Geochemistry and genesis on saturated hydrocarbon of crude oils in Gas Hure Oilfield in Qaidam Basin

Global Geology, 14(1): 7-14(2011)10.3969/isn16739736.2011.01.02Article Il:16739736(201l)01-0007-0Geochemistry and genesis on saturated hydrocarbonof crude oils in Gas Hure Oilfield in Qaidam BasinMO Xiaoye and ZHANG Min1. Department of Geochemistry, Yangtze University, Jingzhou 434023, Hubei, China2. Key Laboratory of Exploration Technologies for Oil and Gas Resources, Ministry of Education, Yangtze UniversityJingzhou 434023, Hubei, ChinaAbstract: Gas Hure Oilfield is the biggest one in the Qaidam Basin, Through intensive sampling and GC-MS a-alysis, the authors analyzed the geochemical characteristics of the crude oils in the Gas Hure Oilfield. The re-sults show that the crude oils have even-odd carbon predominance for n-alkanes. In shallow reservoir, part ofcrude oils suffered biogenic degradation. As for normal oils, they have low Pr/Ph ratios with abundant gammatrane and Cas homohopane, which indicate that the crude oils are formed in strong reducing salt water environ-ment. Compared deep(E])reservoir with shallow(N, -N2)reservoir, the ancient salinity of sedimentary environment of N- N; crude oils is slightly higher than that of E]crude oils; the C2/C* ratios of crude oils are1. 02-1. 71 with a higher abundance of Cz7 sterane, which shows that their mainly origin matrix are bacteria andalgae; the C2g sterane 20S/(20S+20R)is generally low, the ratio in deep E;reservoir distributes between 0. 31and 0. 40, but in shallow reservoir is relatively low with 0. 29-0. 36. At the same time, Czg sterane aBB/(aBB+aaa)and Ts/Tm both are extremely lower. All the above show that the crude oils in gas hure are mainly lowmature oils, although the maturity of deep reservoir crude oils is slightly higher than shallow oneKey words: Qaidam Basin; Gas Hure Oilfield; crude oil; saturated hydrocarbon; geochemical characteristics1 Introductionsource rocks and crude oils from Qaidam Basin havebeen reported( Zhu, 2004; Huang, 1989, 1991The biomarkers analysis of saturated hydrZhang, 2004; Zhang, 2008; Li, 2008). Under thisfraction is the essential study area in oil-gas geochemcircumstances, the biomarkers study of oil samples isistry, which refers to many kinds of compounneeded more analysis resources to enrich it. Based onrated hydrocarbon fraction is one of the important the results of previous studies, the purpose of thischemical compositions in crude oils and source rocks, study is to analyze the distribution characteristics andcompositional diversities of saturated hydrocarbons indepositional environments, source input, and thermal Qaidam oils from saline lacustrine depositional envi-naturity, oil-source rock correlations. Qaidam Basin ronments. Focusing on the different series of biomarkis one of the most important oil fields in northwestern ers in saturated hydrocarbon, the 25 oil samples wereChina, representing saline lacustrine depositional en-the nomnneitional charractenstcsvironments. Many studies of saturated hydrocarbons in中国煤化工to provide clues toCNMHGReceived 12 December 2010, accepted 11 January 2011ported by the National Natural Science Foundation of China( No. 40973041)Mo x.y and Zhang mthe genetic classification of oilsblock is east-west trend. Based on acquainting thecharacteristics of Tertiary strata saline lake faces2 Geological settingChen et al., 2006; Zhu et al., 2004: Wang et alower Oligocene reservoirs of this basin were de- 2005: Huang et al., 1989; Jin et al., 2001), 25posited in high calcium-polyhaline interior lake de- crude oil samples from two reservoirs were well septrital sedimentary under the dry climates. Gas Hure rated and analyzed by column chromatography andoilfield, located in the east of Gas Hure Lake, deposit GC-MS. This paper will further figure out the geohuge Tertiary System strata( Zhang et al., 2008; chemistry characteristics of saturated hydrocarbon andYuan et al., 2006), which is the most oil and gas aGas Hure oilfield combined its geo-bundance zone in western Qaidam Basin with high relogical background and analysis resultsserves and yIIt formed deep(Ey)eservoirand shallow(N,-N, )reservoir( Fig. 1). Deep El 3 Samples and methodsreservoir is a well-structured program, anticline struc-Oil samples(25)were used in this study. Afterture by nearly north-south axis, with even axial re- collecting, those oil samples were carried back to lagion, asymmetry flanks, widely gradient eastern side boratory and kept at low temperature. Oil samplesnd narrowly precipitous western side. The shallow Nwere isolated by chromatogram and mass spectrumN; reservoir structure was divided into upper andChromatogram and mass spectrum conditionsdown two parts by Youshashan Fault. Down-dip block The equipment is HP6890GC/5973MSD GC-Ms ap-has remarkable inheritances and tectonic axis with paratus, HP-5 MS (30 m x0. 25 mm x 0. 25 um)north-south trend. whose structural feature is familiarcapillary column. Temperature-programmed of saturat-with the E)reservoir; the tectonic axis of upper-faulted ed hydrocarbon as follow: keeping the temperature atNShizigeFaultstructural belttarget zoBeiwustYuejinOilfield D]target zoneFig 1 Structural geographic location of Gas HrHH中国煤化工CNMHGGeochemistry and genesis on saturated hydrocarbon of crude oils in Gas Hure Oilfield in Qaidam Basin50C for 2 min, then the heating rate from 50C to immature-low mature oil or source rocks in saline la-100℃is20℃/min,foml00℃to310eis3℃/ custine facies show an even carbon number predormin, finally keeping it at 310C for 15.5 min. The nance(Zhang, 1998). The distribution characteris-injector temperature is 300, He as carrier gas, flow tics of n-alkanes in this region show predominance inrate at 1. 04 ml/ min, the m/e range of 50-550 Amu. low carbon number. The CM/Cn ratios are from 1. 02Detection method is multiple ion scan, ionizing power to 1. 71 (Table 1), which reflected that the sourceis 70 evmaterial comes from the homoneneae aquatic organism4 Results and discussion( Welte, 1973 ) Compared with it, some imparitiesexist between the normal oil and biodegradation oil as4. 1 Normal alkanesfollow: just like the Yue378, Yue82, Yue 54, Yue 975Normal alkanes are the important chemical con- and Yue 976, 5 crude oil samples from shallow Nstituent in oil and extract of source rocks. Through the N2 reservoir, their baselines in total ion currencyanalysis, crude oils in this region could be divided in-(TIC)profile all rise up, and the n-alkanes sufferedOne is normal crude oil, and another varying degree loss. All of these express that the oilsone is biodegradation oil. In Gas Hure oilfield deep experienced varying degree biodegradation( Fig. 2)E] reservoir, the normal paraffin hydrocarbons in satu- In this article, according to the distribution character-rated hydrocarbon is familiar with the shallow N, -N2 istics of saturated hydrocarbon biomarkers and classifireservoir. Both of their chemical compounds present a cation from Peters Moldowan( 1993), the oils ingle peaked distribution, and the carbon target zone could be divided into different level. As ismber distributed between nC1o-nCa( Fig. 2), with shown in Fig. 2, the n-alkanes from crude oils in relaeak even-odd carbon predominance between Cutively shallow burial depths get diversity degree los-Cx. The main carbon peak is from nCu to nCn, CPI ses, but still have marked remains. Pristanesnearly 1. Generally, in China, the n-alkanes from the phytanes isoprenoid biomarkers hardly sufferedYue541490-1540mN-N2Yue58189742556mN-N2BiodegradationShallow reservoirue13-2532426-3401.8mEYue752060-20752mN-N2BiodegradationDeep reservoir中国煤化工Fig 2 Distribution characteristics of saturated hydrocarbonCNMHGOilfield(TIC)Mo x.Y. and Zhang Mdegradation. So the degree of these kind oils is ex- phytane ratios are generally low, among 0. 24-0. 42 intremely weak, that is in 1-2 level. Relatively, the nnormal oils with smooth alteration( Table 1& Fig. 2)Alkanes from crude oils in deeper burial depths were which show that the crude oil formed in relativelywiped out because of biodegradation, pristane and strong reducing environments. In deep e3 reservoirphytanes isoprenoid biomarkers only fewer remnantsPh/nCis ratios are lower ranging from 0. 80 to 0. 99So the degree of these kind oils is weak, which is in Pr/nC17 is among 0. 25-0. 32; in shallow N-N,reser-2-3 levelvoir,the ratios is tiny higher than deep e,reservoir,4.1 Isoprenoid alkanesPh/nCIs higher, majority between 0. 89-1. 52, whileIsoprenoid biomarkers are used commonly in the Ph/nC, is among 0. 28-0.67(Table 1), which isstudy of petroleum geochemistry. From the distribu- probably related with its maturitytions of pristane and phytases, the pristine toTable 1 Analytical data of n-alkenes and isoprenoid in oils from Gas Hure OilfieldType Pr/Ph Pt/nCn Ph/nCis Ph/nCis-Pr/nC1 CPl24-4C2-Cn+1490-15400.241.42695.05Yue3825460-1547.6N1-N20.000.000.000.000.00Yue153980-1727.0N1-NV0.28040655.6-1709.6N0.261.215.31v0.280.401,461.291739.5-1741.7N1-性20.280.391.46Yud41768.2-1837.8N1-N0.280.421.52177.5-171.9N1-N△0.280.672.Yue87518412-19597N1-NV0.280.381Yue4751870.3-1978.70.39Yues-818974-2556.0N0.320.280.891.26Yue9839976-2040.6N1-NV1,022060.0-20752N-N0.422.ll>100Yue shi73212.6-3388.6V0.320.280.890.1.0lYuel3-253242.6-3401.80.320.280.863258.4-3422.40.320.270.861.36Yue14-293286.8-3476.80.320.260.82Yue shi30.320.260.83L353330.6-3547.0l,02ue6-343371.8-3556.6V0.310.250.80Yuexin3-53448.04-3474.00.300.290.970.290.290.99Yuc -393498,0-3597,4V0.310Yue19-93538.4-35560.290.中国煤化工a1yYuco1-I3633.2-3822.60.250.3CNMHGOI 1O8Geochemistry and genesis on saturated hydrocarbon of crude oils in Gas Hure Oilfield in Qaidam Basin4.2 Terpene hydrC3 to C3s(Table 2, Fig 3). Besides, Cn long-chainIn crude oils, the hopanoid is the mainly constittricyclic terpenes/C3o hopanoid ratio is used reflecteduent of pentacyclic triterpane, the distribution range the ratio of long-chain tricyclic terpenes and pentacyof carbon numbers is Crr-C3s, Gaclic triterpane. The ratios vary from 0. 3-0. 43, andpanoid ratio is among 0. 7-1. 3 in deeper E3 reservoir the samples whose long-chain tricyclic terpenes are( Table 2), with a higher content of gammaceranegher than Cy hopanoid hac not been found. C3owhich illustrate the crude oil formed in terrestrial bamoretane/ hopanoid ratios are from 0. 15 to 0. 19sin saline lake facies or salty environment Li, with high content of hopanoid. The relative abun-2008). Meanwhile, the C3s/Cu hopanoid is distribu- dances of C2 secohopane/Cns tricyclene show slight dited among 1. 29-1.51, that is, Cu>c3>C3