Diagenesis and Restructuring Mechanism of Oil and Gas Reservoir in the Marine Carbonate Formation, N

Vol. 83 No 6 pp. 1173-1181ACTA GEOLOGICA SINICA (English EditionDec.2009Diagenesis and restructuring mechanism of oil and gas reservoir inthe marine Carbonate Formation Northeastern Sichuan:A Case Study of the Puguang gas reservoirDU Chunguo.2, WANG Jianjun, ZoU Huayao?, ZHU Yangming and WANG Cunwu'I Research institute of Petroleum Exploration and Development, Beijing 100034, China2 Petroleum University of China, Beijing 102249, China3 Department of Earth Science, Zhejiang University, Hangzhou 310027, China4 Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, ChinaAbstract: Based on the technology of balanced cross-section and physical simulation experimentsassociated with natural gas geochemical characteristic analyses, core and thin section observations, ithas been proven that the Puguang gas reservoir has experienced two periods of diagenesis andrestructuring since the Late Indo-Chinese epoch. One is the nuid transfer controlled by the tectonicmovement and the other is geochemical reconstruction controlled by thermochemical sulfatereduction (TSR). The middle Yanshan epoch was the main period that the Puguang gas reservoirexperienced the geochemical reaction of TSR. TSR can recreate the fluid in the gas reservoir, whichmakes the gas drying index higher and carbon isotope heavier because Cu(ethane and heavyhydrocarbon) andC (carbon 12 isotope)is first consumed relative to CH, and C(carbon 13isotope). However, the reciprocity between fluid regarding TSR(hydrocarbon, sulfureted hydrogen(H2S), and water) and reservoir rock results in reservoir rock erosion and anhydrite alteration, whichincreases porosity in reservoir, thereby improving the petrophysical properties. Superimposed bylater tectonic movement, the nuid in Puguang reservoir has twice experienced adjustment, one in thelate Yanshan epoch to the early Himalayan epoch and the other time in late Himalayan epoch, afterwhich Puguang gas reservoir is finally developedKey words: diagenesis and restructuring mechanism, marine carbonate formation, thermochemicalsulfate reduction, Puguang gas reservoir1 Introductiondenudation or the area has thick effective caprock,reservoir reconstruction is one of the most critical factorsPrevious field studies have shown that oil anto control petroleum distribution. It also provides the basicreservoir in marine carbonate formation isinformation to forecast petroleum distribution inaccumulated at a steady tectonic environment andsuperposition basins. At present, the oil and gas fillingduring the late accumulation period(Maksim 1992: process has been researched worldwide(Dewers andMcGillaway and Husseini, 1992; Pollastro et al., 1998). In Ortoleva, 1994; Wang and Zhang, 1997; Wojcik et althe Chinese marine carbonate formation, four conditions, 1997; Parnell, 1998), but research of oil and gas reservoirincluding multi-accumulation, diagenesis, restructuring, diagenesis and restructuring is still at the beginning stageand destruction by multiple tectonic movements, are especially the mechanism research of diagenesis andessential for oil and gas accumulation and reservation. Oil restructuring. Compared with oil and gas reservoirs in theand gas reservoirs are destroyed by uplifting, groundwater clastic formation, reservoir reconstruction in the marinemotion, and fault activation, and have been investigated carbonate formation in Chinese superposition basinsextensively(Geoffrey et al., 1999; Qin et al., 2004: Hao et alwayH中国煤化工 lysis, and fluid-rockal., 2005; Zhao, 2007). If the area has no extensive reciprigate the diagenesisCNMHGzas reservoirs in the*CorrespondingauthorE-mail:Duchunguo126@163.commarine carbonate formation in terms of two aspects1174Diagenesis and Restructuring Mechanism of Oil and Gas ReservoirDu et alincluding fluid transfer controlled by the tectonic had commercial gas flow. It has recently been discoveredrement and geochemical reconstruction controlled by that the Puguang, Maoba, and Dukou gas fields are part ofthermochemical sulfate reduction (TSR) by taking the marine carbonate rock formation within the Xuanhan-Puguang gas reservoir in the Xuanhan-Daxian area, Daxian area. At the end of 2005, the cumulative provennortheast Sichuan as an examplerecoverable gas reserves of Puguang gas field were2510.75x10m', making it the biggest marine gas field in2 Regional Geology and ExplorationChina until nowNortheast Sichuan is located at the northern margin of 3 Fluid Transfer Process Controlled bythe Yangtsensis Plate and can be divided into five primary Multistructure Superpositionstructure parts: (I)the thrust zone at the MachangMountain front; (2) the decken zone at the Daba The seismic data of the Xuanhan-Daxian area showsMountain front; (3)the gentle zone at middle Sichuan; (4) that the faults in the middle petroleum system almostthe Tongjiang-Pingchang structure zone; and (5)the disappeared in the thick T2j-T2l anhydrite cap-rock. FewXuanhan-Daxian folded zone. Puguang gas field is faults can penetrate into the regional cap-rock. The upperlocated at the Xuanhan-Daxian folded zone, which is a and lower fluids of the regional cap-rocks have no geneticduplex, superimposing structure area by the east Sichuan relationship, which can also been proven by the differentfolded zone and frontal Daba Mountain decken zone. The natural gas geochemical characteristics in the marine andspecial tectonic geographysical position determines its non-marine formations. Thus, the fluid transfer of the gascomplicated history of tectonic evolutionreservoir in the P2ch-TIf Formation consists mainly ofThe Xuanhan-Daxian area has experienced two large lateral migration along the reservoir and is controlled byevolution stages: the marine carbonate rock deposition multistructure superpositionstage from the Paleozoic Era to the Middle Triassic, andthe non-marine clastic rock deposition stage from the Late 3. 1 Evolutional tectonic history of the Puguang-Triassic to the Jurassic. In a vertical direction, it can be dongyue Zai structureinto the upper, middle, and lower petroleum In the present study, we selected 13 lines ofsystems, that is, the Upper Triassic-Upper Jurassic sections from the Puguang-Dongyue Zai structure. On theLower Silurian-Middle Triassic basis of erosion thickness recovery and decompactionpetroleum system, and the Sinian-Upper Cambrian calibration, the top and bottom surface depths of eachpetroleum system. The source rock of the middle period formation within the Puguang-Dongyue Zaiwas Silurian mud shale rock and structurethe balamian dark mud rock, limestone. The hydrocarbon technology. The evolutional tectonic history of the Lategenerating potentiality is enormous. The reservoir is Indo-Chinese epoch, Middle Yanshan epoch, Latemainly dolomite rock (including anhydrite), which is Yanshan epoch-Early Himalayan epoch, and Latethick, high-quality reef and shoal facies in the Changxing Himalayan epoch has been restituted in 3-D. Each periodFormation( P2ch)and Feixian Guan Formation(Tu). The of structural characteristics is shown in Fig.1thick anhydrite rock(over 1000m)in the fourth member ofBecause of the unstable uplifting of early buriedthe Jialing River Formation(T2)) and the second member basement faults in the Indo-Chinese epoch, the big risof the Leikou Slope Formation (T2/) is regional cap rock. and depression and northeast orientation occurred in theThe good matching condition of source, reservoir, and cap cap-rock of northeast Sichuan. For example, the Luzhouof the middle petroleum system makes it an important palaeohigh formed along the southern part of the Huayingexploration target in northeast Sichuan.Mountain fault and the Kaijiang palaeohigh formed alongGround oil geology exploration in the Puguang gas the northern part. The Puguang-Dongyue Zai structurefield, part of the Shuangshi Miao-Puguang structure belt, was the subpalaeohigh at the northwest slope of thestarted in the 1950s. From 2003 to 2004, it completed a Kaijiang palaeohigh. The structure feature was higher456.06 km high-resolution 3-D seismic survey. In 2003, northeast and lower southwest, and the height differencethe Puguang 1 well gained high commercial gas was almost 200 m( Fig. la). Relative to the Late Indproduction in the TU-Tv Formation, and the gas zone Chinese epoch, the Early-Middle yanshan epoch was athickness of logging interpretation was 12.2 m, which relating中国煤化工 east Sichuan. In thisshowed very good oil and gas exploration foreground. At pericthe end of 2004, there were seven wells completed at the ZaiCNMH Ge(ig 1b). BecauseShuangshi Miao-Puguang structure belt, and five wells of the Paleo-Pacific Plate's underthrusting from the LateVol. 83 No 6ACTA GEOLOGICA SINICA(English Edition)Dec.200911751000-20005×103.495×10°-30003485×10°18761876X101.877×10(b)/3485×10息3000-40003495×10°500013.495×10°1.876×101.876×101.877×10(c)3485×10°3.485×10°1.877XFig. 1. 3-D configuration of the Puguang and Dongyue Zai structures during their geologic history.Yanshan epoch to the Early Himalayan epoch, northeast 3.2 Physical simulation experiments of fluid transfersoutheast. Controlled by the regional compressing motion, period of the puguang grvoir strike changing, wethe highest point of the Puguang-Dongyue Zai structure implemented the physical simulation experiments of fluidbecame the lowest position, as seesaw motion. The height transfer and accumulationdifference was 600 m (Fig. Ic). Due to the collisionbetween the Indian and Asian Plates and continuousExperiment model and methodcontinent-to-continent convergence activity at the late Quartz sand saturated by red ink is used to simulate theHimalayan epoch, the Sichuan Basin comprehensively Puguang reservoir in the P2ch-TI Formation because thechanged. Daba Mountain intensively compresses from the porosity in the reservoir is pore space or vugs. If thenortheast. which madenorthwest and northeast (nitrogen gas) accumulates into the reservoir, thestructures superposed. with the uplift of the superposition will become light so we can see the gas migration andstructure, the sea level reached its highest point at 800 m, accumulation by the quartz sand color. The experimentand the Puguang structure near the Puguang 2 well instrument is a 2-D simulated experiment instrument fromposition uplifted a little(Fig. ld)the Key Laboratory of Petroleum AccumulationThe 3-D configuration of the Puguang structure had Mechanism, Ministry of Education, Petroleum Universitybeen changed considerably due to structure superposition of China. The main part of the experiment instrument isin the Late Indo-Chinese epoch so the potential energy steel中国煤化工 property. The borderfield inside the gas reservoir had also changed. In order to andbalance the new potential energy field, the fluid potential pressiCNMHGnolics glass clothed glass. Theenergy should also charexperimental phenomena can be seen directly through the1176Diagenesis and Restructuring Mechanism of Oil and Gas ReservoirDu et alseal bedgaswater contactFig. 2. Fluid adjustment and accumulation simulation experiments of Puguang gas reservoirgeology evolution section of Puguang structure and filleFig. 1b shows the Puguang structure characteristics at degree of gas reservoir, the experiment method ande Middle Yanshan epoch(Fig. 1b), and the 2-d process was as follows (Fig. 2). First, the gas wasexperiment model is shown in Fig. 2a. The experimental transferred into Puguang reservoir through F fault until themodel size is 50 cm x 30 cm x 2 cm. In side of the model, water/oil contact paralleled with that of the gas outlet. Thethe high permeability bed (SI)represents the high-quality gas outlet was then sealed off. The model of Fig. 2bdolomite reservoir in the P2ch-TUFormation. The SI is on simulated the initial Puguang reservoir before the paleo-oilthe lower left and upper right, and the azimuth magnitude reservoir restructuring. Second, the model is rotatedis 8. The low permeability bed (S2)represents tight clockwise to 23(Fig. 2c), which is the same strikelimestone without dolomitization. F stands for fault, which magnitude as the reservoir at the Late Yanshan epoch tois a good migration path for gas. The sealed formation is Early Himalayan epoch(Fig. Ic). Third, the model wasused by rubber, and each parameter is shown in Table 1. continuously rotated clockwise by 5. more(Fig. 2c)There is a gas outlet in the middle of the Si sand bed, 22.5 making the strike the same as the reservoir strike incm high relative to the instrument bottom. Under the F Late Yanshan epoch( Fig. Id)fault is a gas-charging inlet, 0 cm high relative to theinstrument bottomExperiment resultsAfter clockwise rotating Fig. 2b model toTable 1 Experimental parametersermeability((×10mcapillarity, and then migrates along the030.35reservoir from right to left by buoyancy. Due to the left0050.1seal bed blocking, gas accumulated at the left reservoir.The left gas volume gradually became larger, so the colorof the quartz sand saturated by red ink becomes light andBefore the structure reverse restructuring, crude oil was the中国煤化工155 minutes, gastransformed to gas inside the palaeo-oil reservoir at the waten. Gas migrates fromMiddle Yanshan epoch, thus the paleo-oil reservoir did not theHCNMHnd water migratestake part in the fluid transfer. According to the historical inverselyVol. 83 No 6ACTA GEOLOGICA SINICA(English Edition)Dec.20091177T.工0400960971.01gas drying indexCH,content(%)◆■▲xx0.120.04(c)005-29-30-31-32-33-34-35-36CH content ( %)Fig 3. Relationship between natural gas geochemistry characteristics and acid gas index in the Xuanhan-Daxian area.The balance status of gas and water is broken again in accumulates in the relatively higher structural part. Thisthe model when the model is rotated 5 more clockwise. period of fluid transfer lasts to the end of early HimalayanGas migrates continuously to the left and the volume epoch when the structure motion is quiet. In the lateincreases. At last, the status is balanced again, which lasts Yanshan epoch, southwest Puguang-Dongyue Zai3 hours and 28 minutesstructure uplifts almost 200 m again relative to northeastAfter rotating twice, most of the gas on the right side part, so the fluid inside the Puguang structure is adjustedmigrates to the left and accumulates in a new gas again. It is because of the fluid adjusted by the tectonicreservoir. Meanwhile, the right side, the initial gas movement that the initial gas reservoir of northeast part ofaccumulation area, becomes the water accumulation area. Puguang structure becomes nowadays high waterproduction reservoir(415. 17 m water production per day3.3 Fluid remigration and accumulation inside thein TU formation, Puguang 3 well)Puguang gas reservoir controlled by multiphasetructure superposition4 Chemical reconstruction processBased on the structure inversion and physically. Controlled by tSRsimulated experiment research, we conclude that thePuguang structure in late Middle Yanshan epoch succeedsTSR is the reaction between sulfate and hydrocarbonthe structure feature of the Late Indo-Chinese epoch which (mainly gaseous hydrocarbon). In the process, sulfate isis higher northeast and lower southwest, so the gas mainly reduced and hydrocarbon is oxidized (Krouse et al., 1988;accumulates in the northeastern part. In the late Yanshan Worden et al., 1996; Heydar, 1997; Cai et al., 2003; Zhuepoch, controlled by different uplift on the regional et al., 2005, 2007), which is an important reason that highcompressional action, the southwest part of Puguang hydrogen sulphide content in gas. The TSR reactionstructure rises much higher than its northeastern part, so equati中国煤化工initial potential energy inside gas reservoir is broken. Gasmigrates from the northeastern high potential energy areaCNMHGNOto southwest in the low potential energy area and driven by thermochemical dynamics. The temperature at1178Diagenesis and Restructuring Mechanism of Oil and Gas Reservoir Du et alwhich thermochemical dynamics trigger a TSR reaction TSR relative to CH4. Therefore, under the normalhas been a contested issue for a long time. most scholars condition, the weighting range of heavy hydrocarbon isnow believe that the critical temperature of TSR reactionobviously larger than CH4is higher than 120oC(Machel, 1987, 2001; Emst, 2001)The Tu sedimentary environment of the Xuanhan-Daxian 4.2 Reservoir petrophysical quality reconstructionarea in northeast Sichuan is mainly evaporate platform controlled by TSRfacies, so the anhydrite content is higher. The earthIn the marine sulfate-bearing carbonate formation,temperature inside paleo-oil reservoirs, such as the under buried conditions, two kinds of fluid react withPuguang reservoir, Tieshan Slope reservoir, and Dukou reservoir rock. One is organic acid and CO2 released byRiver reservoir were over 120C in the Late Indo-Chineseource rock in the period of hydrocarbon emissionepoch, thus starting the diagenesis controlled by the TSr (Spirakis and Heyl, 1988; Moore, 1989); and the other isreactionfluid regarding TSR, including hydrocarbon that takesparticipation into TSR and acid gas generated by TSR, for4.1 Hydrocarbon reconstruction controlled by TSRexample, H2S etc(Hill, 1995). The acid fluid released byBased on Fig. 3a, the gas drying index of TV in the source rock reacting with reservoir rock, has been wellXuanhan-Daxian area is obviously positively correlated totudied in marine or non-marine formation(Spirakis andthe acidic gas index(H2S/[H2 S+SC H2n 1) That is, the Heyl, 1988; Moore, 1989). In this paper, we emphasize onmore H2S content, the more CH4 in hydrocarbon. The the fluid regarding TSR reconstruction of reservoir rocksC2H6 and C3 Hs proportions are reversed, which isnegatively correlated to the acidic gas index(Fig. 3b,4. 2. 1 Porosity increasing in reservoirs by anhydritec). In the high H]S content of natural gas, the C2H6alterationcontent is lower than 0.2% and the C3Hs content is lower Based on theoretical calculations, 1 mol anhydritethan 0.05%. For example, the CaHg content in the Puguang volume is almost 47 cm, and 1 mol calcite is almost 37and Luojia Zai gas reservoirs is zero, but if there is nocm. As the TSR reaction equation, we know that after 1TSR reaction, the C2H content in natural gas is almost mol anhydrite changing into I mol calcite, the pore spacehigher than 0. 23% and the C3Hs content is higher tharin reservoir rock will almost increases by 10 cm. During0.08%. Heavy hydrocarbon (CHlo and much heavier TSR, after anhydrite crystalline grain or alteration, manyhydrocarbon)in T f natural gas which has been subjected anhydrite moldic pores begin to form(Fig. 4a). It isto TSR is zero, but it is still a few proportions indisplayed that the average porosity is less than 2% in nonatural gas. It is thus evident that C2+ heavy hydranhydrite moldic pore reservoirs, which has been provenis consumed with higher priority in TSR reactionby core and thin section observations and a petrophysicalto CH4. The computational solution shows that at 120 c property test of Puguang 1 well, 5420-5430 m interval inthe activation energy between anhydrite and ch4, c2hTy formation. But in 5424.8-5428.7 m interval withC,Hs, and Ch,o is respectively.74, -102.01, -159.8, much more anhydrite moldic pores, the average porosityand -261.64 k/mol. The result is that the greater the in reservoir is over 5% with maximum overhydrocarbon, the lower the activation energy, and the 8%. Moreover, anhydritic micrite dolomite in theT,greater the active gas chemical property, the easier it is toFormation, Jinzhu I well in northeast Sichuan, is a goodobtain a TSR reaction. that is the important reason whyreservoir because anhydrite tubercle is dissolved to vugsTSR reaction selectively consumes hydrocarbon and the Thereby, in the course of TSR, it is significant to improvegas with high H]S content has high gas drying index.reservoir storage in anhydrite-bearing reservoirs becauseBased on Fig. 3d, the acidic gas index of natural gas in of pore space increasing action caused by anhydritethe P2ch-T Formation in the Xuanhan-Daxian area is alteration to calcite and anhydrite moldic poresobviously positively correlated to the carbon isotope valueof CH4; in other words, the more H2S, the more heavy 4.2.2 Reservoir corrosion by H]s, the TSR productioncarbon isotope and the average weighting range can be Hill (1995) pointed out that the corrosion mechanism byreached 1.5%0. The weighting range of heavy hydrocarbon H,S includes two types: (1)H2S reacting with oxois much higher; for example, average C2H weightingrange can reach to 3.5%0. It is because C=C bond formation water produces sulfuric acid, and sulfuric acidsrelative to C=c bond has less bond energy. Indissolve carbonate rock to form corrosion vugs; and(2)period of TSR reaction, C reacts with TSR first, so吧中国煤化工mation water mixescontent is relatively richer in the remnant hydrocarbon, formation water, and thenwhich results in the heavier carbon isotope in natural gas. theCNMHGed to carbonate, so itAt the same time, heavy hydrocarbon easily reacts with has a strong corrosion effect on the reservoir rockVol. 83 No 6ACTA GEOLOGICA SINICA(English Edition)Dec.2009Fig. 4. Characteristics of dissolved pores and vugs at Puguang gas reservoir during the P2ch-Tuf FormationResearch shows that T -T2? thick anhydrite cap rock in tested inclusions(Fig. 5a). As shown in Fig. 5b,withnortheast Sichuan(statistics shows that pure anhydrite increasing temperatures, the formation water salinityrock thickness is 300-550 m) has an effective sealing recorded by fluid inclusions decreases. When thethe oxo formation water cannot migrate to the temperature is lower than 120 C, the salinity in inclusreservoir in the P2ch-Tif Formation(Pang et al., 2003: is relatively higher and the maximal value reaches almostShen, 2005; Ma et al., 2006), so the corrosion mechanism 21%. While the temperature over than 120C and theshould be the second type. However, for a long time, in temperature increases, as Fig 5b displaying, the salinity isthe close system of the P2ch-Tff Formation, the other mainly lower than 10%formation water source to dilute initial formation water inOur study shows that the inclusion salinity of gashigh H2s content gas reservoir is still an unresolved issue reservoir varies with temperature, mainly caused by TSR(Shen, 2005; Ma et al., 2006)reaction intensity in different buried depth phase. WhenIn this paper, primary inclusions in 35 samples of the formation temperature is lower than 120oC, the TSRsecondary calcite are taken from seven high H2S content reaction is weak or does not react a little or no fresh watergas reservoirs, such as Puguang, Maoba, and Dukou River is generated, and the salinity recorded by inclusion isgas reservoirs in northeast Sichuan. The salinity record relatively higher. That the salinity is 21% represents initialcan prove that there is abundant fresh water charge in the salinity of primary formation water. While the temperatureP2ch-TUf Formation during the TSR process. In general, is over 120C, which meets the TSR reaction, abundantformation water salinity in the fluid inclusions taken from hydrocarbon is consumed and meanwhile a lot of freshthe Pach-Tf Formation samples has a large range, with a water will be generated. The fresh water mixes withvalue from 0.18% to 21%, and salinity almost less than primary formation water and dilutes the primary formation10%, mainly 1%to 9%, consisting of 66.9% of total water, making the salinity lower. Because the TSR叶-…32120℃二3579111315171921Fluid inclusion salinityHomogenization temperature of fluid inclusion cc)Fig. 5. Relationship between fluid inclusion salinity distribution and中国煤化工 TIf Formation,northeast Sichuan(a)anhydrite moldic pores in crystalline powder dolomite. Part of the pores areCN MHGm depth, T . plainfiled with asphalt. -guang 6 wel,5 320 30 m dept, Pxch plainlight, 2xloMust uI the big vugs were noDiagenesis and Restructuring Mechanism of Oil and Gas ReservoirDu et alreaction is chiefly the reaction between hydrocarbon and epoch to Early Himalayan epoch and Late himalayansulfate driven by thermochemical dynamics, with deeper epoch. The Puguang gas reservoir is now formed by lateburial and the higher gothermy, the TSR reaction is more migration(twice) and accumulation after early gasintense and generates much more fresh water. So with the reservoir adjustment.geothermy increasing, the quantity of lower salinity pointsrecorded by inclusions will be more, and the salinity point Acknowledgementsin the Figure will be denser(Fig. 5b). It should be pointedout that the salinity range of formation water varies. The These research results are part of a key project carriedpossible reason is due to the heterogeneity of carbonate out in 2005-2007. The project was financially supportedreservoir which makes the differential mixture of primary by the 973 State Project( Project no. 2005CB422105).Theformation water and fresh water generated by TSR authors would like to thank Cai Xunyu, Director, and GuoTonglou, General Geologist of SinoPec South ExplorationBased on the above analysis, the fresh water mixing and Production Company, Zheng rongcai, Professor ofcontinuously and diluting primary formation water makes Technical University of Chengdu, and Cai Zhongxianthe formation water unsaturated to H2S and carbonate. Vice-Professor of China University of Geoscience, forThus, H]s reacts with reservoir rock and keeps the their help and guidance during the researchcondition of corrosionIt is discovered by the detail observation of 137 m coresManuscript received Nov 19, 2007and 293 thin sections of Puguang 6 well in the P2ch-Tufaccepted June 6, 2008Formation that within the dolomite reservoir of the pchdited by Fei hongcaiFormation, there are only fracture and few small diametervugs filled by asphalt. Most of big diameter vugs are Referencesclean, with no asphalt filling(Fig. 4b). This phenomena Cai, CF, Worden,RH. Bottrell, S H. Wang, L, and Yang, C,show that there are at least two stages of dissolution in2003. Thermochemical sulphate reduction and the generationPuguang gas reservoir. One is the vugs filled by asphaltof hydrogen sulphide and thiols (mercaptans)in Triassiccarbonate reservoirs from the sichuan Basin China. chemicalformed before petroleum charging or at the same timemainly dissolved by organic acid. The other is non asphalt Dewers, T, and Ortoleva, P, 1994. Nonlinear dynamical aspectsfilling vugs formed after petroleum charging, deduced byof deep basin hydrology: fluid compartment formation andthe forming time and high H2S content in P2ch natural gas, episodic fluid release. American Jounal of Science, 294: 713-mainly dissolved by HS generated by TSR reaction. Fig4b shows that relative to primary porosity and vugsErnst, A., 2001. Raman Micrspectrometry of fluid inclusionLithos,55:139-158generated by organic acid dissolution, the vugs generatGeoffrey, w.O'Brien, Mark, L, lan, R D, and Simon, K Mby His dissolution are more important for Puguang gasconvergence, foreland development and faultreservoir to form the high quality reservoirreactivation: primary controls on brine migration, thermalhistories and trap breachin the Timor Sea, Australia. Marine5 Conclusionand Petroleum Geology, 16: 533-560Hao Fang, Zou Huayao and Fang Yong, 2005. The researchdifficulties and frontiers of subtle reservoir. earth ScienceThe reservoir rock of the Puguang gas reservoir and Frontiers, China University of Geoscience(Beijing), 12(4)accumulation of fluid inside it has experienced TSR481-488(in Chinese with English abstract)diagenesis. Relative to CH4, heavy hydrocarbon has lower Heydari, E, 1997. 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