Gas formation mechanism of marine carbonate source rocks in China

Science in China ser. d earth Sciences 2005 Vol 48 No, 4 441-453Gas formation mechanism of marine carbonate sourcerocks in chinaZHAO Wenzhi!, WANG Zhaoyun, HE Haiqing, ZHANG Mingjie, WANG Hongiun'WANG Yunpeng*& QIN Yong1. Research Institute of Petroleum Exploration Development, PetroChina, Beijing 100083, China2. Petrochina Exploration& Production, Beijing 100011, China;3. Department of Geology, Lanzhou University, Lanzhou 730000, China4. Guangzhou Geochemical Institute, Chinese Academy of Sciences, Guangzhou 510640, China5. China University of Mining and Technology, Xuzhou 221000, ChinaCorrespondenceshouldbeaddressedtoZhaoWenzhi(email:Zwz@petrochina.com.cn)Received March 8. 2004Abstract By breaking through the conventional ideology on the study of gas-formationmechanism of source rocks which focused on merely the characteristics of source rocks themselves, and based on the development and distribution characteristics of marine carbonatesource rocks in the medium-to-bottom parts of the chinese superimposed sedimentary basins aswell as various processes in the course of gas generation and reservoir forming, the complexhydrocarbon-generating model of carbonates is established. Such a model concerns not only thethree-stage"gas formation characteristics of carbonates, but also the gas generation processdue to a thermal cracking of oil and the re-generation process of gas from heated secondaryorganic matters during the gas-generation and reservoir-forming course. The high-quality hy-drocarbon generating matter of marine carbonate source rocks determined a high gas-oil ratioand tended to form paleo-oil-reservoirs, representing an enrichment process of organic mattersThe thermal cracking of oil in palaeo-oil-reservoirs is an important means for the effective formation of gas reservoirs. The secondary organic matters in carbonate reservoirs may regenerategas after a secondary deep burial, thus beffectiveKeywords: carbonate, three-stage hydrocarbon generation, inhibitive effect of maturity, oil-cracking gas, secondDOI:10.1360/04yd011lIt has been proven in exploration practice that the carbonates can not be the same as that for clasticcarbonates in China not only can generate hydrocar- rocks, and it is important to establish a special hydrobons, but also form commercial reservoirs. The car- carbon generation mechanism and model of our ownbonates are different from clastic rocks in view of their for carbonatedeposition environment as well as their sedimentary gas generatio中国煤化工and diagenetic processes. Therefore, the evaluation bonates of dCNMHPto the complecriteria and hydrocarbon generation mechanism for ity of gas generation from organic matters in carboCopyright by Science in China Press 2005442Science in China ser d Earth sciencesates in three aspects as followswas the material base for oil generation in the earlystage. while the saline-alkaline environment of water1 Re-discussion on the " three-stage hydrocarbongeneration model in carbonatesbody was its external condition. The research reprojeIn 1997, Wang Zhaoyun, together with other co- and Reservoir-forming Conditions of Immature Oil inauthors, published a paper entitled"Study on the China"also indicates that the salinized-meta-alkalinMechanism and the Three-Stage' Model of Hydro-carbon Generation in Carbonates", advancing a point scale formation of immature and low-mature oil 5). Inof view that the carbonate hydrocarbon source rocks inaddition, as for the modern or relatively young carChina experienced three master hydrocarbon genera- bonate sedimentary profiles in the world(e.g, Austintion stages(see curves 1, 2 and 3 in fig. I)U. Owing to Chalk of the Cretaceous System in south Texas),thetheir efforts in an in-depth study in recent years, theprofile of the early diagenetic stage has quite a highunderstanding has been further deepeneddegree of hydrocarbon conversion o demonstr1. 1 Generation of the immature and low-mature oil that the matrix type and sedimentary environment ofcontrolled by the type of organic matters and the marine carbonate source rocks determined thesedimentary environment of marine carbonate source huge-quantity generation of immature and low-maturerockorganic matters in marine carbonates are 1.2 The Ro value of kerogen for oil generationprimarily composed of low aquatic organisms. Excel- shifted downwar0.3 due to the inhibitive effectlent oil generation matrix (including the matrix un- of marine environment on thermal evolution of ordergoing saprofication due to bacteria transformationganic mattersEvolutionHydrocarbonHydrocarbon generationeneration of source rocks Tempe- Oil-cracking geby secondaryStage R(%)Oil0.60.850℃Fig. 1. Hydrocarbon generation model of marine carbonates in China. 1, Immature and loH中国煤化工ough kerogen thermal degradation, and Ro is shifted downwards to 1.6%0: 3, gas generation through thermaCNH Gy inclusion organicmatters;4, gas generation from organic matters in source rocks, 5, gas generation through oil cracking; 6, gas generation by cracking secon-dary asphalt; 7, gas generation from secondary inclusion organic matters through thermal pressurized crackingas formation mechanism of marine carbonate source rocks in ChinaComparitive study on the maturity of organic the coal in the Taiyuan Group is lower than that of thematters in coal and limestone deposited in the Carbon- overlying Shanxi Group due to the inhibitive effect offerous -Permian marine -terrestrial transitional envi- the marine environment on the thermal evolution ofronment in the southern part of North China was con- organic matters. The Taiyuan Group was depositedducted. The result indicated that under the same heat- mainly in an epicontinental marine carbonate plat-ing conditions in the history, the thermal evolution form-near-shallow sea clastic environment. The limerates of organic matters in the coal and limestone of stone in the group was developed in the epicontinentalTaiyuan Group and the coal of Shanxi Group were sea environment, while the coal was formed in thdifferent. The samples studied were mainly collected half-saline water-saline water peat palustrine. As indi-from the southern area of North China, including the cated by the analysis of organic matter components,southern part of Henan Province, as well as the west the coal has a high content of sulfur and hydrogen,regions of Shandong and Anhui provinces. In order to affected apparently by marine water. However, theobtain a sample with lower maturity than the studied coal of the Shanxi Group was formed primarily in thesamples, an additional sample was collected from freshwater peat palustrine, and contains low sulfur andShanxi Province. The analytical data on samples from high oxygen, indicating that basically, the Shanxi20 comparative sites are shown in fig. 2. The coal ma Group has never been immersed in seawater. There-turity (Ro)for the Taiyuan Group varies between fore, under the same heating conditions, the thermal0.54%-4.88%, that of overlying limestone between evolution rate of organic matters in the marine0.34%-4.90%, and that of the Shanxi Group coal atbonates is somewhat lagged in comparison with that of0.58%—4.57%the non-marine sedimentary source rocks, and it willbe more reasonable to shift the ro of kerogen to theAccording to the one-to-one comparison and value of 1.6% in marine carbonate rocksarrangement of the horizons and ages of the different1.3 The organic matters in primary and secondarysamples, it is found that in the samples with Ro<2.0%, inclusions of carbonate rocks cracked at a high-evolutwo phenomena exist: One is that the maturity oftion phase and thus expulsed hydrocarbonslimestone is apparently lower than that of a coal seamon the same horizon with the maximum differenceDuring the deposition and diagenetic processes ofreaching 0.38%; and the other is that the maturity of carbonate rocks, abundant primary organic inclusionsR。(%25050070kmTaiyuan GroupHarT±一 Shanxi GroupYinchuan Taiyuan,210Lhasa Chor15GuangBaofeng 456中国煤化工CNMHGFig. 2. Sampling locations and maturity comparison between limestone and coal. Upper, Sampling locations: lower, maturity combetween limestone and coalScience in China ser d earth scienceswere formed due to recrystallization and metasoma- highly thermally matured, featuring gas generationtism. In addition, the reservoir characteristics of car- However, more oil than gas will be generated duringbonates(e.g, solution voids, caverns, fractures, etc. the hydrocarbon generation process because of the lowformed during the epigenetic phase led to the forma- aquatic organisms in marine carbonates. It is nottion of secondary organic inclusions(the amount of argue here that during the deposition and diageneticwhich is controlled by the quantity of hydrocarbon process, the meta-oxidized environment may humifi-fluids and by the media surrounding the carbonate cate organic matters, thereby deteriorating the type ofrocks)in the later reservoir formation process. Organic organic matters. It should be pointed out here that thismatter inclusion is the unique component in carbon- process is not in a controlling position. During the oilates,and they may coexist in the rocks as the single-, generation phase in carbonates(Ro: 0.6%-1.6%),adouble-or tri-phase of solid, liquid and gas. Refer to large amount of oil will be undoubtedly formed, whichthe charts(fig 3(a()may, under proper conditions, form paleo-reservoirsUpon the thermal decomposition and conversion or be scattered in some reservoirs, where the oil satuof inclusion organic matters(including hydrocarbonration is lower than that for commercial reservoirs Ingenerated by kerogen, and gas generated by oil or as-both of the two situations, the organic matters in carphalt), the conversion from high-density organic matbonates experience an accumulation process, and ifter to low-density liquids (oil or gas)may lead to any further heating process occurs, the formation of avolumetric expansion, thus increasing the pore fluid gas pool by organic matters can then be drasticallyncreased. The marine carbonates in China are devel-pressure. Particularly, gas generation tends to formhigher pressure than oil generation does due to itsoped at the very bottom of the superimposed basin, oldin age, exposed in long thermal evolution and high inrather low densityBy applying the hydrogenmaturity, making a favorable condition for the earlybalance principle and calculating the volume andnange of oil as the oil in reservoirs crackedstage formation of reservoirs Under high-temperatureinto gas, Barker(1990) pointed out that, under stan-conditions, the paleo-reservoirs or the scattered hy-drocarbons(crude or asphalt) may generate gas easilydard temperature and pressure conditions, a barrel ofthrough cracking, and form gas pools. This is theoil(158.987X10'm)may generate, through cracking, So-called the secondary accumulation of organic mat-3000cft(85 m) gas, and some graphite slag In addi-ters in hydrocarbon source rocks and the process oftion for a reservoir full of oil, if 1 %o of the oil is conefficient gas pool formation in marine carbonatesincrease caused will be high enough to increase the 2.1 Study on the crude oil instability under highpressure of the enclosed reservoir by 1.0 Psi/ft(22.6 temperature conditionskPa/m). If all the oil in the reservoir was converted toIt is commonly agreed that crude oil is unstablegas through cracking and the pressure of the enclosed with time and temperature 4-27. As indicated by thereservoir was to be maintained, then about 75% of the statistics concerning the burial temperatures of knowngas would be dispersed, or the gas-water contact hadreservoirs across the world. most of the oil exist at ato be lowered so as to increase the effective pore space. temperature of 65.5-149C. As the reservoir tem-matters is effective not just after the decomposition of perature exceeds 150'C, oil will be thermally crackedcarbonate minerals, and the thermal pressurized into gas, causing an apparent incase of thecracking is the main way of its hydrocarbon expulsionratio in the reservoirs. When reservoir temperatureexceeds 200中国煤化工substitute oil2 Oil cracking is the main way to gas generationTherefore. tIH Gres are referredin marine carbonatesCNtoas“oiwiILl wyIuo. cude oil is unstaThe marine carbonates in China are generally ble under a high-temperature environment (>150C),as formation mechanism of marine carbonate source rocks in ChinaFig 3. Photograph of primary and secondary inclusions and multiple-phase asphalt. (a) Well Wei 117, 29x, (-), Z. 3047.21 m, primary gas andasphalt inclusions in solution-void dolomite; (b) Well Bao 10, 450X, () P2, 2783.30-2783.51 m, primary inclusions in bio-limestone; (c)Well Wei99, 450X, (-) Z4, 2903.00--2903 14 m, primary inclusions in solution-void dolomite are distributed along cleavage cracks; (d) Well Wei 99, 450X-) Zo, 3029.02-3029.12 m, secondary inclusions in grayish white dolomite;(e) Well Chang 21, 450x,(-),P1, 2357.09-2357.22 m, inclusions insalt lime veins of limestone; (f) Well Zi 1, 450X, () Z2, 4047.58 m, deformed mobile inclusions in grayish white doldsil tite; (g)Well Zi 7, 71x,(),Z2, 3936 m, secondary asphalt fill in grayish white dolomite;(h) Well Bao 10, 71X,(), P2Well Wei 117 29x,(), Z4 3047. 21 m, multiple-phase asphalt developed in solutionWei 112, 29x, (-), Z4, 3075.26 m, as phalt in salt lime veins of grayish white doldsiltite.TH”erns:()Well446Science in China ser d earth scienceswhich, with the chronic passage of time, may be con- perature compensation principle emphasizes that timeverted into gas through crackinand temperature compensates each other during theThe previous simulation experiments on gas gen-thermal decomposition process of organic matters 251the temperaeration through oil cracking and its components conducted by other researchers show that gas generationby oil in the reservoirs must reach the temperatureare impacted by various factors such as temperature,required by the minimum value of the normal distribucatalyst, pressure, medium, etc.tion of crude oil activation energy. That is, if the tem-perature in a reservoir is lower than such a temperature(1)In 1991, Mango published a paper inrequired, the crude oil in the reservoir will tend to bepointing out that, during the thermal decom- quite stable, and no thermal cracking will occur noposition process (100--150C), liquid hydrocarbons matter how long the thermal evolution lastedare relatively stable. This viewpoint is different from 2.2 Study on oil-cracking gas generation mechanicsthe previous study, which stated that the hydrocarbonsfor Well Lungu 12. the lunnan region tarim basinare unstable and will progressively decompose intomethane and pyrobitumen. Naphthalene, for instance,It is affirmative that the marine carbonate sourcehas been quite stable during the thermal decomposi- rocks are developed in the platform of the Tarim Basintion that lasted several billion years. During the higher and contributed to the oillgas pool formation". Thetemperature period (150--190C), however, the high- detailed oil-source correlation analysis indicated thatmolecular-weight hydrocarbons may generate gasthe crude oil in Well Lungu 12 of the Lunnan regionthrough cracking, and the half-life of each hydrocar(for well sites see the index map for fig. 6)was generbon component in oil ranges from millions to billions ated from marine source rocks. Therefore the samplesIn the paper published in Nature inin the well are selected for the study of oil-crackingMango demonstrated the catalysis effect of transition gas generation mechanics. In order to compare themetal ions in carbonates on gas generation. As indi- parameters of hydrocarbon generation mechanics becated by the oil cracking experiment with NiO/SiO, tween marine oil and marine source rock kerogen(activated by 400C sulfur hydride) as the catalyticsamples with a relatively low thermal maturity at theagent, the oil was rapidly converted into gas at 175C. mid- and upper-Proterozoic outcrop in the Zhangjia-The reaction mechanism is that the transition metalkou Region of North China were collected to preparekerogen. Table 2 presents the physical properties ofpromotes the reaction of hydrogen and n-alkane the crude and the geochemical analysis data on the(formed by thermal decomposition of kerogen) to formkerogenlight hydrocarbons and gas2) Table I presents the experimental results ofThe experiment was conducted by the gold-pipegas generation through cracking of some hydrocarbonenclosed system, with the temperature-rise rates at 2Ccomponents in crude oil under different conditions/h and 20 C/h respectively. Refer to fig 4 for theThe experiment based on one compound in oil indimethane yields. Table 3 presents the calculated pacates that its thermal decomposition conforms to the rameters of gas generation mechanics. The oil in Well12 is viscous crude subjected to bicRank-1 mechanics equation or to the low tempera tion and oxidization, featuring relatively low gas yieldture-high pressure reaction mechanism. Pressure restrains the thermal decomposition of crude oilly highters are closely related to the property of crude. As forThe authors believe that, although the time-tem- the compari中国煤化工 -ration dynamicCNMHG1)Liang Digang, Zhang Shuichang, Wang Feiyu et al. Study on oil source rocks and oil Sourcebasin. I ne state /Ninth Five-Year PlanKey Research Projectas formation mechanism of marine carbonate source rocks in ChinaTable 1 Effects of temperature, pressure, and catalyst on cracking gas generation by hydrocarbon components in crude oilExperimental conditionsSampleExperimental resultsData sourceSystem Temperature/'c Heating Pressure/MPUnder 12MPa, thermal decomposition conformsto Rank-I mechanics equation. In the producBehar and VanDry combustion 325-425 1-360 h 20, 40, 80 saturated hydrocarbon is primarily composed of denbrouckeCIo-C22, and the gas conversion ratio is from19961<2%to80%The thermal decomposition reaction of a singleradicals77-550℃;21,200,600MPahydrocarbon compound conforms to the low3 single com-N-hexane. 2temperature-high pressure reaction mechanism.290-360℃.21-1560MPaand the thermal decomposition rate is restrainedby high pressureTable 2 Base data on experimental samples for gas generation mechanicsRDepth/m HorizonPhysical properties and geochemical parametersCrude Lunnan, Tarim 401000 Density, Viscosity/mPa.s Set point/C IBP/C Paraffin con- Sulfur contenttent(%)9.03718.10.17R。(%lH/mg.g (TOC)Kerogen North China Outcrop51.754300.562℃h2℃m8Simulation temperature/CFig. 4. Methane yield curves of curde and kerogen at different temperature-rise rates. (a) Crude; (b)kerogenTable 3 Parameters of hydrocarbon generation mechanics of marine crude and kerogenTyComponent Pre-exponential factor A/S"Activation energy(kcal/mol),rangevalue/main peak valueYield ratio(mL/g sample65-79/70Marine crudeC-Cs63-70/66Marine kerogen4.04×101.84×1057-78/70parameters between crude and kerogen, the activation 2.3 Case study of cracking-gas by the Paleozoicenergy of heavy-hydrocarbon gas from the thermal crude in the East Tarim Regioncracking of crude has a narrower distribution range中国煤化工than that of kerogen. In other words, once the crackintemperature is reached, a huge quantity of thermined by thCNMHG history of hy-will bedrocarbon kitchen and the heating history of reser-448Science in China ser d earth sciencesvoirs.The East Tarim region, tectonically, is part of the the Middle and Upper Ordovician underwent at theManjiaer Depression. The most extensively distributed end of the Caledonian Movement a large-scale hyhydrocarbon source rocks are the platform-facies car- drocarbon generation and reservoir formation probonates, followed by pelite and marl, in the Cam- cess. These hydrocarbons might exist in three typesbrian-Lower Ordovicianof geological bodies:(1)In structural-lithologicAs indicated by the figures of burial, thermal traps developed on large uplifts adjacent to hydroevolution and hydrocarbon generation history for the carbon generation kitchen;(2)in a series of stratiWell Weima 1(for well site see the index map for figgrafic-lithologic traps developed on the slope from6), the Cambrian source rocks were at the late Ordo-hydrocarbon generation kitchen to uplifts(e. g, bavician Period(about 455 Ma ago), rapidly buried at a sin bed fans, slope fans, onlap pinchout zone, etc.depth of 7500 m(fig. 5). During a short period of and(3) in the reservoirs adjacent to the hydrocarbonmerely several million years, its Ro was drastically generation horizon within the hydrocarbon generaincreased up to higher than 3. 0% from <0.6%, andtion kitchen area. The liquid hydrocarbons thereprocess of a larnt ofexperienced primary accumulation, but not signifihydrocarcant enough to form commercial reservoirs(refer tothe predicated profile of reservoirs in fig. 6). TheThe conversion of oil into cracking-gas at that above-mentioned three types of hydrocarbon accutime was closely related to the horizon at which the mulation are developed chiefly within the Cam-crude is reserved. In the East Tarim region, the ar- bria-Silurian formations The reservoirs formed overganic matters in the carbonates of the Cambrian- paleo-uplifts suffered intensive destruction durinLower Ordovician and those in the clastic rocks of the Caledonian Movement. whereas the reservoirsQ150℃400010000Era/MaFig. 5. Burial, thermal evolution and hydrocarbon generation history of well Weima 1.0中国煤化工oil-cracking gas generation; (2 Upper limit of the Ordovician System stratum with oil-cHC NMHGevonian Period andwithout further generation thereafter; @3 Lower limit of the Ordovician System stratum without oil-cracking gas generation after the Tertiaryas formation mechanism of marine carbonate source rocks in China4490 10 20 km Funke 1Weima IYingnan 2KzKZongque riverxihai strucure02tructure02Weima structurestructureManjiaer Depression 0Lingdongburied I-9001structureHetianYuanbaoshan12000nose-faulted structureFig. 6. Profile of geological structure and predicated reservoir for Manjiaer Depressionformed in stratigrafic-lithologic traps within the Devonian. it has been sited above the 150C isother-slope area, and the hydrocarbons concentrated and mal line, and no further large-scale oil-crackingdispersedly distributed adjacent to the source rocks, generation happened; and thirdly, the Ordovicianhave been reserved, becoming the main gas sourceg. 5 experiencematters when these reservoirs are deeply re-buriedil-cracking gas generation about 458 Ma ago, andlateremains such even today (below Line 3), or noAs indicated by the study on crude stabilityoil-cracking gas generation for part of the Ordovician150C is the critical temperature for oil thermal after the Tertiary(the formation between Line @2 andcracking. The oil in the Silurian accordingly, has never Line 3 ). It should be noted that the compensationexperienced the oil-cracking gas generation process effect of time on temperature is eglected in the above(fig. 5). By contrast, the oil in the Cambrian and onlap discussionpinchout zone underwent oil-cracking gas generationas early as about 460 Ma ago, and continues such evenLow productivity gas flow was produced in Welltoday. As for the oil in the Ordovician reservoirs, theWeima 1. In Well Yingnan 2, which is similar in oillate stage changes are quite complex, and three kinds genesis to that in Well Weima 1(see fig. 6 for the wellof situations are observed. Firstly, the Upper Ordovi-site), the daily gas production is 145000 m, the dailycian formations above Line in fig 5(depositedcondensate oil 1. 33 m. for the yingnanafter 452 Ma or so. and the present burial depth at the predicated reserve of condensate gas is 52750Xabout 4500 m) has always been sited above the 150C10m, with a content of condensate oil being 52 g/misothermal line, and no oil-cracking gas generation has(ii) Features of oil-cracking gas. Generally, theoccurred; secondly, the Ordovician formations be- gas in the East Tarim region is wet gas. In the gas pro-tween Line and Line @2 in fig. 5(deposited after duced from中国煤化工 ent of methane455-452 Ma, and the presentreaches 68.9CNMHG.07%; the con-5300 m) experiencedration betent of heavy-lll gas t2+ Is relatively highfore the Devonian After an uplift at the end of the too, ranging generally between 9. 17% and 14.01%Science in China ser d earth sciencesThe aridity coefficient of gas is less than 0.95. The varies between 2. 4% and 15.7%0carbon isotope value for methane is between -36Carbonates with enriched asphalt contain a largeand-40%o and that for ethane is between.5 %and amount of organic matters 31. 321. In Well Gaoke 1,the-309o The gas is generated mainly from the sapro- content of organic carbon in the Sinian and Cambrianmaterial in marine source rocks of the Lower samples is 2. 14% and 1.55% respectively. The analytiPaleozoic. According to the gas genesis model based data on pyrolysis chromatograph and the extractionon the composition and formation temperature of gas and group composition of dissoluable organic mattersby Rooney(1995)and Rrinzhofer (1995), the Well indicate that such asphalt is of substantial potential forYingnan 2 gas, with its 8C2-8CI value betweenUnder th2.6 and 5.4, and the 8C3-8C2 value between 7.5 conditions, the secondary asphalt may re-crack toand 8.6, should be the oil-cracking gas under high generate gas. The secondary asphalt can be an effec3 Regeneration of gas by secondary organic mat- po as sourcetemperature conditionstive gas source, and contribute to the formation of gasters in reservoir carbonates3.3 Gas generation potential of organic matters in3. 1 Carbonates featuring both reservoirs and sourcesecondary inclusionsrocks contain organic matters with different originsThe vacuum cataclastic-mass spectrography ap-dopted to study the gas composition ofCarbonates that can be hydrocarbon source rocks organic matter inclusions in carbonates of varioustervals from the sinian to the triassic in the sichuanerefore, are of a complex origin and ocBasin. The selected inclusion samples and the analytcurrence. In the Sichuan Basin, carbonate core samresults are shown in table 4ples were collected at Weiyuan, Longchang, LingyFigure 7 shows the total gas volume released byTemple, Panlong, Ziyang, Kongtan, Jieshichang and inclusions in carbonates of different ages: theother areas(see table 4 for well sites) for the Sinian, of the gas released by the Sinian-Silurian carbonatesCambrian, Ordovician, Silurian, Permian and Triassic. the largeest, while the gas released by the triassicThe microscopic thin section and optical section stud- carbonates is the least. Also, the composition of theies of these samples indicated an extensive existence gas released from carbonates of different ages is quiteof primary/secondary inclusions, primary and secon- different, which is related to thermal evolution, typesdary asphalts and other organic matters. The occur- of organic matters, gas generation, reservoir formationrence and shapes of some organic matters are shown in process, etcthe charts4 Conclusions3.2 Gas generation potential of secondary asphalt(1)The study on gas generation mechanism ofphalt is an important type of organicarbonates source rocks is usually based on the fea-matters in carbonates. As indicated by the bulk rock tures of source rocks, and focuses on their gas generaasphalt analysis of dark gray dolomite and gray dolo- tion and expulsion mechanisms. Comprehensivelymite of the Sinian, Cambrian and Ordovician in the studied in this paper, however, are the developmenSichuan Basin, the asphalt content in the Well Gaoke 1 and distribution of marine carbonates source rocks inis 5.41%,2.46% and 4.86% respectively, and that of China, as well as the various reactions during the gasthe sinian and ordeViciandolomite in Well Nuji is generati8.12%0 and 3.70% respectively In the Well Moshenl, in it isWell Nushen 5, and Well Anping 1, the asphalt content of carboaYH中国煤化 I and establishedCNMHn the Sinian, Cambrian, and Ordovician dolomites generation feature of carbonates themselves but alsoas formation mechanism of marine carbonate source rocks in Chinadescribes the oil generation process through oil crack- sedimentary basins, the marine carbonates in Chinaing during gas generation and reservoir formation are characterized by old age and a long history ofprocess, and the gas re-generation process due to the thermal evolution. Accumulated usually in the earlyheating up of organic mattersstage reservoir formation are liquid hydrocarbons(2)The deposition environment and the sourcewhile the late-stage reservoir formation is featured byaterials in carbonates determined the generation of a accumulation of gaseous hydrocarbons. The oillarge amount of liquid hydrocarbons during the im- cracking gas generation under high temperature condinature and low-mature phase. In addition, the oil gen- tions(150C)isway to gas generation ineration through thermal decomposition of kerogen has marine carbonates. The formation of paleo-reservoirshysteresis due to the thermal decomposition process ofis actually an accumulative process of organic mattersorganic matters, and the Ro is shifted downwards to which may become a high-quality gas source kitchen1.6%for gas pool formation in marine carbonates(3)Since the hydrocarbon generation materials in(4)The carbonate is a chemical rock, whosemarine carbonates are mainly composed of low sedimentary environment, deposition and diagenetaquatic organisms, the oil/gas ratio is high in carbon- mechanism are different from that of clastic rocksates during the hydrocarbon formation process. DeAbundant primary and secondary organic inclusionsveloped atvery bottoms of the superimposed are formed due to recrystallization and metasomatismTable 4 Gas composition of inclusions in carbonates of the Sichuan Basin at different eras(%)CH, C2H, C3H C2H7916.070.000.0060.6710.010.63ng1Zam20.010.020.340.000.000.0010.931.130.1385.352100.050.0015.192.47Zi 6Z240.02.250.040.000.0566.661.341.0227.420.98Wei 117 Z0.020.001.500.020.0193.313.45Laolong 1 Z40.020.000.900.040.000.0020.300.190.276.012280.98Wei 117 Z,0.010.0292.30Wei 117 Zy0.020.001.940.00.0030.960.4563.41.74wei106∈12+11330.0013.760.500.000.690.075.526.940.95ong 32 O0.210.0019.620.182.870.040.074.582390.99ing I F12A30.240.0075413.010.283.824960.0200411.081.120.95Wei 106 S0.190.0011.690.980.111.289410.420.1573.472.300.900.120.0120.900.000.520.0033.631.980.060.0049.750.390.000.457970090.1040001.190.990.190.0066.620.510.00.790.7929740.450.008.350.240.000.710.0700086.753.000.960.003.6500.030.0383.30.460.0015932.240.283.281.190.071.80.637.480.890.00中国煤化工4860.85Kong 23 T,0.00CNMHG3.35e blank sample is determined through same-duration blank strikes bnagnetic sampler, of which the gasScience in China ser d earth sciences10200画08g50⊥a6Z2 Z2 22 Z3 74 Z4 24 cl 02 S1 P1P1 Pl P2 P2 Tf TcTcTc TrFig. 7. Total gas volume(column)released by inclusions in carbonates of different ages and the Ci/Cis values(line)in the Sichuan Basinated by studies, these organic inclusions areleum Geochemistry and Source Rock Potential of CarbonateRocks, AAPG Studies in gecarbonates, and the thermal pressurized cracking is the 7. Barker, C Development of abnormal and subnormal pressures inmain way to its hydrocarbon expulsion. The secondaryreservoirs containing bacterially generated gas, AAPG Bulletin,asphalt existing in pores and caverns in carbonates1987.1:1404-1413may thermally crack into gas upon being heated, thus 8. Barker, C, Calculated volume and pressure changes during thebecoming a type of effective gas sourceacking of oil and gas in reservoirs, AAPG Bulletin1990,8:1254-1261Acknowledgements Special thanks to Dr Zhang Guangya of RIPEDResearch Institute of Petroleum Exploration and Development), who9. Al-Shaieb, Z, Puckette, J. 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