Geochemical characteristics and possible origin of natural gas in the Taibei Depression, Turpan-Hami

Chin J Geochem. (2010)29: 307-312DOl:10.1007/sl1631-010-0461-7Geochemical characteristics and possible origin ofnatural gas in the Taibei Depression, Turpan-HamiBasin ChinaZHAO Hongjing 23, ZHANG Min", and WANG ZhiyongChina University of Geosciences, Wuhan 430074, ChinaGeochemical Department of Yangtze University ingzhou 434023, ChinaP The Key Laboratory of Exploration Technologies for Oil and Gas Resources / Yangtze University), Ministry of Education, Jingzhou 434023, ChinaTuHa Oil field, Hami.CorrespondingauthorE-mailzhaodiana@sina.com;**zmjpu@163.nerReceived March 16, 2009; accepted May 4, 2009o Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2010Abstract 292 chemical composition data and 82 isotopic composition data of gas samples collected from the TaibeiDepression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas ispoor in most samples whereas abundant nitrogen in some samples is positively correlated with 8CI. Althoughethane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected inmost samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-edas8C1:45.5‰%to-335%,8C2:-302%to-10.5%,and8"Cg:27.6%to-112%,respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showingnormal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormaldistribution. Based on the isotopic composition, the disagreement between the relationship of A(8Cr-8C2)and8C2 and that of A(8Cr-8C2)and 8C2, and the calculated Ro, there are oil-associated gas, coal-derived gas andmixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of theTaibei Depression have been degraded by organismsKey words Taibei Depression; natural gas; carbon isotope; mixed gas; biodegraded gascult to constrain the origin of natural gas, in most1 Introductioncases based on its chemical composition and isotopiccomposition. The origin of natural gases in the TaibeiThe Turpan-Hami Basin, West China, is famous Depression is still an open question. Xu Yongchang etfor the emergency and evolvement of coal-derived oil al. (2008)proposed that the gas in the Turpan-Hamitheory in China(Huang Difan, 1999; Cheng Keming,Basin was coal-derived gas of low maturity. However1997; Cheng Keming et al, 2002; Zhao Changyi et al, Su Chuanguo et al. (2008)demonstrated that the gases1998; Chen Jianping et al, 1999; Wang Xiang and from the Qiudong and Wenjisang areas were mixtureZhang Min, 2008; Ding Anna et al, 1999). As more of oil-associated gas and coal-derived gas. The pur-than 0.3710 2 of gas was observed in the Tur- pose of this paper is to investigate the chemical com-pan-Hami Basin(Zhou Haihun, 2007), scientists have position and isotopic composition of natural gasespaid more attention to the origin of natural gas there. from the Taibei Depression of the Turpan-Hami BasinGeochemists put the focus of their study on the in order to constrain the gas origin.origin of natural gas since the 1980s(Schoell, 1983,1988: James, 1983: Jenden et al., 1993: Prinzofer and 2 Geological setting and samplesluc, 1995; Prinzofer and Pernaton, 1997; Clayton,1991; Chung nai Jinxing, 2000: Xu Yongchang et al., in theal., 1988; Liu Wenhui and Xu Yong中国煤化工 intermontane basinfold mountain sy1990: Zeng Fangang et al., 1998). Gas is simpler in temC Ghe first-order tedcomposition than oil. Therefore, it is relatively diffi- tonic units of the Turpan Basin include the Taibei Dewww.gyig.ac.cnwww.springerlink.comringerChin J Geochem. (2010)29: 307-312ression, the central uplift and the Tainan Depression. The distribution of higher hydrocarbons varies5%of exploration is focused on the Taibei Depres- depth( Fig. 2), they are generally low in shallown with an area of 9600 km. The Turpan Basin is ervoirs, and increase to the highest value nearasymmetrical, with the deepest and thickest sediments depth of 1600m, and then decreases gradually within the north, and the shallowest and thinnest sediments depth.in the south the mesozoic and cenozoic sediments inthe main depocenters are over 7000 m in thickness.Several potential source beds are distributed in theTaibei Depression: Carboniferous-Lower Permianmarine carbonates and clastics, the paralic lacustrinesediments of the Upper Permian Taodonggou Groupthe lower-Middle jurassic coal-bearing strata(mainlin the Badaowan and xishanyao formations), and theMiddle JiQiketai Formation(Yuan Mingshengetal,2002)There are 292 gas samples treated in thisChemical compositions and stable carbon and hydro-gen isotopic compositions of all the gas samplesFig. 1. Relative abundance of nitrogen and isotopic composition oflyzed by the PetroChina Tuha Oilfield Companyavailable in this study3 Results and discussion3.1 Chemical composition and possible originHydrocarbon gases are predominant in gasfrom the Turpan-Hami Basin in general, with a con-centration higher than 65%. The concentrations ofhydrocarbon gas in most samples are higher than 90%雪gPCOz and N2 are predominant in nonhydrocarbongases.The origin of nitrogen can be identified in termsdapo自of the ratio of N2/Ar( Chen Shijia et aL., 2000). Theratios are far higher than 84 with the minimum as 129,excluding atmospheric derivation of N2. Nitrogencontents in natural gas depend on the type of sourcerock, thermo-evolution degree and capture condition,and high nitrogen contents almost coincide withhighly mature and over-mature stages(Littke andKroos, 1995). There is an obvious positive correlationeen the logarithm of nitrogen contents and thetopic composition of methane when nitrogentents are higher than 0. 1%(Fig. 1), indicating the nitrogen contents in the Taibei Depression are possiblyrelated to gas thermal evolution. However, biodegra-dation can lead to an increase in the isotopic ratioFig. 2. Relative concentrations of higher hydrocarbons with depth innatural gas (Dai Jinxing, 2000). Therefore, whetherthe abundance of nitrogen is affected by biodegradetion or thermal maturation must be distinguished inThe abundance of methane, ethane and procombination with other informationfrom experimental simulation in a confined system ofMethane is the major constituent of hydrocarbon source rock can be used to characterize the sourcegases.However, heavy hydrocarbons are also in- type and maturity( Behar et al., 1992). In general, thecluded. The contents of higher hydrocarbons decrease range中国煤化工 nan that of C:Cwith carbon number. The wetness(C2-s/CI-s)of most and-om-l.02to5.92samples is lower than 50%, whereas samples with mainCN MH Gwith an average ofhigh humidity are always collected from oil reservoirs. 2.07, but the ratios of In(C2/C3)vary from-1.59 toChin JGeochem. (2010)29: 307-3124.77, mainly within the range of 0.0-1.5 with an av- 8C2, 83C3, respectively. All samples can be groupederage of 0.6, indicating the gas generation occurred in into 2 families in combination with Dais criteria(Figthe thermal degradation period. Abnormal high ratios 6). One group is characterized by normal carbon iso-of CyCy and C2/C3 for some samples indicate thetopic composition, located in the upper part. It is suggration over long-distance because the abundance of gested that the gas from Well Ling 2 is an oil-associmethane increases and that of higher hydrocarbons ated gas while the others are oil-associated anddecreases along the migration pathway. On the other coal-derived or mixed gases. As to the other group, 6hand, abnormal alkane composition with the negative samples from the Yanmuxi oilfield and Well Ba 18atios of In (CI C2)or In(C/C3)for some samples and Well Ge 8, located in the lower part of Fig. 6,suggests biodegradation(Fig. 3). In a word, natural have obviously heavier carbon isotopic compositions,he Taibei Depression are mainly primary indicating biodegradation of the samplescracking gases with the exception of migrated gas andCH4CH。CHCHFig. 4. Carbon isotopic compositions of gases in the Taibei DepressionBiodegraded?-150Fig 3. Plots of C,/Cz and Cy/, for gases in the Taibei Depression. IL.2Type Il organic matter. Ill. Type Ill organic matter3.2 Isotopic composition of gases and possiblegenesisCarbon isotopic compositions of methane for theC.H.gases in the study area vary from -45.5%o to -33 5% Fig 5. Hydrogen isotopic compositions of gases in the Taibei Depression.with an average of -40.4%o(Fig 4)while 8 Cis val-ues of most samples are -42%0-39%0, lying in theIt is difficult to constrain the origin of most samusual thermogenetic domain. The pattern exhibits ples merely based on the carbon isotopic compositionsgeneral positive slopes(8C; <8C with isj)(Fig. 4), of methane, ethane and propane(Fig. 6). Carbon iso-indicating organic origin. The change trend of hydro- topic composition of hydrocarbons becomes heaviergen isotopic composition of the gases in the study area with the process of maturation( Dai Jinxing, 2000)andis similar to that of carbon, i. e, 8D <8D2<8D3. The the difference between them decreases(rooney et al8Ds values of methane, ethane and propane in most 1995). Therefore, if hydrocarbon gas was not affectedsamples are within the ranges of -260.3%-219.7%0, by biodegradation or mixing, the difference between-230.0%‰--179.0‰,and-24.3‰-166,2% o resp8 C3-8C, and 8C2-87 shows this type of relationship between 8Cy8C2values of methane, ethane and propane of 2 samples and 8C2. However, the difference between"C2 andfrom Well Yan 6-17 and Yan 6-09 are abnormally Ch decreases with increasing 8C2, indicating thatheavy, >-150%o(Fig. 5), indicating the biodegradationof natural gas(Fig. 7), resulting from the mixing of methane withnormal&C, and methane with lighter8Cl. In order3.3 Diseussionto minimize the effect of biodegradation, samples withrevC∴38)areThe data on the distribution of carbon isotope of中国煤化工methane, ethane and propane can be used to study gasgenesis.Dai Jinxing(2000) proposed the criteria for Le IdCNMHGcalculate the r ofclassifying hydrocarbon gas types based on 8Cl. source rocks(Table 1). Rol, Roz, and Ro3 in Table I areChin J Geochem. (2010)29: 307-312estimated based on the regression equations between ence between Ro2 and roy but a considerable deference6C1,618 C3, respectively, for between Rol and Ro2 and Ro, respectively, for thecoal-derived gas proposed by Dai Jinxing (2000 ). The sample from Well Pu 6-1, indicating different genesesRos calculated by using the regression equations based of methane. No coal-derived gas and possibleon 8C1,8C2, and8 C, should be similar to those oil-associated gas can be deduced from the consider-of hydrocarbons of the same origin. For a sample from able differences among 8CI,8C2, and 8 C3 in theWell Le 101, three Ros are estimated to be ca. 0.61%, 2 samples from Well Ling 2 and Le 10suggesting coal-derived gas. There is a little differ.Ling2 Oil-associated gas & C(%)oil-associatedA Mixing gas, coal derived gas?Coal-derived gas Ge&22Fig 6Plots ofc and&C, vs &C;(after Dai Jinxing, 2000).6C2(%)-6℃:(%)Fig 7 Plots of 8Ca and 8Cr0"CI vs 8CSome isotopic ratios of methane, ethane propane contents of hydrocarbon gases do not decrease draand butane of samples listed in Table 2 are abnormally maticaon number and nitrogen contentshigh. These samples have a reverse carbon isotope are hig中国煤化工: may be less thandistribution and a high abundance of branched hydro- thosecarbon gases, i.e. iCnCa. 44. Moreover, chemical thanC Biodegradation ofcompositions for most samples are abnormal. The hydrocarbons led to a decrease in their contents andChin J Geochem. (2010)29: 307-312enrichment in heavy isotope, and shows a highimplof result from the biodegradation of one or more types ofbranched hydrocarbon to normal hydrocarbonhydrocarbon gasesing that the gases with heavier carbon isotope mayTable 1. Calculated Res of the typical samplesDepth(m)2748-275843.I6.5-2409Note:Rol.Roz,and R,3 were based on the regression equations between R, and 8C1.8C2, andC,, respectively for coal-derived gasWell No. AgeTable 2. Chemical and isotopic compositions of some gases from the Taibei Depressionepth(m)Esh16116-6291.220431.5313.32.2473.52-33.5-109-17.0-18.2594Yan6-28Esh1596-16165.010.978472-353-105-168-17.7Ba 1840.8Chen Jianping. Huang Difan. Li Jinchao, and Qin yong(1999)Main source4 Conclusionsocks of petroleum from Jurassic coal-bearing strata in Turpan HamiBasin []. Acta Geologica Sinica. 73, 140-152.In general, gas from the Taibei Depression is Chen Shijia, Fu Xiaowen, Shen Zhaoguo, Zhao Mengjun, and Huang Difanpoor in nonhydrocarbon. A few gas samples are rich in(2000) Molecular nitrogen genesis in natural gases and relationshipN2, whose high N2 contents are positively correlatedwith gas accumulation history in Tarim Basin [] Acta Sedimen-with 8CI. Hydrocarbons are enriched in most gases,tologica Sinica. 18, 615-623(in Chinese with English abstract)and their depth is corresponding to that of oil layers, Cheng Keming (1997) Hydrocarbon generation models of coal and maceralsindicating abundant higher hydrocarbons resultantfrom Turpan Hami Basin [] Chinese Science Bulletin. 42from existing conditions. Carbon and hydrogen iso.2102-2105(in Chinese with English abstract)topic compositions show positive slopes(8 Cm-1<8m, Cheng Keming. Xiong Ying. Zeng Xiaoming, Moldowan JM, and ToddDCm-1