Natural gas generation model and its response in accumulated fluids in the Yinggehai basin

Vol. 46 No, 11SCIENCE IN CHINA(Series D)November 2003Natural gas generation model and its response inaccumulated fluids in the yinggehai basinHAO Fang(郝芳), ZoU Huayao(邰邹华耀)& HUANG Baojia(黄保家)1. Faculty of Natural Resource and Technology Information, Petroleum University, Beijing 102200, China;2. Nanhai West Institute, China Offshore Oil Corporation Research Centre, Zhanjiang 524057, ChinaorrespondenceshouldbeaddressedtoHaoFang(email:haofang@cug.edu.cn)Abstract The natural gases found in the Yinggehai basin have nitrogen gas content ranging from3.5% to 35%, and carbon dioxide content ranging from 0. 1% to 93%. The nitrogen and co2 con-tents show a wide variation even in the same gas field. The natural gas generation and evolutionmodel has been established based on the relationship among the contents of nitrogen, carbondioxide and hydrocarbon gases in the discovered gas fields, and the results of simulation experiments. The thickness of the Tertiary and Quaternary in the Yinggehai basin is over 17km, and gasgeneration stages from diagenesis(microbial methane)to metamorphism have been developedNitrogen in this basin is organic in origin, and has been generated mainly in the catagenetic stage,roughly synchronous with hydrocarbon gases. Carbon dioxide is mainly inorganic in origin, and hasbeen generated from high-temperature decomposition of carbonates in the metamorphic stageThe Dongfang gas field shows strong inter-reservoir compositional heterogeneities, and differentfirmed that the complexity in natural gas composition in the Yinggehaigas reservoirs have recorded the products of gas generation at different stages. It has been con-the combined resultsof multi-stage natural gas generation and multi-stage gas injection and accumulationKeywords: Yinggehai basin, non-hydrocarbon gases, gas generation, reservoir fluidsDOI:10.1360/02vd0013The generation of natural gases is much more complicated in comparison with liquid petroleum in that natural gases could be generated from both humic and sapropelic organic matter atdifferent stages and that natural gases could be organic and inorganic in origin. Carbon dioxideand nitrogen are non-hydrocarbon gases commonly found in natural gases. The origins of CO 2 hasbeen widely studied.2, and effective parameters for distinguishing CO2 of different origins havebeen established. Because of the wide variation of isotopic composition for nitrogen of the sameorigin/ -s, the origin and genesis of nitrogen, and parameters for distinguishing nitrogen of differ-ent origins have remained the most vulnerable spot in natural gas generation studies. Natural gasesfound in the Yinggehai basin display a wide variation in the contents of hydrocarbon gases(7%-85%,mgn(35%-35%) and carbon dioxide(0山中国煤化 an ideal case forstudying the origin and genesis of nitrogen, and for estCNMH Gration model ofnatural gases of both organic and inorganic origins. The purpose of this paper is to establish thenatural gas generation model and its response in the accumulated fluids in the yinggehai basin bySCIENCE IN CHINA (Series D)combined geological and geochemical studies as well as simulation experiments.1 Geological settingThe Yinggehai and Qiongdongnan basins, which are separated by the No. I Fault, are im-portant Cenozoic, petroleum-bearing basins developed in the northern continental shelf of theSouth China sea. The Qiongdongnan basin trends east to northeast and is a rift basin with characteristics of multi-stage rifting. The Yinggehai basin trends northwest. It is a transform-extensionalbasin whose development was controlled by the combination of the lithosphere extension and thetrike-slip movement along the red river fault zone l6lThe Yinggehai basin is characterized by high subsidence rates (500-1400 m/Ma), and themaximum thickness of Tertiary-Quaternary is over 17 km. The Neogene-Quaternary is dominatedby fine-grain sediments and no faults except small-throw faults associated with diapirs have deeloped. The thermal gradient of the Yinggehai basin is about 46C/km, obviously higher than theaverage thermal gradients(30 C/km)of sedimentary basins of all ages in the worldAs a result of rapid loading and the resulting undercompaction, as well as hydrocarbon generation and thermal expansion of pore fluids, strong overpressures developed over a large part ofthe basin[6, 8, 9. The maximum pressure coefficient(fluid pressure/hydrostatic pressure ratio) so farmeasured is up to 1.9, and pressure prediction from geophysical data indicates that the pressurecoefficients of deep-buried rocks are even higherThe Miocene Sanya and Meishan formations are the most important source rocks in theYinggehai basin 6, I0. The richness of organic matter of each interval in the Yinggehai basin is usually low, with average total organic carbon (toC) content less than 0.6%. Most stratigraphic n-tervals of the Yinggehai Basin are dominated by Type Ill kerogen, with atomic H/C ratio less thanO, and Rock-Eval hydrogen index(Hi)less than 200 mg/g TOC. The Meishan and Sanya formations contain inorganic carbonate, with maximum inorganic carbonate content up to 15%2 Association of hydrocarbon gases, N2 and CO2 and their originThe hydrocarbon gas content in the Yinggehai basin ranges from 7.0%o to 85%. All the gasehave low heavy hydrocarbon gas(C t)contents, with C /2Cn ratios higher than 0.95. The ethane8C values vary between -21.8%cand-26.9 %o reflecting an origin of higher plants .Thereforethese gases should be classified as"coal-type gases", which is consistent with the fact that thesource rocks in this basin are dominated by humic kerogen. Most gases display methane 8Calues ranging from -40 %ooto-29 %o These gases are the thermal degradation products of humicorganic matter in the catagenensate/wet oas window [sourcerock vitrinite reflectance(Ro) between 1. 3% and 2.0%中国煤化工The CO, content of the yhai basin gases raCNMH Gnd CO, contentdecreases with methane content (fig. I(a)). C028C values are effective parameters of the originNo, lIGAS GENERATION MODEL IN THE YINGGEHAI BASIN105and genesis of CO. In the Yinggehai basin, CO2 in gases with its content lower than 10%o has8C values lighter than-10 %g and is mainly organic in origin; whereas in the gases with COcontent higher than 10%, CO, has 8C values larger than -10 and therefore is mainly inor-ganic in origin (fig. I(b))88100CH4(%)CO2(%)Fig. I. Variation of carbon dioxide content with methane content(a), and variation of carbon dioxide 8 C values with carbondioxide contents for natural gases from the Yinggehai basin(b)The content and isotopic composition of helium are sensitive indicators of mantle gasesNatural gases from the Yinggehai basin have very low helium content. The He/He ratios rangefrom 0.049x10 to 0.679x10, and the vales of the ratio of He/ He for gas samples from theYinggehai basin to that for the atmosphere(ra=1.4x10-6)range from 0.035 to 0.485 (fig. 2).Sucha helium isotopic composition indicates that the carbon dioxide is mainly of crustal origin and isthe product of high-temperature decomposition 12of inorganic carbonateThe commercial gas reservoirs in the 0.Yinggehai basin have a nitrogen content rang- a-ing from 3. 5% to 35 %, and the non-commercial 36-gas shows a nitrogen content up to 58.31% Onthe whole, the nitrogen content decreases asCO, content increases(fig 3(a)) and increasesas methane content increases(fig. 3(b). Thereare many possible sources for nitrogen-rich Fig. 2. Helium isotope distribution for natural gases fromgases, including atmospheric nitrogen! 13)the Yinggehai basin. R/R=Ratios of He/He for gas sammantle degassing 4, metamorphism I and ples to He!He for atmosphere(RA=1.4x10)thermal degradation of organic matter l5. Because of中国煤化工pIc compositionfor nitrogen of the same origin3-5I there are yet no efftCNMHGifying the originand genesis of nitrogen! 4, I5. Nitrogen in the Yinggehai basin has 8N values ranging from -15 %0SCIENCE IN CHINA (Series D)(b)30e810CH;{%)Fig 3. Relationship between nitrogen content and carbon dioxide content(a) and methane content(b) for natural gases fromthe Yinggehai basinto +10 %dfig. 4), which do not permit a satisfactory distinction among different nitrogen sourcesThe association among hydrocarbon gases, nitrogen and carbon dioxide, however, provides important information about the sources of the nitrogen. Firstly, the molar ratio of molecular nitrogento argon is about 80 in the atmosphere. The measured molar N,/Ar ratios for gases from theYinggehai basin range from less than 200 to 3600, which excludes the atmospheric originSecondly, nitrogen-rich gases derived from mantle degassing are usually characterized by highhelium concentrations l6) and high He/He ratios 4. As discussed above, natural gases from theYinggehai basin display low helium concen-tration with He/ He ratios considerably smaler than that of the air, indicating that nitrogencould not have derived from mantle degassingAs discussed above, in the Yinggehai basin the nitrogen content decreases as CO, content increases (ig. 3(a)), which indicates thatnitrogen and co, have different sources. Allgases with nitrogen content higher than 15%have CO28C values lighter than -10 %o(figFig 4. 8 N values for gases from the Yinggehai basin. 5(a), which suggests that nitrogen co-existswith organic CO2. The positive correlation ofnitrogen content with hydrocarbon gas content (ig. 3(b) and the co-existence of nitrogen withorganic CO, indicate that nitrogen in the Yinggehai basin is mainly organic in originIt is widely believed that, during thermal evolutionmainly generated in the over-mature stage(with sourceH中国煤化工 n-rich gases areCN MH Ggher than 3. 0%)when hydrocarbon gas generation almost ends and sedimentary rocks turn into metamorphicrocks5. For the gases generated under such conditions, nitrogen content would have reversedNo, lIGAS GENERATION MODEL IN THE YINGGEHAI BASIN10orrelation with hydrocarbon gas content, and with increasing maturity of hydrocarbon gases(isotopic composition becoming heavier), nitrogen content would increase. Yet, exactly the oppo-site is observed in the Yinggehai basin: nitrogen content displays positive correlation with hydrocarbon gas content (ig. 3(b)and, as methane becomes more mature(8 C becomes heavier),nitrogen content decreases(fig. 5(b). Moreover, almost all the gases with nitrogen content higherthan 15 hathane8C values smaller than -35 %d(fig. 5(b)). Obviously, nitrogen-rich gasesin the Yinggehai basin could not have been generated in the metamorphic stage. Instead, the nitrogen-rich gases have been generated from organic matter within the catagenetic stage(vitrinitereflectance <2.0%) Most nitrogen-rich gases co-exist with a small amount of condensates, whichfurther confirms that the nitrogen-rich gases in the Yinggehai basin have been generated withincatagenetic stage0Organic CO°Inorganic CO2-25CO26"C(‰)(b)Methane-404CH4C(‰)CO2(%)Fig. 5. Variations of nitrogen content with CO, 8C values(a) and methane 8C values(b), and variations of methane andethane 8C values with CO, content for natural gases(c)from the Yinggehai basinAs cO, content increases.8 3C values of methane中国煤化工(fig. 4(d)), whi-ch is contrary to the correlation of nitrogen content withCNMHGINFecting that inorganic CO2 was generated at an even higher temperature/maturity level and that hydrocarbon gasesSCIENCE IN CHINA (Series D)and nitrogen were generated earlier than inorganic CO23 Simulation experiments on non-hydrocarbon gas generationSimulation experiments on hydrocarbon gas generation have been carried out by many investigators. Simulation experiments on the generation of non-hydrocarbon gases, however, havebeen rarely reported. In order to determine the source and genesis of the nitrogen and carbonoxide in the yinggehai basin and establish a combined generation model of both hydrocarbonGas content(%e)Gas content(%010203040506070l0203040506070simulation ex- periments were carriedWell LG20-]-11644. m(Yinggehai) o 100-FP 2534.4 m(Meishan)out using shale samples from MeishanTOC-0.33%TOC=0.39%R=0.38%and Yinggehai formations. The varia300aR.-0.732400btion of the com- position of the ex-periment products is…CO2It can be seen from fig. 6(i)that mostnitrogen was generated at temperad1000(b) tures between300℃and600℃,withFig. 6. Variation of the product composition of the experiments on peak generation temperature for thede from the ymarine shales from the Yinggehai basin with temperaturebeing at 450C, slightly higher thanthat for the sample from the Meishan formation (fig. 6(b));(ii)that the significant carbon dioxidegeneration began at temperatures higher than 450C, with peak generation temperature between600C and 700 C; (ii) and that the range of temperature for CO2 generation was wider than thatfor nitrogen, and the peak generation temperature for CO, generation was higher than that fornitrogen generation. As a result, nitrogen generation should be earlier than CO2 generation for thesame source rockNo reliable nitrogen isotopic measurement on the experiment products was obtained in thisstudy. CO2 generated in the experiments had a measured 8C value of.6 %o similar to the 8cvalues for the inorganic CO2 from the Yinggehai basin gases(cf. fig. 1(b). The measured CO28C value confirms that the carbon dioxide was the product of high-temperature decompositionof inorganic carbonate. The experimental result also confirms that shales with average inorganiccarbonate content of about 10% could generated a large amount of CO2, which indirectly suggestthat the inorganic CO in gases in the Yinggehai basin could have been generated fromtemperature decomposition of inorganic carbonates in the Sanya and Meishan formations4 Natural gas generation and evolution model in th中国煤化工Simulation experiments confirm that the potentialHCNMHGhai basin couldgenerate large amounts of nitrogen and carbon dioxide(fig. 6). What is more important is that, thesimulation experiments confirm that nitrogen was generated earlier than CO,which is consistentNo, lIGAS GENERATION MODEL IN THE YINGGEHAI BASIN109with the association among hydrocarbon gases, nitrogen and Co2, i.e. nitrogen content has a posi-tive correlation with hydrocarbon gas content(fig. 3(b)and nitrogen co-exists with organic COTherefore, both simulation experiments and analysis of natural gas origin in the Yinggehai basinconfirm that organic, nitrogen-rich gases could be generated not only in the metamorphic stage(source rock R,>3.0%)5, but also within the catagenetic stage(source rock R >2.0%). This newmechanism of nitrogen-rich gas generation is a key to understanding non-hydrocarbon gas distribution in the Yinggehai basin On the basis of combined analysis of the origin of the naturalgases and the results of simulation experiments, the natural gas generation and evolution model inthe Yinggehai basin can be established (fig. 7)Composition of accumulated fluidsin Dongfang I-I Gas FicldAmountCO ( CI, P 8 C, (6) R Gaseneral0204002040600306090-30-50 Resery.Microbial methaneVDST-28/ST-39/DST-8/ DST-29/DST.2DS1-34DS144/DST. 39(4300 m/Thermal4DST-lgenetie2/DST.4methane5/DSI-3(50N05/DST-22/DST-22/DST- 3Inoreanic Co7/DST-37DST-2Fig. 7. Natural gas generation model in the Yinggehai basin and its responses in the accumulated fluids of different injectionstages in the Dongfang l-l gas field. R Gas Reserv. = representative gas reservoirs(1) Diagenetic stage (R<0.5%, burial depth <2800 m). The main product of the early di-tic stage was microbial methane the depth for microbial methane generation was no deeperthan 2000 m Each stratigraphic interval was dominated by Type Ill kerogen, and therefore hadrelatively high biogenetic gas potential. Strong gas sho中国煤化工WeLd301|Awas biogenetic methane. The main product in the lateCNMHGnIC CO2,whichwas the result of organic matter decarboxylation. The generaOI organc tU2 sustained untilvitrinite reflectance reached about 1.3%(fig. 7). However, most organic Co, could not accumul110SCIENCE IN CHINA (Series D)late because of its high solubility in water and large amount of formation water in the generation(i) Early catagenetic stage(R=0.5% to 1.3%o, burial depth from 2800 m to 4300 m)main products of this stage were thermal genetic hydrocarbon gases, nitrogen and a small amountof liquid hydrocarbons, with the generation of organic CO2 still ongoing. Because no major faultsdeveloped plus rapid sedimentation of fine-grained sediments, the overpressured source rockslong remained a closed or semi-closed fluid system! l0, I/. As a result, the products of the majorsource rocks(the Maishan and Sanya formations) at the early catagenetic stage had long beenretained, and were expelled along with natural gases generated later Ol(ii) Late catagenetic stage (Ro=1.3% to 2.0%, burial depth from 4300 m to 5000 m). Themajor products of this stage were hydrocarbon gases dominated by methane and nitrogen. Late atthis stage, inorganic carbonate decomposition began and a considerable amount of inorganic cowas generated(iv) Metamorphic stage(Ro>2.0%, burial depth >5000 m). The major product of thisstage was CO2 generated from the decomposition of inorganic carbonates and methane from hightemperature cracking of kerogen and/or liquid hydrocarbons5 Response of the natural gas generation and evolution model in the accumulated fluidsThere are considerable compositional variations among different reservoirs in the same gasfield in the Yinggehai basin. We call these variations inter-reservoir heterogeneities. Comparedwith the lateral compositional variations in the same reservoir or in-reservoir heterogeneities, n-ter-reservoir heterogeneities have the advantage of being unaffected by in-reservoir mixing processes after accumulation, and therefore "record"the original differences in the composition ofpetroleum that filled the reservoir in different periods. The Dongfang gas field is the largest gasfield found in the Yinggehai basin. The composition and characteristics of the natural gases indifferent fault blocks and different reservoirs reflect the products of different gas generation stagesand therefore provide a basis for examining the established natural gas generation modelGases from dst2 and dst3 in Well dF118 and those from dst2 and dst in Well dF119are dominated by methane, with nitrogen content of 15%--23% and methane 8 C values smallerthan -50 %o(fig. 7). These gases represent the mixture of biogenetic gas with a small amount ofthermal genetic gas generated within catagenetic stage(fig. 7)Gases from dst2 and dst3 in Well dF119 and those from dst3 and dst in Well DF114are dominated by methane, with nitrogen content of 18%-33%. These gases have very low CO2content with CO28C values ranging from -12.5 %do -19.9 %dfig. 7) suggesting organic originThe methane 83C values range from -38 %oto-40%中国煤化工es generated inearly catagenetic stageCNMHGGases from dsti and dst2 in Well dfl14 those from dst in Well df112 and thosefrom DST2 and DST3 in Well DF115 have relatively larger methane 8C values(33.0%otoNo, lIGAS GENERATION MODEL IN THE YINGGEHAI BASIN111137.0%, and the hydrocarbon gases display a higher level of maturity. These gases represent thegases generated in late catagenetic stageGases from dst2 and dst3 in Well DF112 and those from dst3 and dst2 in Well DF117are dominated by carbon dioxide, with nitrogen content lower than 10%. The carbon dioxide has8C values larger than -10 %g and is mainly inorganic in origin. The methane 8C values be-come larger(>-32.0%3, suggesting a higher maturity level. These gases represent products of themetamorphic stage6 ConclusionsBased on the analysis on the origin and genesis of the natural gases in the Yinggehai basinand simulation experiments on potential source rocks, the following conclusions have been drawn(1)Because of the huge thicknesses of the Tertiary and Quaternary, natural gas generation stagesfrom diagenesis to metamorphism have been developed in the Yinggehai basin.(2) Organic nitrgen-rich gases can be generated from organic matter not only at metamorphic stage(source rockR>3.0%), but also within the catagenetic stage(source rock R, <2.0%0) Such a mechanism oftrogen-rich gas generation is a key to understanding and predicting the distribution of nonhydrocarbon gases in the Yinggehai basin. (3) The inter-reservoir compositional heterogeneities ofthe gas fields found in the Yinggehai basin have recorded the products generated at different gasgeneration stages, and the complexities in the composition of the natural gases in the yinggehabasin are a combined result of multi-stage gas generation and multi-period natural gas migrationAcknowledgements This work was supported by the National Natural Science Foundation of China( Grant NoReferences1. Dai, J.X., Song, Y, Dai, C. S. et al., Geochemistry and accumulation of carbon dioxide gases in China, AAPG Bulletin1996,80:1615-16262. Dai Jinxing, Selected Works of Natural Gas Geology and Geochemistry (in Chinese), Beijing: Petroleum Industry Press,20003. Jenden, P. D, Kaplan, I.R., Poreda, R. J. et al. Origin of nitrogen-rich natural gases in the California Great Valley: Evidence from helium, carbon and nitrogen isotope ratios, Geochimica et Cosmochimica Acta, 1988, 52: 851-861M, Pineau, F, Demaiffe, D, Nitrogen and carbon isotopic composition in the diamonds of Mbuji Mayi(Zaire)Earth and planet science letters, 1984. 68: 399-4115. Exley, R. A, Boyd, S.R. Mattey, D. P. et al., Nitrogen isotope geochemistry of basaltic glasses: Implications for mantledegassing and structure? Earth and Planet Science Letters, 1987, 81: 163-1746. Gong Zaisheng, Li Sitian, Continental Margin Basin Analysis and Hydrocarbon Accumulation of the Northern South China Sea(in Chinese), Beijing: Science Press, 19977. Hunt, J M, Petroleum Geology and Geochemistry, 2d ed, San Frr中国煤化工%48. Hao, F, Sun, Y. C, Li, S. T. et al., Overpressure retardation of oCN MHGarDon generatoncase study from the Yinggehai and Qiongdongnan basins, offshore9. Zhang Minqiang, Li Sitian, Active hydrothermal fluid flow and pool-forming studies in the Yinggehai basin, in NaturalGas Geology Studies and Application (in Chinese)(eds Song Yan, Wei Guoqi ), Beijing: Petroleum Industry Press, 2000.l112SCIENCE IN CHINA (Series D)177-1870. Hao, F, Li,S. T, Dong, w. L. et al. Abnormal organic matter maturation in the Yinggehai basin, offshore South ChinaSea: Implications for hydrocarbon expulsion and fluid migration from overpressured systems, Journal of Petroleum Geolgy,1998,21:427-411. Xu Yongchang, Shen Ping, Liu Wenhui, Theory and Application of Natural Gas Generation (in Chinese), Beijing: Science12. Dai Jinxing, Identification of Hydrocarbon Gases, Science in China, Ser. B, 1992, 35(10): 1246-125713. Marty, B, Criaud, A, Fouillac, C, Low enthalpy geothermal fluids from the Paris sedimentary basin (I)Characteris-tics and origin of gases, Geothermics, 1988, 17: 619 63314. Hiyagon, H, Kennedy, B M, Noble gases in CHA -rich gas field, Geochimica et Cosmochimica Acta, 1992, 56: 1569-158915. Littke, R, Krooss, B, Idiz, E, et al, Molecular nitrogen in natural gas accumulations: Generation from sedimentary a-ganic matter at high temperatures, AAPG Bulletin, 1995, 79: 410-43016. Chen Jianfa, Zhang Tongwei, Wang Xianbin et al, The genesis of non-hydrocarbon gases in classical gas fields in Easternand Middle China and its geological and geochemical significance, in Natural Gas Geology Studies and Application (inChinese)(eds Song Yan, Wei Guoqi), Beijing: Petroleum Industry Press, 2000, 139-14517. Price, L C, Basin richness and source rock disruption: A fundamental relationship? Journal of Petroleum Geology, 19081:222-248中国煤化工CNMHG