studiesgeochemical study on natural gas in the shallow res- on natural gas can provide important information forervoir in Binhai area of the Bohai Bay Basin, Zhang gas genetic types and gas accumulations which mayLinye and Li Xuetian(1990) found that propane turn, help guide future exploration. This study wasseemed to be selectively attacked, resulting in anoma- taken to examine the microbial alteration of naturallously heavy propane carbon isotopic compositions. gas accumulations within the Xinglongtai OilfieldBacterial attack of propane has been found in Dagangfield of the Bohai Bay Basin and the Junggar Basin, 2 Analytical methodstoo Dai Jinxing et aL., 2004). Besides, Wang Wanchunet al.(2005) proposed that propane and normal butaneTwenty five gas samples from Xinglongtai fieldwere attacked by microorganism, whereas carbon iso- were collected and analyzed in this study(Fig. 1)topic compositions of methane and ethane were not They cover both a wide geographic and stratigraphicchanged in the bamianhe field of Jiyang Depression. range, with sampled reservoirs ranging in depth fromAnd Kinnaman et al. (2007) pointed out that all Cl-C4 1249 to 1934 m. Samples were collected from thegases were oxidated in coal-oil point of hydrocarbon production well using 500 mL stainless steel cylindersseep field of offshore Santa Barbara, with an apparent with double valves, and were analyzed at the Keypreference for propane and butane over methane and Laboratory of Natural Gas Exploration and Develop-ethane. The simulating test of microbial degradation ment in the Research Institute of Petroleum Explora-of natural gas has proved this point of view (Wang tion and Development-Langfang(RIPED-Langfang),Wanchun et al., 2008)PetroChina within 20 days after collectionThirdly, the smaller the carbon number of hydroIndividual gas components were performed usingcarbon molecules is, the easier the bacterial alteration Agilent 6890 N gas chromatograph(GC) fitted with awill be( Gao Gang et al., 2008). Based on the analysis thermal conductivity detector(TCD ). With helium asof geochemical characteristics of natural gas in Yan- the carrier gas, a 30 mx0. 32 mmx20 um PLOT Q or amuxi field of Tuha Basin, Gao Gang et al.(2008) 30 mx0.32 mmx20 um PLOT 5A MS column wasproposed that components such as methane, ethane used. The inlet temperature was set at 150C, and theand propane are most easily attacked, then the normal TCD temperature was 200C. The oven temperaturebutane and pentane, and isoparaffins such as was initially set at 40C for 7. 5 min isothermallyiso-butane and iso-pentane have strong ability to resist Then it was programmed to 90C at 15C/min, finallymicrobial alterationfrom9o0tol80℃at6℃/min. Data are reported asHowever, the questions as to how the alteration mole-percentactually occurs and why the bacteria seem to removeThe stable carbon isotopic compositions ofone or more components selectively are not clear yet. gaseous hydrocarbons were performed on a Delta PlusIt is possible that there exit different kinds of bacteria isotopic mass spectrometer(Thermo Fischer Scientific)in different geologic settings(Dai Jinxing et al., 2004). equipped with an Agilent 6890 N GC. A capillarySo we have to study the characteristics of microbial column(30 mx0. 32 mmx20 um PLOT Q)was used toalteration in a certain geologic setting in order to pro- separate individual hydrocarbon gas componentsvide important information for genetic identification (C1-C5). The inlet temperature was set at 150 C, andof natural gas and gas-source correlation In this arti- the temperature of oxidation oven was 950C.Thecle, we focused on the recognition of microbial altera- initial gas chromatograph temperature started at 30Ction in natural gas accumulations within the Xin- for 5 min isothermally, ramped to 80C at 8C/minglongtai Oilfield of the Bohai Bay Basin, Chinthen rose fre中国煤化工n, and finally itThe Xinglongtai field, located in the central was prograLiaohe Depression of the northeastern Bohai Bay Ba- ported in staCNMHGO. Data are re-e to thesin(Fig. 1), was discovered in 1969 and began pro- Formation belemnite(PDB) standard and theChinJ Geochem. (2012)31: 055-063sion of analysis is #0.1%0.very low, with carbon dioxide of less than 0.34% andThe light hydrocarbons of the gases were meas- nitrogen ranging from 0.01% to 0.37%. The gas dryured on HP 5890 A GC with a HP PONA capillary ness indexes C/CI-s ranging from 0.945 to 0.999 andcolumn with dimensions of50m×0.25mm×25pmC/(C2+C3)ratios ranging from 21 to 831 display thatHelium was used as the carrier gas, and the inlet tem- examples can be classified as dry gases except forperature was held at 120 C. The components were Xing 283. Dry gas may be biogas, thermal crackingcollectedcold trap for 5 min, and the eluting gas(vitrinite reflectance Ro>2.0%)or secondary gashydrocarbons were detected using flame ionization migrating from the deeper reservoir for a long dis-detector( FID)at the temperature of 320C. The oven tance, and it should be noted that microbial alterationtemperature was initially set at 25C isothermally for of natural gas can also reduce the gas wetness througl15 min, then rose to 70'C at 1.5C/min, and then pro- the preferential removal of the wet gas componentsgrammed to 160℃at3℃/min, finally from160ames and Burns, 1984)280℃at5℃/ min with a final hold time of20minThe iCa/nC4 ratios ranging from 1.78 to 34.0 aremuch higher than 1.0. This ratio is thought to decrease3 Analytical resultswith increasing thermal maturity because of the generation of n Ca, and it remains constant of-075 when3.1 Gas compositionthe thermal maturity levels attain the oil-window(Heroux et al 1979). However, no trend in theMolecular compositions of gases from the Xin- iC/nCa ratio with depth is observed in Xinglongtaiglongtai field were listed in Table 1, and nearly all of field(Fig. 2), and the iCa/n Ca ratios of ten samples arethe gas samples are dominated by methane, ranging higher than 10, which indicates that the gases havefrom 96.4% to 99.8% of the total gas and averaging experienced some alterations. As with the gas wetness,97.9%. Furthermore, higher molecular components, this ratio can be altered through microbial processesfrom ethane to pentane in most samples are detected. which preferentially remove nC4 and therefore causeEthane accounts for 0.12%-2.29% with an average an increase in the iC/nCa ratio or even lack of nCAvalue of 1.42%. There is also a small amount of pro- component, thus making the samples appear less ma-pane, butane and pentane with the max proportion of ture. Most gas samples of Xinglongtai field appear to0.95%. The proportion of non-hydrocarbon gases is show this effect (Fig. 2, Table 1)Table 1 Gas composition data of the Xinglongtai oilfield: normalized air-free mole-pcenDepthReservoiriCsH1? nCsH12绑)(%)·(‰)(%)(%)(%(%)Xing 11-10c1534.9Ed0.000.1797.050.040010.0000197012.170.130.440.02001Xing 2011414.70.000.1597781.720030.30Xing 2151511.8000.1197371.850.12001Xing 215-21846250.320.1096481.940.340.360.150021407550000.1598.30Xing 251420.20.160.1298.640740020.310010.00Xing 2650000.1197561.700.090.340.02008Xing 281l5656E0.3400998420750.040290.010.0Xing 2831.01043.570.950.44Xing 408E:510000.2699.520.180.020020010000.00Ing148850000.2297891560.14Xing 459c1403.80.000.3199460.220.Xing 5-6124980.000.1299.760.120.000.00.001674.40.3300596532290.130.3900200lXing 9-11148290.150.0997.010.350.10Xingbei 31435.3Xingqi 11151145Ead0000.1097541.840.090.341826.75Easy0.180.0698.390.970.050.190.010000.1098.181.300.070Xingqing0000.2397721.750.080.100.03004001Xingqianqi 101550.5500002497561.730.130.0.0Xingqianqi 161542.30000.3799.500.13中国煤化工Xingqianqi 6583.8Eyd0.000.1897.211.9752End0.000.2197.4CNMHG 0.0Chin J Geochem. (2012)31: 055-063Proven gas reserveGas sampleFault2kNole. The contour of XinglongtaiFig. 1. Location of the Xinglongtai field and the gas sampling3.2 Stable carbon isotopesthere are large differences between methane and eth-ane carbon isotopes in the Xinglongtai field, with theThe stable carbon isotopic composition of indi- largest one being 26.2%0 of Xing 5-6 gas samplevidual alkanes in Xinglongtai gases was listed in Table Similarly, the difference between ethane and propane2. The methane carbon isotopic compositions have a' carbon isotopes is as large as 17.3%o(Xing 201 gasbroad range from-52. 4%o to-42 4%0, with an average sample). Thus the gases of Xinglongtai field discussedof-45.9%o The ethane 8 C values range from.8%0 herein are rare and it's very difficult to determine theirto-17.4%0, with an average of -26.58%0. Except for genetic typessome C3+ gas components are lack of isotopic databecause of their low contents, the propane 8C valuesrange from -22%0 to%0, the n-butane 8 3C valuesiCa/n Cahave a range between -24%0 and-17.4% and then-pentane 8C values have a range between -24.8%02030and-22. 1%. In contrast, the ranges of the carbon iso-1200topic compositions of iso-butane and iso-pentane aremuch narrower, from-28.9%0 to -25.2%0 and-262%0to-24.2%0, respectively.1400James(1983)and Galimov (1988)have pointedout that the differences of carbon isotopes between thenormal alkane components of unaltered gas from one目160source type had a limited range. At lower maturities(Ro <0.5%), the difference between the carbon isotopeof methane and that of ethane ranges from 15% to80018%o, and the difference between ethane and propanecarbon isotopes is around 6%0. With the increase ofmaturity, the separations reduce gradually. At over-2000mature stage (Ro.5%), the difference between中国煤化工methane and ethane carbon isotope is about 7%0, and2200CNMHGthe difference between ethane and propane carbonisotopes is around 2%0. However, as shown in Table 2Fig. 2. The iC,/nCA ratio indicating depth.Chin J Geochem. (2012)31: 055-063Dai Jinxing et al. (2005)has pointed out that tive proportions of Cs, C6, C7 n-alkane, iso-alkane andnatural gas with ethane 8C values greater than cycloalkane show that the gas in Xinglongtai field is27.5%o and propane 8C values greater than-25.5%0 coal-derived gas accordingo Hu Tilings criteriaoriginated from coaly sources, suggesting the possi-(1990)(Fig. 3b). The abundances of the Cs, C6, Cbility that most of the gas samples examined are n-alkanes which are believed to be derived from thecoal-derived gas. However, the methane 8C values sapropelic with very low organic matter, have the avare indicative of oil-associated gas. It should be sug- erage percentages of 8.1%, 5.3% and 1.6%, respecgested that if partial microbial oxidization has octively. However, the abundances of iso-alkanes thatcurred, the carbon isotopic compositions of the residare mainly originated from the humic organic matterual gas components would be heavier and the genetic are relatively high with the average proportions oftypes of natural gas might be misinterpreted( Coleman 68.1%, 78.0% and 43.8%. The responsible reason foretal,1981)the two contradictorystatements may be bacterial alteration. Bacterial alteration may affect the compos3.3 Light hydrocarbonstions of light hydrocarbons, and n-alkanes are moreeasily depleted by biodegradation than branched alAlthough the content of the light hydrocarbon kanes and cyclic alkanes(Lin Renzi, 1992; Zhang(Cs-Cn)is usually low in natural gases, it can provide Linye et al., 1996; George, 2002). So the low contentuseful information for identifying the genetic type and of n-alkane and relatively high iso-alkane concentra-origin of the natural gas. One remarkable characteris- tions of Cs-C7 light hydrocarbons may be misleadingtic of the gases in Xinglongtai field is the low content in indentifying the gas genetic typeof Cs-C, aromatic hydrocarbons that is believed to bederived from the humic organic matter (Leythaeuser et 4 Discussional.,1979). As shown in Fig 3a, the relative percent-ages of Cs-C aromatic hydrocarbons have a rangefrom 0.5% to 4.7%, whereas the concentrations ofThe analytical results showed that gas compo-alkane and cycloalkane are relatively high, ranging nents, carbon isotopic compositions of gaseous hyfrom 44.8% to 81.7% and from 17. 8% to 50.5% redrocarbons and light hydrocarbons of Xinglongtaively. And this figure shows that the gas herein field were obviously abnormal. Microbial alterationhas characteristic of oil-associated gas. But the rela- discussed below is attributed to these phenomenaTable 2 Stable carbon isotope composition of the Xinglongtai oilfield: PDBReservoir10 nC4H1o iCsH1? nCsH12(%)(%o)Xing11-10c153494,46-22.020.6Xing 14-26,2Xing 201414.744.21511843.8-26.114.8196Xing21521846.2542725.6-199274-20,8Xing248c1407.5544.42589626.91420.2E3d-424-2217.0Xing2651529643.7-26.114.6280Xing 2811565643.3-22.514.524.8Xing2831690.440.2-26023.2274-23.1-26.1-51.3Xing 4261488.5-3181403.850.6277Xing 5-6443.6Xing 6642.8-26.-13,2289253-22.1Xing 9-1143.726.128.225322944.1-259-100-28.243.6-26.028.1Xingqi 128-255-18.721.01465413.5-28.0249Xingqianqi 152419824.0Xingqianqi 10 1550.5551.018,2中国煤化工481649028317.2-2CNMHGXingqianqi 6 1583.851.030.7-21.328.024026.2ChinJ Geochem. (2012)31: 055-063Alkane (%InC(%)70010203040506070809001020304050607080Cycloalkane ("Aromatic ( n)C(%)Cyc (Fig 3, Distribution of light hydrocarbons in Xinglongtai field. The relative content of aromatic hydrocarbons is low(a) and that of the nC (Cs. C6or C)is very low(b)(nC. normal alkane; iC iso-alkane; CyC cycloalkane)The carbon isotopic compositions of natural gas microbial alteration could lead to heavier propanecomponents in normal, unaltered gases are known to carbon isotope but not apparently decreasing thechange with increasing molecular weight. Two types abundance of propane, as indicated by five samplesof carbon isotopic compositions have been observed: a Xing 11-10c, Xing 215-2, Xing 283, Xing 426 andnormal order (i.e, 8 C1<8 C2<8C3<8C4)andXingqianqi 8 in Xinglongtai field(Fig. 5).Further-reverse orrder (i. e,8C1>8C2>8C3>0 C4)(Fuex, more, carbon isotopic compositions of propane from1977; James, 1983; Dai Jinxing et al., 2004). Howeverthose five samples are less enriched inC ( Table 2),the gas samples in Xinglongtai field do not display indicating that the degree of alteration is less extenthis regular carbon isotopic variation but display apartially reversed isotopic order, with anomalouslyIt is generally believed that the isotopicheavy propane carbon isotopic compositions(Fig. 4, tioon of ethane may also be altered by bacterial proc-Table 2). Dai Jinxing et al. (2004)presented that par- esses, although to a much less degree (James andtially reversed isotopic order of hydrocarbon compoBurns, 1984; Wang Wanchun et al, 2005).Thisnents in natural gas can result from four mechanisms; pears to be the cases for at least the Xing 5-6 and Xingincluding the mixing of biogenic and abiogenic al- 254c gas samples, with the carbon isotopic composikanes, mixing of humic and sapropelic gases, mixing tions of ethane are-17.4%0 and -22. 1%o, respectivelyof gases from source rocks of different maturity, or (Table 2). They seem to be more enriched in 13c inbacterial alteration of alkane components. As abio- contrast to the normal gas(Fig. 4). Compared withgenic gas has never been discovered in this sedimen-ry basin, the mixing of biogenic and abiogenic al-propane, ethane seems to be not attacked or slightlyattacked by bacteria. As a result, carbon isotopicanes is excluded firstly. If it is supposed that the par- compositions of ethane will not change or be enrichedtially reversed isotopic order herein is caused by mix- in C slightly, while that of propane become moreing of gases from sapropelic and humic gases or mix- enriched inC. According to James and Burns( 1984),ing of gases from source rocks of different maturity,the enrichment in the carbon isotopic compositionsthere should be one type of gas with the carbon isemay exceed 20%. In Xinglongtai field, 8c values oftopic composition of ethane heavier than-17.4%0 and the ethane and propane appear to be enriched by aspropane carbon isotope heavier than -7%0. But this much as 13%0 and 21%0 in comparison with the nortype of gas has never been discovered in Liao Depresmal gas, respectively. That is why the differences ofsion, and it's rare all over the world. So, the partially carbon isotopes between methane, ethane and propanereversed order for gases in Xinglongtai field can not are greater than those of normal gas. The genetic typesbe well explained by above three mechanisms. Micro- may be misinterpreted by this enriched carbon iso-bial alteration seems to be much more reasonable for topic compositionsthis unusual gas. Compared with butane(Table 1, Fig5), it is evident that microbial alteration has preferen-Another evidence for bacterial alteration is pre-tially attacked propane. In the process of microbial contents ofsented within the carbon isotopic compositions andTLe n-butane analteration, the isotopically light propane was priorly n-pentaneremoved, leaving the residual propane enriched inc display unus中国煤化工 inglongtai fieldICNMHGositions relativeand resulting in heavier carbon isotopic compositions to the corresponding iso-butane and iso-pentane (Ta-of propane. However, it should be pointed out that the ble 2, Figs 4 and 6). Microbial alteration of gases ap-Chin J Geochem. (2012)31: 055-063pears as the preferential removal of isotopically light almost depleted, causing the misinterpretationnormal alkanes, resulting in isotopically heavier genetic types. Above all, microbial alteration ofn-butane and n-pentane components than that of can affect the Normal alkanes As bacterialiso-butane and iso-pentane. Also, there have been ob- tion proceeds, the C normal alkanes will be exviously decreases in the n-butane and n-pentane con- tremely eliminated resulting in a significant decreasecentrations(Table 1). Consequently, the iCA/nCa ratios in gas wetness. The extensively altered gas could bebecome so great that make the gas samples seem less confusing easily with overmature dry gas whose wetmature. This phenomenon is similar with the studies components have been destroyed largely by extensiveof Katz et al.( 2002)and larter et al. (2005)thermal cracking ( Jaames and burns, 1984). Take theXing 254c gas sample for example, if microbial al-teration had not been recognized, the origin of this gascould have been misinterpreted from molecular com-positions only. But it is unlikely that thermal crackingcould result in the apparently selective degradation ofpropane, they also exhibit a progressive increase in thecarbon isotopes with increasing molecular weight.Microbial gasRegular gasiCa CFig. 4. Compound-specific carbon isotopic composition of gas sam-ples in Xinglongtai field displaying enrichment in c as a result ofmicrobial alteration001The last but not the least evidence is the charac-teristics of light hydrocarbon compositions. Lin Renzi Fig. 5. Components of wet gases in the Xinglongtai field. The solid(1992)proposed that light hydrocarbon compositionsline indicating the extensively altered gases whose propane contentscould be used to indentify microbial alteration, among, are lower compared with that of butane; the dotted line displaying thewhich the isomers of hexane varied obviously. Theless extensively altered gases with unapparent decrease of propaneconcentrations of iso-hexanes display in a certain or-der of unaltered gases as they are not affected by theorganic matter, maturity or migration. The orderIt is evident that microbial alteration actually atshould be 2-MCs >3-MCs >2, 3-DMC4 >2, 2-DMC4, tacked the natural gases in Xinglongtai field throughwhereas it would be changed evidently by microbial changing the unusual characteristics of gas compo-alteration(Lin Renzi, 1992; Zhang Linye et al., 1996). nents, carbon isotopes and light hydrocarbons. PropaneIn Xinglongtai field, four gas samples(Xing 201, is believed to be the most extensively attacked com-Xing 248c, Xingbei 3, and Xingqi 12) appear to show ponent herein, and normal alkanes seem to be preferthis feature(Fig. 7). The sequence of hexane isomers entially attacked compared to the correspondingchange obviously with relatively high content of iso-alkanes in the microbial process. However, the in-2, 3-DMCa, which indicate that they have been in the triguing questions as to why the bacteria seem to re-fourth level of extensively bacterial alteration accord- move propane selectively over the other componentsing to Lin Renzi (1992). It should be noted that mi- and why the normal alkanes are more easily affectedcrobial alteration may change the concentrations of than iso-alkanes are not clear yet. Combined with thehexane isomers, but not necessarily. From the charac- published documents (ames and Burns, 1984; Wangteristics of the molecular components and carbon iso- Wanchun et al., 2007, 2008; Gao gang et al., 2008),tope compositions, it's clear that the other twenty-one the authors suppose that microbial alteration may begas samples were also attacked by microbial alteration influenced by two factors in Xinglongtai field. One isto various extents. But they do not display obvious propane-oxidizing bacteria dominates in this areachanges in concentrations of iso-hexanesleading to extensively alteration of propane. The otherDuring the process of biodegradation, normal al- one is that th中国煤化工 of iso-alkanes iskanes seem to be preferentially attacked, resulting in greater thandecreases in the contents. As shown in Fig. 3b, the molecular ceCNMHtheir different.bviously reduc-content of n-pentane, n-hexane and n-heptane were tion ofChin J Geochem. (2012)31: 055-063o iso-pentane16● n-butane●0.50.15Fig. 6. Observed relationship between the stable carbon isotopic compositions and abundance of iso-butane and n-butane, iso-pentane and n-pentandmicrobial alteration, which is manifested in the carbonisotopic compositions and light hydrocarbons. Firstlythe isotopic compositions of n-butane and n-pentaneare abnormally heavier than that of the correspondingiso-butane and iso-pentane. Secondly, the iC,/nCa ratios are elevated as a result of the n-butane reductionFinally, the n-alkanes of Cs-C7 light hydrocarbons aredepleted by bacterial alteration, which may be misleading in indentifying the gas genetic typesThe light hydrocarbon compositions of four gas2-MCs 3-MCs 23-DMC4 22-DMC4samples display evidence for microbial alteration. TheFig. 7. Relative content of hexane isomers in the Xinglongtai field.sequence of hexane isomers vary obviously with highThe solid line indicating the extensively altered gases with the ordercontent of 2, 3-DMC4, indicating that they have beenvaried; the dotted line displaying the less extensively altered gasesin the fourth level of extensively bacterial alterationwith the normal order of iso-hexane. 2-MCs. 2-methylpentaneAbove all, the geochemical characteristics of the3-MC3. 3-methylpentane; 2, 3-DMC4 2,3-dimethylbutane: 2, 2-DMC4natural gas may be attacked by microbial alteration2, 2-dimethylbutaneRecognition of microbial alteration in gas accumulations is very important for interpreting the natural gas5 Summary and conclusionsgenetic typesAcknowledgements The authors are grateful toMethane contents of twenty five gas samples the reviewers for the helpful and constructive com-examined range from 96.4% to 99.8% of the total gas. ments. Thanks are also given to Engineer MinIn addition to the Xing 283 sample, all the samples Zhongshun from the Liaohe Research Institute of Ex-can be classifieas dry gases. The iCA/nCa ratios ploration and Development, PetroChina, for his assis-range from 1.78 to 34.0, and no trend with depth was tance in gas samplingobserved indicating that the gases have experiencedsome alterationsReferencesThe carbon isotopic compositions of natural gascomponents display a partially reversed isotopic order,with anomalously heavy propane carbon isotopesBaily N.J. L, Krouse H.R., Evans C.R., and Rogers M.A.(1973)AlterationCombined with the abnormally lower content thanof crude oil by waters and bacteriaEvidence from geochemical andbutane, propane seems to be selectively attacked. Theisotope studies [J]. AAPG. 57, 1276-1279Blair N.E. and Aller R C (1995) Anaerobic methane oxidation on the Ama-ethane isotopic composition may also be altered byzon shelf []. Geochimica et Cosmochimica Acta. 18, 3707-3715.bacterial processes to a much less degree. This altera- Blanc PH and Conman J.(1992) Origin and occurence of 25-porhopanestion can result in the enrichment of ethane and pro-Geochemistry. 18, 813--828pane ofc by more than 10%o. That is why the geColeman DD.中国煤化工 fractionation of carbonnetic types may be misinterpreted by the carbon iso-topic compositionsCNMHGiZing bactenia可GeochThe C gas components can also be altered by Dai Jinxing (1990)A brief discussion on the problem of the geneses of theChin J Geochem. (2012)31: 055-063carbon isotopic series reversal in organogenic alkane gases [J]. Natu-Wuhan(in Chinese)a Gas Industry. 10, 15-20 (in Chinese with English abstract)Ma Anlai, Zhang Shuichang, Zhang Dajiang et al. (2005)The advances inDai Jinxing, Xia Xinyu, Qin Shengfei, and Zhao Jingzhou(2004)Origins ofthe geochemistry of the biodegraded oil []. Advances in Earthpartially reversed alkane 8C values for biogenic gases in China []Science. 20, 449-454 (in Chinese with English abstract)Organic Geochemistry. 35, 405-411Milner C W.D., Gogers M.A., and Evans CR(1977)Petroleum transfor-Dai Jinxing, Qin Shengfei, Tao Shizhen, Zhu Guangyou, and Mi Jinkuimations in reservoirs [J] Journal of Geochemical Exploration. 7,( 2005)Developing trends of natural gas industry and the significant101-153.progress on natural gas geological theories in China [J]. Natural Gas Pallasser R.(2000)Recognising biodegradation in gas/oil accumulationsGeoscience. 16, 127-142 (in Chinese with English abstract)hrough the 8C compositions of gas components [J] Organic Geo-Fuex A N.( 1977) The use of stable carbon isotopes in hydrocarbon explorachemistry.31,1363-1373.tion [J]. Joumal of Geochemical Exploration. 7, 155-188Rice DD and Claypool E. G(1981)Generation, accumulation, and resourceGalimov E.M.(1988)Sources and mechanisms of formation of gaseouspotential of biogenic gas [J]. AAPG. 65, 5-25.hydrocarbons in sedimentary rocks [J]. Chemical Geology. 71,77-95Roeling W.F.M., Head I M, and Larter SR(2003)The microbiology ofGao Gang, Huang Zhilong, Liang Hao et al. (2008)Analysis on selectivityhydrocarbon degradation in subsurface petroleum reservoirs: Perspec-biodegradation of natural gas in Yanmuxi Oilfield of Tuha Basin pItives and prospects []. Research in Microbiology. 154, 321-328Acta Petrolei Sinica. 29, 494498(in Chinese with English abstract)Rowland S.J., Alexander R Kagi R L et al. (1986)Microbial degradation ofGeorge S.C., Boreham C.J., Minifie S.A. et al. (2002) The effect of minor toaromatic components of crude oils: A comparison of laboratory andmoderate biodegradation on Cs to Co hydrocarbons in crude oils [J]field observations [J] Organic Geochemistry, 9, 153-161Organic Geochemistry. 33, 1293-1317Sassen R, Roberts HH, Carney R. et al.(2004) Free hydrocarbon gas, gasHeroux Y, Chagon A, and Bertrand R. 1979)Compilation and correlationhydrate, and authigenic minerals in chemosynthetic communities ofof major thermal maturation indicators [J]. AAPG. 63, 2128-2144the northem Gulf of Mexico continental slope: Relation to microbialTilin, Ge Baoxiong, Zhang Yigang, and Liu Bin(1990)The develop-processes [J]. Chemical Geology. 205, 195-217ment and application of fingerprint parameters for hydrocarbons ab- Wang Wanchun, Zhang Linye, Liu Wenhui et al. (2005)Effects of biodeg-sorbed by source rocks and light hydrocarbon in natural gas [J]. Peadation on the carbon isotopic composition of natural gas-A casetrolewm Exploration and Experiment. 12, 375-379 (in Chinese withtudy in the bamianhe oil field of the Jiyang Depression, eastern ChinaEnglish abstract)U]. Geochemical Joumal. 39, 301-309.James A T(1983)Correlation of natural gas by use of carbon isotopic Wang Wanchun, Ren Junhu, Zhang Xiaojun et al.(2006)Geochemicaldistribution between hydrocarbon components J]. AAPG. 67.characteristics and origin of low-mature oil-associated1176-1191south region of Kongdian, Huanghua Depression []. Natural GasJames A.T. and Burns B.J.(1984)Microbial alteration of subsurface naturalgeoscience. 17, 153-159 (in Chinese with English abstract)gas accumulations J). AAPG. 68, 957-960Wang Wanchun, Li Junyuan, Tang Hongsan et al. (2007)Genetic characterKatz B., Narimanov A. and Huseinzadeh R(2002)Significance of mi-istics of associated gases from crude oil biodegradation in Gudao oil-crobial processes in gases of the South Caspian basin []. Marine andfield, Jiyang Depression []. Oil and Gas Geology. 28, 427-432 (inPetroleum Geology. 19, 783-796Chinese with English abstract)Kinnaman F.S., Valentine D L, and Tyler S C(2007) Carbon and hydrogen Wang Wanchun, Li Nengshu, Liu Wenhui et al. (2008)Simulating test ofotope fractionation associated with the aerobic microbial oxidationmicrobial degradation of natural gas [] Natural Gas Industry. 28,of methane, ethane, propane and butane [] Geochimica et34-37 (in Chinese with English abstract)Cosmochimica Acta, 71. 271-283Ward B B, Kilpatrick K.A. Novelli P C, and Scranton MIMeth-Kniemeyer O, Musat F, Sievert S.M. et al. (2007)Anaerobic oxidation ofane oxidation and methane fluxes in the ocean surface Id deeshort-chain hydrocarbons by marine sulphate-reducing bacteria [1anoxic waters [J Nature 327, 226-229.Nature.449,898901Zehender A.J.B. and Brock T D. (1979) Methane formation and methaneKotelnikova S.(2002)Microbial production and oxidation of methaneoxidation by methanogenic bacteria [J] Journal of Bacteriology. 137,deep subsurface [J]. Earth Science Reviews. 58, 367-395420432Larter S.R., wilhelms A, Head I et al. (2003) The controls on the composi- Zehender A.J. B. and Brock T D.(1980)Anaerobic methane oxidationtion of biodegraded oils in the deep subsurface, part 1: Biodegradationoccurrencelogy [J]. Applied Environmental Microbiology. 39,rates in petroleum reservoir []. Organic Geochemistry. 34, 601-613Larter S.R. and Diprinio R(2005)Effects of biodegradation on oil and gas Zhang Linye, Li Juyuan, Li Xiangchen, and Tang Hongsan(1996) Lightfield PVT properties and the origin of oil rimmed gas accumulationshydrocarbon constituents of the biodegradation gas and gas-source[]. Organic Geochemistry, 36, 299-310correlation []. Experimental Petroleum Geology. 18, 88-95(in ChiLebedew w.C., Owsjansikow W.M., Mogilewskij G.A. et al.(1969)ese with English abstract)fraktionierung der kohlenst off isotope durch mikrobiologis Prozesse Zhang Linye and Li Xuetian(1990) The origin of natural gas in the shallowin der biochemische zone [J]. Z Angew Geology. 15, 621-624reservoir in Binhai area in Jiyang Depression [J]. Petroleum Exploraeythaeuser D, Schaefer RG, and Corndord C(1979)Generation andtion and Development. 17, 1-7(in Chinese with Englishmigration of light hydrocarbon(C2-C7) in sedimentary basin [].yakun(1985) Caron isotop中国煤化工 Geokhimrya9Lin Renzi(1992)Light hydrocarbons Technique and its Application in OilcYH291-29CNMHGand Gas Exploration [M]. China University of Geosciences Press,