Origin of Natural Gas in Kekeya Field, Tarim Basin, China Origin of Natural Gas in Kekeya Field, Tarim Basin, China

Origin of Natural Gas in Kekeya Field, Tarim Basin, China

  • 期刊名字:中国地质大学学报
  • 文件大小:797kb
  • 论文作者:Hou Dujie,Xiao Zhongyao,Tang Y
  • 作者单位:Energy Resource Department,Tarim Petroleum Exploration Bureau,Geochemistry Research Center
  • 更新时间:2020-09-13
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of China university of Geosciences, Vol. 15, No. 1, p 91-98, March 2004ISSN1002-0705rinted in ChinaOrigin of Natural Gas in KekeyaField. Tarim Basin, ChinaHou dujiEnergy Resource Department, China University of Geosciences, Beijing 100083, ChinaXiao ZhongyaoTarim Petroleum Exploration Bureau, Kuerle 841000, ChinaTang Youjun Zhu Junzhang Li XianqingGeochemistry Research Center, Yangtze University, Jing zhou 434023, ChinaABSTRACT: This paper is mainly concentrated on the geochemical characteristics and origin of gas ofKekeva field in the Tarim basin. Nw China. This study shows that Permian mudstone is the main sourcerock of oil and gas. Based on the carbon isotopes of Ci-Ci, the carbon isotope of gas in Kekeya field isa little heavier than that in the typical marine-derived gas. The relationship between carbon isotopes ofmethane and ethane is coincident with Faber equation of gas derived from organic mattermajority of gas maturity is estimated, based on the formula, at 1.8%-2.2 %besides K2 and K18wells. In addition, the gas derived from 0.9. 2 %R, source rocks may also be mixture. #Ar/3ArandHe/He ratios from the gas samples also support the mixing process. Moreover, the gas in this re-gion is mainly generated from more mature source rocks although the low mature gas exists.KEY WORDS: natural gas, origin, Kekeya, source rockINTRODUCTIONmay lead to this situation. The three different sourceThe Tarim basin is one of the largest basins in rocks, i. e. Carboniferous-Permian mudstone andthe world that has not been well explored. The ex- limestone, Jurassic coal-bearing strata, Upper Cretaploration programs that are special for natural gas ceous-Eogene source rocks have been proposed bysystems are becoming increasingly important in the many researchers. Li et al.(1999) deduced that thesearch for new hydrocarbon resources of the region oils and condensates were from the Carboniferousbecause of higher maturity source rocks. Successful Permian sections and paraffinic condensates were notexploration in turn requires a good understanding of formed by thermal cracking of oil by oil and cond.all elements of the petroleum system, of which the sate analysis. But their deduction is still not appossible source rock kitchen is a basic element proved because of the lack of source rock data. Some(Chen, 2000). Specially, the southwest depression of researchers still believed that the natural gas isis an important and difficult region in Tarim basin. mainly derived from coal-bearing strata(Song,Y.,etThe Kekeya field was discovered in 1977. However, al., 2001. Personal communication). Fortunately, newthe source rocks and origin of gas and oil have not exploration target in this region provides us with a chancebeen clarified yet. The complexity of geological and to obtain new oil and source rock samples for the renewalgeochemical data, relatively higher maturity oil and of our current knowledge. The purpose of this study wasgas samples, lower petroleum exploration activity therefor中国煤化工 of natural gas and con-densateCNMHGd evidence for the conadicting gas origin hypotheses of this regionThis paper is supported partly by the 973 Project (2001CB209101)and partly by Tarim Petroleum Research Foundation (41301050387)GEOLOGICAL SETTINGManuscript received November 10,2003Trounded by TianshanManuscript accepted December 25, 2003.Mountains and Kunlun mountains. located in theHou Dujie, Xiao Zhongyao, Tang Youjun, Zhu Junzhang and Li Xianqingsouthern part of Xinjiang, is the largest sedimentary Kunlun Mountains eastward into the area around He-basin in China. The southwestern depression of Tar- tian. Therefore, five potential source rocks are presim basin is an important depression and contains four ent in this region which include Cambrian-Ordoviciantectonic sub-units: the Maigaiti slope, the Kashi sag, and Carboniferous-Permian marine carbonates andthe Yechang-Hetian sag and the Qimugen salient clastics, Jurassic coal-bearing strata and the Upper(Fig. 1). The depression experienced two different Cretaceous-Neogene marine sedimenthesesedimentary processes that include Paleozoic platform potential source rocks, it is more disputable if coaland Meso-Cenozoic foreland depression. The Eogene plays an important role for gas generation.depression is located in the foreland basin to thenorth of suture zone of collision between Eopaleozoic ANALYTICAL TECHNIQUESTarim plate and the middle of Kunlun island arc. AlThe oil, gas and source rock samples have beenthough far from the suture zone in Neogene, the de- analyzed in this study. The chemical compositions ofpression is still characterized by the foreland basin in gas samples were analyzed with thermal gas chromathe shape of basin with sedimentary and subside char- tography. The composition of gas is given as nitroacters and structure. The stratigraphic sequence is gen, carbon dioxide, methane, ethane, propane, burelatively complete in this region and the thickness of tane and so on. The stable carbon ratios measured ondeposit is about 12 000-20 000 m ( Fig. 2). The Finnigan MAT-251 TM isotope ratio mass spectrom-platform led to the Paleozoic marine deposit that in- eter are reported in 8 notation in pars per mil(103)cludes strata from Sinian to Carboniferous-Permian. deviation. relative to the Peedee Belemnite (PDB)The structure is mainly showing its vertical move- marine carbonate standard. The rock samples werement of lithosphere. The shallow marine plateau faci- pulverized and Soxhlet extracted with chloroform.es strata of Cambrian, Ordovician, Silurian are stead- Then asphaltenes were precipitated with n-hexaneily and widely distributed in the depression. The followed by filtration. The asphaltene-free bitumensCarboniferous- Permian carbonates and clastics are were further separated with column chromatographymarine transgression strata which was also widely into saturated hydrocarbons, aromatic hydrocarbonsdistributed in the depression. The Carboniferous- resins, using hexane, benzene and alcohol as the solPermian strata are thinner in Maigaiti slope and Ba- vents, respectively.chu lift and thicker in region near Kunlun Mountainbecause of Caledonian orogeny and Hercynian oroge- NATURAL GAS COMPOSITION, STABLE ISOTOPny. The sedimentary facies changed from shallow IC CHARACTERISTICS AND THEIR ORIGINSmarine plateau facies into inland lacustrine facies.The natural gases in this region, mostly hydroAnd Jurassic deposit is mainly non-marine coal- carbon gases, include mainly methane, ethane andbearing clastics. Upper Cretaceous-Eogene section is heavier hydrocarbon (above ethane ). The hydrocarcomposed of fluvial flood plain to shallow water, la- bon gases account for above 80 and are mostly asgoon, marine platform facies deposits where the Te- high as above 90 %(Table 1). The nonhydrocarbonthys Sea transgressed along the northern foot of gas compositions are mainly N2, CO2, O2 and someothers (Table 1). thetly below%cept in Well KloKorlafieldm basin, the hydrocarbon gas composition has lowerAkes fanorthenThe wet gas can reach as high as 11.5 and is gen-Kashid to ch的叫mHmkesheng IcushionCNMHGer Ci/(-C,)ratio isfound in this region, compared with the gas of typicalbig gas field in Hetian region in Tarim basin Fig. 3)f the hydFigure 1. Sketch map showing tectonic unit of Tarim ba- carbon components in gases are mostly not in agreesin and location of Kekeya field.ment with the rules that n-alkane gas is of positiveOrigin of Natural Gas in Kekeya Field, Tarim Basin, ChinaLithologya Etianic3401derlround a shawCIU 3I Uin06Dmethane e3000 oil reducts,,,Figure 3. Plot of Cl/(C-Cs) with methanecontent in gas2000∵巴:1702761Figure 4.8C values of Cl -C+ light hydrocarboncomponents in different areas of tarim basinFig. 4). The gas in Hetian gas field is dry whose dryingcoefficient varies from 0.95 to o. 99. Most researchers2651believed that such a gas belonged to marine high andposti2471give us a hint that the gases in this region were not de-自国■rived from typical marine source rocks because of itsheavier carbon isotopic ratio and more wet gas content.GEOCHEMICAL EVIDENCE FOR GAS OF MIXEDturbiditeORIGINFigure 2. Stratigraphic column for southwestern de-As we know, the mixed gas, commonly found inpression, Tarim basinnature, can be a mixture of abiogenic and organogeric gases, or a mixture oil-associated gas and coal-carbon isotope series, i. e.8C1<8C283C4, fractionations during dissipation. But how can wewhich may be related to the gas composed of gases in identify them The following data testify our hypothdifferent origins. Generally speaking, the carbon iso- esis tha中国煤化工 different sources andtopic ratio of methane (83C1), varying from generaHCNMHG43.34×10-o-36×10-3, belongs to typical organogenic gases. The carbon isotopic composition of Carbon Isotopic Composition of Hydrocarbon Gaseshydrocarbon components in this region is heavier Heavier than Long Chain n-Alkanederived from typical Paleozoic marine source rocks propane varies from -23X103 to -31Xloe andthan that of Hetian gas field which is believed to beThe carbon isotopic composition of etharandHou Dujie, Xiao Zhongyao, Tang Youjun, Zhu Junzhang and Li XianqingIn conventional reservoir, such wet gases as ethane, propane and butane were formed primarily inthroughout the entire sedimentary column from diagenesis to metagenesis Hunt, 1996 ). ThereforenH+rssi Basera s hould be no more wet gases in high maturitythereon a data bank, Berner (1989) proposed thatlated to maturity (Ro, %)for the thermal genetic gasderived from I, Il organic1416182022242628303234carbon numberMethane (%)=9. lIn R+93.1Ethane(%)=-6 3In R.+4. 8Figure 5. 8C values of n-alkanes (above nCu)inPropane(%)=-29In R+1.9condensate of Kekeya gas fieldAlthough these relations are not accurate, theymostly in the range between-24X10 and-26X natural gas based on the composition. R, value can be10(Table 2). The theoretical study of fractionation calculated based on the data of natural gas composi-is very complex and the simple model shows that the tion (Table 1). The calculative R. value is in theght isotope involves less energy of association than range of 0.3 %-1.0%, and most of R, values aredoes the heavier one because the breakage of a C- below 0. 6%. This is obviously not real because othCi requires more energy than that of a C2-C2(Borer geological and geochemical data are not in agreedenave, 1993). As a result, the fractionation-based ment with this assumption. The carbon isotopes ofcarbon isotopic composition, the 8C values of the hydrocarbon gases and biomarker analysis of coexis-liquid hydrocarbon, should be heavier than that of ted light oil all show the relatively high maturitythe hydrocarbon gases. Actually, the carbon isotopicMany efforts have been made to interpret plotratio of long chain n-alkline(> nCu) ranges normal- of 013C versus indices of maturity( vitrinite reflecly between about-28 10 and-29 X 10(Fig. tance, R)and to delineate organic sources and iden5). They are lighter than the hydrocarbon gases in tify source rockscarbon isotopic composition, indicating the multiThe maturity of natural gas can be roughly estisource for gas generation in this region.mated and calculated, using carbon isotope values ofmethane, ethane and propane etc.. Many authorsHydrocarbon Gases not in Agreement with Positive proposed the empirical equation. Dai(1993) proCarbon Isotopic Series(8C<8C28C38C4) posed the following isotope/maturity relationshipIn general, the organogenic alkane gas is of posi- based on statistical resultstive carbon isotope series,i. e,83C1<8C2<83C(10-3)=15. 8logR-42 2(natural gas de8C3<8C. The contents of alkane gas components rived from sapropelic OM)decrease successively with the increase of carbondC1(103)=14. 12logRo-34 39 (natural gasnumbers in molecules, that is, CH>C2H>CHs derived from humic OM)CA H1o. When gases of different origins are mixedStahl and Koch (1974) established the empiricalthe original order 8C1<8C2<8C3<8CA of the relationship between carbon isotope and vitrinite re-mixed gas will be broken, resulting, in part, in re- flectance Ro. Faber(1987)made modifications of theverse or total reverse of order. Figure 4 and Table 2 equation for the natural gas derived from type I/II OMhow d C values of hydrocarbon consist of C-Ci83C1(10-3)=15.4logR。-41.3The oC value of propane is above the oC value of butane in most situations, showing that the gas may be deTHa中国煤化工322CNMHGSrived from the mixture of different organic matters (OM)or mixture of gases in different maturation periodslationship for type ll Om as follows8C1(10Different Maturities of Gases Testified by Chemical83C2(10-3)=3.32logR。-25.9Composition and Carbon Isotopic CompositionTherefore. we can calculate and estimate theOrigin of Natural Gas in Kekeya Field, Tarim Basin, ChinaK103 deviate from thee empiricamixing of gases frormKS101K40K701mixing of gases fromdifferent source kitchens6C08"C10Figure 6. Determination of mixing gas distributionbased on carbon isotopic composition of methFigure 7. Determination of mixing gas distribution basedand ethane. Note: The line in the diagram shows on carbon isotopic composition of ethane and propane.the relationship in carbon isotopic composition be- Note: The line in the diagram shows the relationship intween methane and ethane proposed by Faber carbon isotopic composition between ethane and propane(1987)proposed by Faber(1987)maturity of gas based on their carbon isotopic compo- equation and have lower 8 CI values because of thesition. The calculated R. values can be seen in Table mixture of gases from the lower maturity. Based on3 and several deductions can be drawn.(1) Although statistical data( Whiticar, 1998) the fractionation ofthe equations proposed by Dai (1993) have some carbon isotopic composition will lead to carbon isodifferences from the equations by Faber and Berner, topic composition of methane that varies with maturithe calculated results show the natural gases, in ac- ty. The 8C value is about -42 X 10 at Rcordance with the relationship of sapropelic OM, are 0.9 %, X103 at Ro=1.2%,-37 aturity equations, indicating that the gases are derived may be derived from mixture of gases whose Cgfar from real situations by using type Ill isotope/ma- R=2.0 % respectively. Therefore, these gasesfrom either sapropelic OM or cracking of oil. (2)0.9 %-1 2 and sometimes that have a higherThe chemical compositions show different maturity maturity(Ro=1.8%). Another point which wetrends different from isotopic composition. What is should bear in mind is that the difference of carbonthe reason for this The authors believe that the two isotopic composition of methane and ethane cannot breasons may be attributed to this. One is related to used for maturity calculation in the situation of mixthe mixture of gases from different sources or mix- ing process. In summary, the gas in this region canture of gases from different generations. The other be estimated by using the carbon isotopic compositionreason may be related to the diffusion of fractionationof ethane. and the vitrinite reflectance value(R.) ofActually we can get more messages based on the source rock may be in the range of 1.8%-2.2%.relationship between carbon isotopic compositions of There is a gas mixing process in some wells from gamethane and ethane (Fig. 6). Overall, the carbon ses mentioned above with relatively low mature gasesisotopic composition of gases in this region follows of 10.9%-1.2%(R)the linear equation proposed by Faber(1987). ThisSimilarly, the carbon isotopic composition ofphenomenon reveals that this empirical formula can ethane and propane can also be used for thebe better used to reflect the real maturity of gas. cation of the mixing of gases(Fig. 7). In additionFrom Fig. 6, it can be seen that the gases in Well k2 the gases in Wells K2 and K18 have lower maturityand K18 have lower maturity and most gases are gen- with th中国煤化工 nd Fig.7. But it canerated from source rock at the maturity level of Ro- also beTYHCNMH Gfference forThe point which deviates from the empirical line equationcarbon isotopic composition of methane varies from and that is typically a situation of mixing. This isin Wells K410, K18, K243, K8, K401, KSIOl, From Fig. 7, it can be seen that gases in K701Hou Dujie, Xiao Zhongyao, Tang Youjun, Zhu Junzhang and Li XianqingK103, K516 and K428 may be also mixed from dif- gas is derived from more mature source rocksferent sources. Because the difference in content be-Ar/Ar and He/He Ratios Varying from Gas Samplesen methane and ethane varies in different souThe sHe/ He and°Ar/36Athe carbon isotopic compositions of methane and eth- evidences for the natural gas origin. The He/Heane are preferred for us to identify the mixing process value, different with the position of mantle and greaof gases. Although it is difficult to know the exact ter than 1. 4X 10, indicates that a considerableproportion of mixing gas, it can be seen that the main amount of helium in given natural gas comes from theTable 1 Chemical components and isotope composition data(g·L)C+(C2+C3)C6)N10.6585.305.703.400.900.303.300.101.009.370.8985,669,642.261.020,610.397,24.276.150.380.86K3N10.7275.0010.204.002.000.206.500.701.405.280.82K4N10.6486.203.001.100.300.807.900.200.521.020.94K8 N88,279,331.450.430.20.138,196.437.650.430.88K10N10.7171.206.403.900.400.4013.200.603.906.91K18N184.058.991.930.730.20.107,704.667.030.520.88K243K331N185.99.411.70.660.280.137.735.547.050.430.8885.569.882.571.280.510.010.166.873.845.940.410.8685.089.011.570.550.235.747.220.450.88K41587.489.141.740.620.240.548.045.257.120.480.88K42888,898.711.470.450,220,28,735,938.040.450.8987.179.472.03137.584.676.860.420.87K701N9.889.422.652.651.053.990.386.623.554.951.190.83Table 2 Isotope composition dataNo.dph/ma66°c1108C:/002C/108°C./108"c:/108C/0SIOL6807-683538.4K1033829-3873.21N1xs38.524,4K1033855-3878NK23237.9126,45K700625.825.78-26.3427.7K33337.6926.3624.7225.9327.9532.81K3036.0825,53-24.4437.1225.8425.7927.26K223NNNNNNNN37.1425,82-26.4K51625,831.7K41225.9331838.2K415K428N25.8K331N37.3K7013839ni. x38.126.2中国煤化工K839.1425,74CNMHGK18NNNN41.8626,1226.15K410Origin of Natural Gas in Kekeya Field, Tarim Basin, ChinaTable 3 Calculated maturity based on carbon isotopic compositionNo.3C1/103R1/%R02/%8C2/10-3813C3/10-3R03/%R。0/%R。3/%R”/%R/%SIOl38,400.521.7424.902,10K10338.500.511.7124.4027.501.521.27K10338.800,4924.4026.301.45K232-37.910.561.8726,3625,331.661.811.620.73K700636.670.692.2425.8024.561.921.76K33337.690.5824.721.72811.73K3036.080.7694124.442.181.971.79039K22237.120.642.1025.84-24.451.871.911.780.01K22337.140.642.0924.601.861.901.750.01K51637,100.642,1025.8031.7010,80K4121.901.038.200.528.20-251.590.2036.000.772.K7011.8439,140.425,7424,3611.931.80K18K180,921.730.86K24337.701.9325,8024.701.711.921.07K24311.480.3126.1524.790.971.850.00K40139,1625.3223.872.020.00K41043.340.2324.580.741.871.760.89Note: 1). The calculated R, values based on the equation of 8Ci with Ro for type Il OM by Dai(1993); 2). The calculated Ralues based on the equation of 8CI with Ro for type I/I Om by Dai (1993);3). The calculated Ro values based on theequation of aC with R. for type I/I OM by Faber(1987); 4). The calculated Ro values based on the equation of 8 C2 withRa for type I/I OM by Faber(1987);5). The calculated R, values based on the equation of o Ca with Ro for type I/I OMby Faber(1987); 6). The calculated Ro values based on the equation of aC with R, for type Ill OM by Berner(1989);7)The calculated Ro values based on the equation of C2 with R. for type Ill OM by Berner(1987)gassing will lead to the various Ar/ Ar ratio in themantle. In the short-term fast-degassing model, the4o Ar/ Ar ratio is 5-6; in the long-term and slowlydegassing model, it is about 2(Wang et al., 1997).Furthermore, helium and argon in hydrocarbonulatieffect.Thet°Ar/6Aruldwith the00600aging of source rocks. Similarly, He/He decreaseswith the aging of source rocks. This difference meansFigure 8. Ar/Ar distribution in gas ofthat the age of the hydrocarbon source rocks may beKekeya regionestimated by Ar/Ar, He/He values in naturalvolatile component of mantle. 4 Ar/ Ar ratio in theIt can be seen from Table 4 that the natural gasmantle is one of the crucial indices in the model of in Shaya region, northern part of Tarim basin is re-earth degassing. Statistical results show that 40 Ar/ markab中国煤化工 Kekeya region.The36 Ar ratio in the upper mantle, as high as 2 000, canArTHCNMH Natural gas in Kekeyaserve as an evidence of abiogenic gas from the deep region can be divided into two subgroups (Fig. 8)earth after neglecting the influence of the chronologic The first subgroup is in the range of 855. 4-981.6accumulation of cruse radiogenic 40 Ar and those of and the second one is in the range of 604. 6-682.4other factors. Besides. different models of mantle de- indicating various sources for the generation of gas.Table 4 #Ar/Ar and He/He ratios for gas in Tarim basins He/HODCH,/10Kel87,9(7.3)×10Ke2855(885.4)6.1×100.04172Ke243Ke351718.8KekeKe428Ke516604.6144Ke7768.3Ke701682.48.6×10-80.06164Ke87,9×100.06OO1CONCLUSIONStions Technip, Paris. 321--33The aim of this investigation was to address the Chen,J. F, Xu, Y. C, Huang, D. F, 2000. Geochemain origin of gas in Kekeya region by utilizing theCharacteristics and Origin of Natural Gas in Tarim Basircarbon isotopic composition of light hydrocarbons.China, AAPg Bulletin, 84(5):591-606The following conclusions have been drawn. (1) The Dai, J, Song, Y. Cheng,S, et al., 1993. Characteristicsof Carbon Isotopes of Alkane Components and Identificacarbon isotope of gas in Kekeya field is a little heaviertion Marks of Biogenic Gas in China. Acta Petrolei Sinthan that of typical marine-derived gas. The relationca,14(2):231-238ship in carbon isotopes between methane and ethane Faber, E, 1987. Zur Isotopengeochemie Gasformiger Kohleis coincident with that of the gas derived from organicneasserstoffe. Erdol, Erdgas & Kohle, 103: 210-218matters I, II.(2)Most maturity of gas is esti- Hunt, J. M, 1996. Petroleum Geochemistry and Geologmated at 1.8%-2.2%, besides K2 and K18 wellsSecond Edition. W. H. Freeman and Company, Newbased on the formula. In addition, the gas derivedfrom 0.9 %-1.2 %R, source rocks may also be Li, M, Lin, R, Liao. Y,et al., 1999. Organic Geochem-istry of Oils and Condensates in the Kekeya Field, Southmixed.(3) Many geochemical evidences support thatwest Depression of the Tarim Basin. Organic Geochemainly generated from more mature source rocko h isthe gas is mixed. However, the gas in this region isWang,X. B, Li, C. Y, Chen, J. Fa.,1997.OnAbiogenic Natural Gas. Chinese Science Bulletin. 42: 1(ACKNOWLEDGMENTS(in Chinese)This project was supported partly by the 973 Whitcar, M. J., 1998. Corrn of natural gas and sourceProject(2001 CB209101) and partly by Tarim PetroRocks. In: Magoon, L. B, Dow, W.G., eds,, Petroleum Research Foundation (41301050387). We apleum Systems, from Source Rocks to Trap. PetroleumIndustry Press, Beijing (in Chinesepreciate collaborations with and enthusiastic supportsXu, Y. C., Shen. P, Tao, M.X., et al. 1999. Geochemfrom Wang Qinghua, Zhang Guangya, Yang Weristry on Mantle-Derived Volatiles in Natural Gases fromjing, Zhang Qiucha, Li Mei and Xi Qing in Tarim PeEastern China Oil/ Gas Provinces. Science in China (Se-troleum Exploration Research Instituteries D).40(3):126-132TH中国煤化工 Genetic Groups of NaturalREFERENCES CITEDCNMHoir formation in tarimBerner, U., 1989. Entwicklung und Anwendung empirischerN. V, eds, Symposi-Modelle fur die Kohlenstoffisotopenvariationen in MiIm on Petroleum Geology of Tarim Basin in China and Sichungen thermogener Erdgase: Dissertation]. T. Uberia Platform in Russia. Petroleum Industry Press, BClausthal. FrG. 160Bordenave, B, 1993. Applied Petroleum Geochemistry. Edi-

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