Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks--A thermal simul Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks--A thermal simul

Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks--A thermal simul

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  • 论文作者:ZHENG Jianjing,HU Huifang,SUN
  • 作者单位:Lanzhou Institute of Geology
  • 更新时间:2020-06-12
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论文简介

Vol 25 No. 2CHINESE JOURNAL OF GEOCHEMISTRYCarbon isotopic characteristics of hydrocarbongases from coal-measure source rocks-Athermal simulation experimentZHENG Jianjing(郑建京)…, HU Huifang(胡慧芳), SUN Guoqiang(孙国强),and JI Limin(吉利民)Lanzhou institute of Geology, Chinese Academy of sciences, Lanzhou 730000, ChinaAbstract Gaseous hydrocarbon geochemistry research through a thermal simulation experiment in com-bination with the natural evolution process in which natural gases were formed from coal-measure sourcerocks revealed that the8C, values of methane vary from light to heavy along with the increase of thermalevolution degree of coal-measure source rocks, and the 8C2 values of ethane range from -28. 3%e to20%e(PDB).8C value was -28%*(Ro=0.45%-0 65%)at the lower thermal evolutionstage of coal-measure source rocks. After the rocks entered the main hydrocarbon-generating stage( Ro0.65%-1.50%),8C, values generally varied within the range of -26%0--23%0*; with furtherthermal evolution of the rocks the carbon isotopes of ethane became heavier and heavier, but generally lessthan -20%oThe partial carbon isotope sequence inversion of hydrogen gases is a characteristic feature of mixing ofnatural gases of different origins. Under the condition of specially designated type of organic matter, hydrogen source rocks may show this phenomenon via their own evolutionIn the lower evolution stages of the rocks, it is mainly determined by organic precursors that gaseohydrocarbons display partial inversion of the carbon isotope sequence and the carbon isotopic values of eth-ane are relatively low. These characteristic features also are related to the geochemical composition ofmary soluble organic matter.oal-measure source rock; thermal simulation; carbon isotopeIn this paper, through a systematic sealed1 Introductionsimulation experiment on the coal-measure hydrosource rocks collected from the Jurassic systemThe compositional characteristics of natural gases Junggar Basin, we analyzed and measured the gaseousand their carbon isotopic composition are controlled hydrocarbons produced in different evolution stagesmainly by the type of organic matter and the degree of described the carbon isotopic characteristics of gaseousits thermal evolution in the source rocks(Song Yan, hydrocarbons produced in different evolution stages,1995; Xu Yongchang et al., 1994; Zhou Xingxi and and discussed their differences and correlations. OnWang Hongjun, 2000)and are also affected by the mi- the above basis, some meaningful conclusions weregration effect and resource-mixing of natural gases drawn( Wang Yutao and Jiang Shaobin, 1997; Dai Jinxing etal.,1997). In the different stages of coal-type gas 2 Samples and experimental analysisgeneration and evolution and in the variation range ofcarbon isotopic values of ethane, the problem of whethThe samples analyzed in this paper were collecteder the carbon isotope inversion characteristics are nec. from the Jurassic coal-measure strata in the Junggaressarily indicative of mixing-source generation of differ- Basin, two of which were taken from the surface andent types of natural gases is still awaiting for detailed four from the well drilling cores. According to the or-experimental research.ganic geochemistry analysis data(Table 1), all of the中国煤化工 c matter type. InIsSN10009426samples wereThis research project was financially supported jointly by the National Bas. grounCNMHGmesh and then putic Research(973)Program of China( No 2001 CB29)(2)and the Important into a high temperature glass vessel. The vesselDirection Project of Knowledge Innovation in Resources and EnvironmentField sponsored by the Chinese Academy of Sciences( KZCX3-SW-128)(open)was placed in a sealed pressure vessel. Fol-* Corresponding author, E-mail: jizheng(@ ns lzb. ac crlowing vacuum pumping, the vessel was heated. BeZHENG Jianjing et alcause of the limited sample collection, the simulating the constant temperature for 72 hours. Instrumental a-experiment was conducted only on three samples at be- nalysis HP, 5890 Il type chrontween 250C and 500C, and as for the other samplesployed to model the composition of the gases, with he-only low evolution-stage simulating experiments were lium as carrier gas. The carbon isotopic composition ofmade at between 250C and 350C. In the simulating gaseous hydrocarbons was on-line measured on a GCexperiments the temperatures were divided into several C-MS (MAT-252)spectrometer, with the analyticalranges with 50C as one temperature interval and at error being 0. 3%00each temperature point the experiment would be kept atTable 1. Geochemical parameters for the samples under thermal simulation experimentRock RTOCAMaceral (%))(%)(mg/g)(mg/g)Sampling siteVitrinite InerDixi-3J1Mud0.471.130.0751.2521.5072.006.50Sikeshu-I1.170.04Sikeshu-2Coa0.4147.21.5392.02115.268.3723.25Mud 0. 480.0412.078.59.581.5Mud0.540.9860.0210.792.1342.5555.32Lunan-2al0.7247.540.683.2598.432.4Ro- Vitrinite reflectance; TOC. total organic carbon; "A",chloroform bitumen A; S,, the content of dissolved hydrocarbon; S2. the contentof cracked hydrocarbonmarks the initial entry of organic matter in the source3 Results and discussionrocks into the hydrocarbon-generating stage. After thetemperature of thermal simulation experiment is higher3.1The evolution characteristics of vitrinite reflec- almost the same as that at the temperatures ar .l0'than 350C. the variation trend of vitrinite reflectancetance(Ro) in the process of thermal simula- 350C. When the simulating temperature is higher thantion experiment300-350C, the vitrinite reflectance increases rapidlyThe vitrinite reflectance is an effective index toT(℃)measure organic matter maturity and also an optical re-0250300350400450500flection to the composition and structure of organic mat-ter. With increasing evolution degree, the value of vit-minite reflectance( Ro)tends to increase gradually0.5and there is a positive correlation between the evolutiondegree of organic matter and the value of reflectance.The variation characteristics of the measured values ofRo(Table 2, Fig. 1)are very distinctive, which wereacquired in the thermal model experiment on sourcerock samples with humid-type (or humic-sapropel-2.0type) organic matter at different temperature intervals-o- Dixi-3Except for well Cai-17, the variation curve I of vitrinite口- Lunan-2米一Cai-012reflectance for the other samples can be divided△-Cai-17two parts. At low or relatively low temperature intervals(250-300C), the vitrinite reflectance Ro is of unre-中国煤化工markable variation thus the curve extends almost inCNMHGparallel. Atexperimental temperatures of 300ig. 1. Variation trend of vitrinite reflectance of samples350C or higher, the values of Ro tend to increase rap- turesunder thermal simulation experiment at different tempera-lly and the curve shows an obvious turning point. ThisCarbon isotopic characteristics of hydrocarbon gase169which also reflects the evolution of organic matter wasspeeded up, too. According to the rates of gas genera3. 2. 1 The distribution characteristics of carbon isotopestion and the values of Ro measured on the spin natural gases in the low evolution stagemaximal yield of gaseous hydrocarbons appeared in theThe results of carbon isotopic and compositiontemperature range of 300-400T. So it can be considalyses for gaseous hydrocarbons are listed in Tableered that the above curve is a characteristic curve of e- 2, which were acquired in the thermal simulation exvolution of vitrinite reflectance of humid-type (or hu-periment on three series of samples. The 8CI valuesmatter in the thermal simu- PDB)of simulated gases from the same series of sam-lating experiment. It is evidenced that the higher the o-ples become higher with increasing temperature. Theriginal vitrinite reflectance of source rocks in the thermalcarbon isotopic values for gaseous hydrocarbons fromsimuating experiment, the greater the degree of thermalevolution at different temperature intervals will bewell Dixi-3 vary from -49, 4%( at 250C)to38. 2%(at 450C), the values of two samples from3. 2 The carbon isotopic characteristics of thermally Sikeshu are relatively high, and they also increase withsimulated gaseous hydrocarbonssimulating temperature.Table 2. Carbon isotopic composition of gaseous hydrocarbons acquired from the thermal simulation experimentsSample No.T(℃)Hydrocarbon gas(%)Carbon isotopic valueCR0(%)C1(mg/g)61C18"C281C381C449.6520.0011.4932.13-49.4-28.3-28.30.4960.480.0239mudstone34.6512.6717.9238.243460.550.1499350599518.8810.9815.7240.6-28.1-25.3-24.90073.2516.41375.321.891.6144786516.154.502.18-38.2-25.1-17.80.7862.462.178950095.614.370.020.9563.082.711425051.9715,0113.51195200.530.0096mudstone30054.9518.7911.5414.7213580.6811.360.3524Sikeshu-]9.993.540.9134.0-27.2-28.2-25.10.8550.8900mudstone30086.1611.462.150.238.421.5131.7Sikeshu-225079.8014.344.930.9438.1-28.1-29.670.8118.018.003316.536.392.43-33.286.2411.801.710.250.8624Ca01225054.9514.2914.2916.49mudstone54.2718.0912.5215.I0.5430.680.082217.497.2525042.1916.7314.6426.440.4220.640.038ne30067.7414.609.6635084.5910.663.441.3lYHCnMHG0063.8218.849.0.9435081.1610.615.322.890.81161.684.410.540.052258.20ZHENG Jianjing et alVoL 25Obviously, the8 C, values of gaseous hydrocar-( Shen Ping et al., 1991). Their carbon isotopicbons are related to the degree of thermal evolution of acteristics primarily reflect the characteristics ofcorganic matter. Precisely, the results of carbon isotopic rence of the original soluble organic matter. In caseanalysis for ethane in thermally simulated gases indica- that the temperature of thermal simulation experiment isted that the C2 value is -28% in the low evolution higher than 300C, the gaseous hydrocarbons will orig-phase(250C). The 8C2 values are generally within inate from thermal degradation of kerogen carbonsthe range of -26%0 to%0, as revealed in the hysource rocks. These two kinds of gaseous hydrdrocarbon-generating limitation evolution phase(300ed at different evolution stages display significant350C), and the values become higher with increasing differences in carbon isotopic composition between gas.degree of thermal evolution, though they are no more eous hydrocarbons formed at the low evolution stagethan%. In the natural gases resultant from sa- and those at the high evolution stage because of differ-propelitic-or humic-sapropelitic- type organic matter, ent parent sources and different generation mecha-the carbon isotopic values of ethane vary between nisms. Gaseous hydrocarbons formed at the low evolu35%0 and%. But in the natural gases derived tion stage resulted mainly from thedown of freefrom humic-or sapropalitic-humic-type organic matter hydrocarbons in the hydrocarbonthe scope of carbon isotopic values for ethane( mainlytheir C-C bonds consist mainly of -C-Cin the range of lower limit of values)is still a contro. the bond energy being lower than that for the C-Cversial issue( Dai Jinxing, 1993; Zhang Shiya andbonds consisting of -C-Cthe bond dissocia-Zhou Jin, 1994 ). The results of thermal simulation ex- tion energy is lower, too. Sedimentary organic matterperiment showed that the 8C2 values are closely re-enced a process of dehydroglated to the thermal evolution phase and the type of or--aromatizatian in the low evolution stage. During thisganic matter, that is to say, differences in 8C, of process comparatively abundant intermediates of unsatethane in the different thermal evolution phases are a urated hydrocarbons were produced (e. g. alkenes, cy-fundamental factor leading to the said debatesclo-alkenes, etc. ). Because of the existence of unsaturated double bonds in the organic molecular structurethe activation energy was greatly reduced. The allylprinciple of organic stereochemistry indicated( Mclaffer-ty, 1990)that the B-carbon-carbon bond which is aparto Sikeshu-]from one bond with alkene bond more eas数down because of the existence of alkene bond. Owingto the above reason thematter with unsaturate"ed double bonds would produce some low-moleculegaseous hydrocarbons. It is obvious that soluble organicmatter in the source rocks is relatively enriched inCwhile kerogens are relatively enriched inC3. 2. 2 Inversion sequence characteristics of carbon iso-6c(‰)pes in gaseous hydrocarbons produced in thermalsimulation experimentngen△8"Cand8CI of natural gases in the Junggar BasinAccording to the previous study of carbon isotopesin natural gases, in case that 8C3 or 8 C4 valuesAt the low evolution stage the distribution of car-re lower than 8C or8C2 values, the inversionbon isotopes of thermal simulation-derived gaseous hy- sequence characteristics of carbon isotopes will appeardrocarbons from three samples is characterized by anThis phenomenon is regarded as the result of mixing ofinversion sequence of8C3<8 C2. The carbon iso- natural gases of different sources, for example, the oil-topic values of ethane vary between -27 2% and type gases and coal-type gases(Song Yan, 1995: Fa28. 3%0, showing a feature of being light in carbonsotopic composition. According to the analysis of resiImonfordues obtained from thermal simulation experiment, gas-中国煤化工ce. n this studeous hydrocarbons in the low evolution stage resulted therC N MH Conducted under themainly from the low temperature thermal degradation of condition of single hydrocarbon source rock( humidprimary soluble organic matter in the source rockstype)in an airtight system. In the low evolution phaser mature evolution phase( Sikeshu coal-type rock)No. 2Carbon isotopic characteristics of hydrocarbon gases171there would appear a partial inversion sequence, which range of -26.71%0--26.21%0, and the latterswas not the result of the above factors. The chromato. 03%0--20.60%o( Song Yan, 1995; Fan Guang-graphic analysis of source rock samples showed a doub- hua, 1990 ) The compositional and carbon isotopicle peak type or 2C2/2C>1, revealing that a large characteristics are those of dry gas and wet gas, indica-ganic matter of the source rocks. Organic petrology a- in different evolution stages, n natural gases formedquantity of lower aquatic organisms was involved in or- ting that there exist differences in natural gases formednalysis indicated that organic macerals in the coal-type sotope distribution of natural gases always shows invermudstones are mainly of the sapropelitic-humic type sion-sequence characteristics, i. e,8C2>8C,orwith higher contents of exinite. The type of organic 8C>8CA. According to the carbon isotopic analmatter gradually became better from each hidden gib- ysis of thermal simulation-produced gaseous hydrocar-bous zone to the main hydrocarbon-generating depres- bons, we can draw such a conclusion that there are twosion regions. It is believed that there is another possi- forms of origin: the first one is the result of mixing andble origin, i. e, the characteristics of macerals of or- gathering of coal-type gases formed in different evolu-ganic matter in the source rocks determined the charac- tion stages, for instance, natural gases in the lowerteristics of the partial inversion sequence of carbon iso- strata of the Qigu oil gas field( Wang Yutao et altopes in gaseous hydrocarbons. Typical oil-type gases 1997; Fan Guanghua, 1990); the second is that theand coal-type gases generated mainly from the thermal coal-type gases formed in different evolution stagesdegradation of sapropelitic and humic organic matter. were gathered to form gas pools, respectivelyThe major organic petrological component of the formerThe relationship between AC,(8C2-8C1)is exinite but the latter's is vitrinite. The carbon iso-topic characteristics are manifested by80and 8C, of natural gases in the south of the junggarBasin is obviously linear, which is in good consistency8C3<8Ca in each type of gases which were derived with the relationship between AC, and 8CIinfrom thermal degradation( Xu Yongchang et al. 1994; thermal simulating-derived gaseous hydrocarbons( FigDai Jinxing et al. 1997; Da Jinxing, 1993). But in our 2). These distribution patterns are closely related toexperiments we found that organic matter is mainly ofthe form of occurrence of and the thermal evolutionthe sapropelitic-humic type in the non-marine oil-bear- stage of the Jurassic system on the southern margin ofing sedimentary basin because of the restriction of sedi- the basin. At the southern margin of the Junggar Basinmentary environment and provenance, especially theprevailed a thrust-fault loading tectonic subsidencehydrocarbon source rocks formed under the condition ofbackground during the late Mesozoic and Cenozoiccontinental environment. Although organic maceralsAdditionally, due to the influence of later Himalayanare compositionally dominated by vitrinite, they contain movement and uplift, great changes took place in buriquite a large amount of exinite. In the case of exinite al conditions of the Jurassic source rocks in going fromaccouning for>10%-20%, in the process of thermalevolution the same source rock itself is characterized bythe southern margin to the Changji depression, the de-of thermal evolution varied from weak togenerating the oil-type and coal-type gases of mixing and the values of R varied from 0. 45% to 2.0%origin, thereafter displaying partial inversion-sequence (Wang Yutao and Jiang Shaobin, 1997). During thecharacteristics of carbon isotopes in gaseous hydrocar-process of gas generation and accumulation, natural ga-ses formed in different evolution stages were enriched3. 3 Geological implication of the study of carbon i- at the southern margin of the Junggar Basinsotopic composition of thermal stimulationproduced gaseous hydrocarbonsReferencesNatural gases from the Dushanzi oil/ gas fieldDai Jinxing( 1993)Carbon and hydrogen isotopic characteristics of natuwhich lies in the south of the Junggar Basin, were pro-ral gas and identifying various natural gases [J]. Natural Gas Geo-duced from the Taxihe Group of the Upper Tertiary se-science. 13, 1-40( in Chinese with Englishries. The values of C,/CI-s vary between 0. 682 and Dai Jinxing, Wang Tingbin, Song Yan et al.(1997)Formation Condi0.829. But natural gases in the lower strata of the Qigution and Distribution of major Natural Gas Field in China [M]. ppoil gas field were produced during the Middle-Late Trias-中国煤化工 and types of reservoirs ofperiods. The values of C/CI-s are within the range ofCNMHGGas Industry. 10, 7-120.974-0.99( Wang Yutao and Jiang Shaobin, 1997)The former's8CI values vary from -46.46%0 to Shen Ping, Xu yongchang, Wang Xianbin et al.(1991)GeochemicalCharacteristics and Forming Gas Theory of Gas Source Rock and Nat-35. 47%0 and the latters from -31 55%0 toural Gas [M]. pp 117-123. Science and Technology Press29.29%. The former's 8C values are within theLanzhou (in ChineseZHENG Jianjing et alVoL 25Song Yan( 1995)Geological Characteristics of Natural Gas Accumulation Xu Yongchang, Shen Ping, Liu Wenhui et al. (1994)Theory of NaturalZones in Junggar Basin [M]. pp 23-48, Petrol. Press, BeijingGas Formation and its Application[M]. pp. 195-210. ScienceWang Guanghui, Jiang Longfei, Wang Conghui et al.(1990)Mass Spec- Zhang Shiya and Zhou Jin(1994 ) Natural gas classification in China. Intrum Analysis [M]. pp. 54-64. Chemical Industry Press, Beijingeological Corpus of Petrol and Natural Gas( No. 4): Natural GasChinese version from F. w. MclaffertyGeological Sudy in China [C]. Pp 27-42. Geological PublishingWang Yutao and Jiang Shaobin(1997) Characteristics of vertical migra-House, Beijing( in Chinese)tion of the natural gas and analyses of its origin on the southen mar. Zhou Xingxi and Wang Hongjun(2000)Relationship between gas earbongin of the Junggar Basin [J]. Acta Sedimentologica Sinica. 15, 70sotope and maturity in the Craton Basin, Tarim Basin [J].Chinese74 (in Chinese with English abstract)Journal of Geochemistry. 19, 349-357中国煤化工CNMHG

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