Formation and evolution of Sinian oil and gas pools in typical structures, Sichuan Basin, China

PetsSci(2012)9:129-140129DOI 10.1007/s12182-012-0193-Formation and evolution of Sinian oil and gaspools in typical structures, Sichuan Basin, ChinaYuan Haifeng'" *, Liang Jiaju', Gong Deyu', Xu Guosheng-', Liu Shugen.2and Wang Guozhi'' State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Sichuan610059, China2 College of Energy Resources. Chengdu University of Technology, Sichuan 610059. China◎China University of Petroleum (Beijig) and SpringerVerlag Berlin Heidelberg 2012Abstract: The only major breakthrough in the exploration of the Sinian of the Sichuan Basin has beenthe Weiyuan gas field. Taking the typical structures in the Sichuan Basin as examples, an apatite fissiontrack simulation was applied to constrain the thermal evolutionary history of the source rocks in thisstudy. Combined with trap formation and evolution, the formation, destruction, and accumulation historyof the Sinian reservoirs were analyzed from a dynamic perspective. The Sinian reservoirs underwentseveral primary stages: the paleo-reservoir formation and destruction in the late Silurian, hydrocarbonrecharge in the Permian-Trassic, the cracking of oil to gas and dissolved gas in the late Triassic - Mid-LateJurassic, and the exsolution and accumulation of dissolved-gas as a result of episodic uplift since the LateCretaceous. The exsolution process of dissolved-gas is of great significance to the accumulation of naturalgas. The formation of the Weiyuan gas field is also related to this process. The Sinian in the SichuanBasin has a broad exploration prospect, and exploration targets focused on the Leshan-Longnisi Paleo-uplift tectonic zone with weak influences on potential hydrocarbon reservoirs from the late tectonism andtransformation.Key words: Apatite fssion track, Sinian reservoirs, Sichuan Basin1 IntroductionAnpingdian and Longndsi. However, no ideal explorationThe Sichuan Basin is a large hydrocarbon-richeffect was achieved. In 2006, the Dingshan 1 well wasdrilled in Southeast Sichuan by the SINOPEC Explorationsuperimposed basin in China (Fig. !), with an areaof 19 .x Southerm Company. Except for some hydrocarbon discovered10* km2. The sedimentary covers of the basin were completelyin the Sinian, no industrial gas flow was obtained. However,developed, with a total thickness of 6.000-12,000 m. A newabundant bitumen was found in the pores, holes, and fracturesround of hydrocarbon resource asessment completed in 2002 of Sinian cores and microscopic thin sections. This indicatedestimated that the total geological gas resources amount tothat a large hydrocarbon accumulation had once existed5.35 x 10'2 m' in the Sichuan Basin, while the remainingin the Sinian reservoirs, but might have been destroyed,geological gas resources were equal to4.63 x 10' m'. By transformed, or lost.the end of 2009, the total proven reserves of natural gasRecent exploration failed to achieve a breakthroughwere equal to 1.9x 10' m' in the Sichuan Basin, with thein the Sinian of the Sichuan Basin. This paper focuscs onproven reserves of continental clastic rocks and marine layers the formation and evolution of possible Sinian reservoirsaccounting for 19.9% and 80.1%, respectively. However,in typical structures of the Sichuan Basin and provides athose of the Sinian Dengying Formation (Z_dn) onlyreference for furher exploration.accounted for 2% (Liu et al, 2011), indicating that the Sinianreservoirs in the Sichuan Basin still have a huge exploration 2 Characteristics of source rockspotential.The exploration for Sinian reservoirs in the Sichuan BasinOverlaving the Denovine Formation (Z,dn), the Lowerhas continued for more than 40 years, with the Weiyuan Camb中国煤化工mainly consist of blackgas field and Ziyang gas reservoir discovered during this and d; shale. or sandy shale.period, as well as gas-bearing structures such as Gaoshiti-As scCN M H Gng eviroment (Chen,2010), the Lower Cambrian strata are some of the besthydrocarbon sources in the Sichuan Basin, with a thickness of*Corresponding author. email: yhf870@yahoo.com.cn0- 445 m (|39 m on average, with the maximum in SouthwestReceived June 7, 2011130Petsei.(2012)9:129-140Guangyuan50 100kmoChengkouNorh Sichuan low-la bellMianyangWuanFractureoDaxiangasfodChengdu《oNanchongjEast Sichuan migh-slop]WanxianCentral Sichuan low-nat belttulted fold bellWest Sichuan口Suininglow-slope/oroYa'anLeshanWentWanyungn BalPmo5y口Chongqing。口 Zigong0 750kmsw Sichuan low-Aat口LuzhouBeljingSW Sichuan lwslpeAD.TCDinghan1structural belt0SichunBasin /0罩aity Wel临1 Sampling point区Fault Section口刁I Stuturtu unt 口Basin bondayof apatiteboundaryFig. 1 Location map of typical structures of the Sichuan BasinSichuan). The Gongshen I well intersects Lower Cambrian Basin (Liu et al, 2007), the Sinian Dengying Formation (Zzdn)strata 445 m thick. The total organic carbon (TOC) of the mainly consists of algal dolomite and granular dolomite. Thesource rocks generally has a distribution of 0.5% 4.0%. It is existing Sinian core data showed that fractures and holes withhigher in the Weiyuan structure of the Southwest Sichuan, a low matrix porosity and denser reservoirs were the mainNorth and East Sichuan blocks, with values generally above reservoir types in the Dengying Formation (Z,dn).1% for type I organic matter. The black shale of the LowerThe Sinian Dengying reservoirs in the Gaoke I welll andCambrian Qiongzhusi Formation has a thickness of90 -120 mAnping 1 well, Central Sichuan, mainly consist of variegatedin the Anpingdian and Gaoshiti structures of Central Sichuan, algal dolomite and powdery fine-grained dolomite of shallow-where the organic matter is mostly in a post-mature stage. The water carbonate platform facies. The average porosity isR。has an average measured value of 2.86% in the Anping 1.51%-2.49% and 0.64% 2.60% in the fourth and thirdI well with kerogen of type I and an average total organic members, respectively, of the Sinian Dengying Formationcarbon of 2.18%, showing a strong hydrocarbon generation in well Gaoke I. The permeability values of the effectivecapacity. The lower part of the Lower Cambrian Niutitang in reservoir are5x 10*-1.24x 10' μum2 and9.6x 10*-1.43 xthe Dingshan structure of Southeast Sichuan mainly consists 10* μm° in the fourth and third members of the Dengyingof gray black carbonaceous mudstones with a thickness of 124 Formation, respectively. The reservoir property of the fourthm. The organic carbon values at the bottom of the Niutitang member is comparatively good. The average porosity of therange between 0.45% and 1.65%. The average for the original Sinian reservoirs is 1.85% in the Weiyuan gas field (1I wells)organic carbon is 1.34% in the mudstone at the bottom of the and 1.53% in the Ziyang gas-bearing areas (Liu et al, 2008a),Niutitang Formation, which is close to the standard for goodwhere the matrix permeability is I x 10> 4.36x 10' μm',source rocks. The hydrocarbon potential intensity is very high with an average value of0.116 x 10 μum'. The main reservoirin the Lower Cambrian hydrocarbon sources of Sichuan (Xu, spaces consist of solution pores, solution fractures, and1999), with an average of 1.33 x 10' m'. Curently, the R。dolomite intergranular pores.values of organie matter in the source rocks range betweenThe Sinian Dengying reservoirs in well Dingshan 12.0% and 5.0%. This structure is in the post- mature stage, mainly consist of fine-erained dolomite and sparry dolarenite.with kerogen pyrolysis gas and oil-cracking gas generated in The av中国煤化工mples is 1.78% withits geological history.the porCd 3.52%. The averagepermeaCN M H Grock matix porosity3 Reservoirs, cap rocks, and overburden and permeability are rlaively low, and the main reservoirrocksspace is the fracture-cavity type.Compared with the measured reservoir properties data inWith a general thickness of 300-1,200 m in the Sichuan the Weiyuan-Ziyang area, it is shown that among the typical132Pet.Sei.(2012)9:129-140Table 1 Analysis results for apatite fssion track in typical stnuctures. Sichuan BasinSample No./Sample depth/ NumberP()Central PooledLithology 1 FormationWell No.leaiom ofgpins 10^cm2 10~cm 10^.m’% oeg,Ma ae.Ma Lm(+10) (1 0M24-11Sandstone I Xujahe Fm (T;x')2021.620.865 18.236 13.415 39.9 12+12+1 1.4+2.5Mo24 WellM24-2/Sandstonc I Xujahe Fm (T;x*)2239.00.735 15.450 13.605 63.4 12+1 12+1 10.8+2.1M58/Sandsone 1 Xujiahe Fm(T,x)2141.01:0.442 12.289 13.794 52.7 10+19+111.8+2.2Mo58 WellWS30Sandstone 1 Shaximiao Fm(Js)4372(3.051 26.058 17.083 98.137+337312.9+1.8WS40Sandstone 1 Xujahe Fm(T;x)4712:2.116 14.582 17.2987.46+547+4 12.4+2.1WS43Sandstone 1 Xxujiahe Fm(T,x)592210.967 10.837 17.009 75.5 28+228+2 13.4+1.5WS70Sandstone 1 Xjiahe Fm(Tx)436121.823 14.379 17.305 57.6 41+5 41+5 13.21.7WSI:Sandstone 1 Xujiahe Fm (Tx)595201.118 12.889 17.082 71.5 28+2 28+2 13.1+1.9WS25Sandstone 1 Xujiahe Fm(T,x)6421.177 14.015 17.157 94.5 27+227+2 12.6+2.4WS46Sandstone 1 Shaximiao Fm (I]3).41210.575 17.010.542+443+312.3+2.1WS47Sandstone 1 Shaximiao Fm (]s)1.0459.90117.52766.8 35+3351312.2+2.3DSASandstone 1 Longtan Fm (P:I)80262.31110.860 13.9845.452+556+511.9+2.5DT8Sandstone 1 Penglaizhea Fm (J,p)1098221.6929.208 15.12068.252+452412.0+2.7DT6Sandsone 1 Suining Fm (Jso)9522.996 15.827 15.68842.255t511.7+2.8ADX4Sandstone 1 Shaximiao Fm (2s)9041.637 10.233 16.06799.948+311.8+2.9 .DTSandstone 1 Xujahe Fm (Tx))502.368 12.967 15.309 35.8 49+5 52+512.0+2.4Notes: P, spontancous fssion-track density; P, induced fission-track density; Po standard fsson-lrack density; P(x), probability of inferredPoissonian distribution of single grain age; L, track length4.2 Formation and evolution of Sinian reservoirs inSichuan, when a large amount of oil generation occurredtypical structures(Fig. 3, Fig. 6). In the Early-Mid Triassic, the Cambrianhydrocarbon sources experienced a peak in oil generation4.2.1 Anpingdian structure(R。of the source rocks was approximately 1.0%). The wetThe thermal evolutionary history of the source rocks in the gas generation stage began in the Early Jurassic (R。wasCambrian Qiongzhusi Formation, in well Anping 1. Centralapproximately 1 .3%). In the Early-Middle Jurassic, theSichuan (Fig. 3) showed the following results. HydrocarbonSinian reservoirs were mainly charged by wet gas generatedgeneration occurred in the Cambrian Jiulaodong Formation in the Cambrian source rocks (R。of the source rocks wassource rocks in the Late Ordovician-Late Silurian (the R。1.3%- 2.0%). In the late Mid-Jurassic, a large amount ofvalue was greater than 0.5%). This period was considered todry gas began to be generated in the source rocks (R。of thebe the initial hydrocarbon generation stage for the Cambrian source rocks was 2.0%). In the early Mid-Jurassic, the Siniansource rocks, as well as the cnude oil charge stage.reservoirs had a roof buried at a depth of around 4,500 mA specific trap morphology existed in wells Anping 1 and a formation tempcrature of 160。C, where the crackingand Gaoke 1 of the Anpingdian-Gaoshiti structure, which of oil to gas occurred (Schenk et al, 1997), leading to theenabled the capture of crude oil generated in the Cambrian formation of pyrolysis bitumen. Until the Mid-Late Jurassic,source rocks. This period was considered to be the formation the roof of the Sinian reservoirs was buried at a depth ofstage for early paleo-oil pools (Fig. 6). During the end of 6.,200 m, a paleo-temperature of 215。C, and the maximumthe Silurian sedimentary period, the strala were upliftedamount of oil cracking to gas was obtained. Because of theand denuded as a result of the Caledonian Movement (Liu multi- stage hydrocarbon generation of the Cambrian sourceet al, 2010). Early paleo-oil pools were destroyed, and this rocks,中国煤化工-rge exists in the Sinianprocess is recorded by the chromatographic features of the reserv_peratures of the fluidsaturated hydrocarbon of the Sinian reservoir bitumen in the inclusYHC N M H Gmainly concentrated inAnpingdian and Gaoshiti structures (Fig. 7).the ranges of 130-170 °C, 210 -220 °C, and 230 -260 °C. ThisThe Cambrian source rocks experienced continuous indicates that at least three stages of fluid charge existed inburial in the Early Permian. Until the Mid-Late Permian, the the reservoirs, i.e. the late Triassic- Early Jurassic, Mid-LateR。value of the Cambrian source rocks was 0.7% in Central Jurassic, and Mid-Late Cretaccous (Yuan, 2008).PetSei:(2012)9:129-140133Time-temperature historyTime temperature historyM24-2A " omarn 1080仙|9M58estTime, MaTrack length, umWS40mmn1242121WS43> ," t Motrswon 1240高CWS25| WS470 f MunL1250mDSA。 ADX4Fig. 2 Simulated rsults of apatite fssion track data for typical structures, Sichuan BasinTimeDepth6406005605204804404003603202802402001601208040 0(Ma)(m)oSD1 PgNgFm100030'C45'CJ60C20000.75%R.-90C3000105CTo120C1 3%R,4000P1.70%R5000200%R2 50%R,600003.00%R,3.5%R7000中国煤化工Fig. 3 Themal history of well Anping 1. Anpingdian-Ci.MHCNMHGThe Anpingdian-Gaoshiti structure began to uplift and pressure in the uplift process, natural gas that was earlierdenudate since the Late Cretaceous, and very few changes dissolved in the formation water exsolved. Because there is nooccurred in the trap morphology during the uplift process. dissolved gas in the current gas pools, it has been speculatedHowever, because of the reduction in temperature and that the solubility changed by 30- 50 m/m' in the exsolution.134PetSei.(2012)9:129-140TImeDepth6406005605204804404003603202802402001601208040 0(Ma)(m)b zPaNgFm30C一100045'Coc75C0.60%R, "30000 80%R,1.00%R40001.20%R,1. 40%R50001 80%R,2.20%R60002 60%R20C300%R。25t70003.40%8000Fig. 4 Thermnal histry of well Wei 118, Weiyuan structure, South SichuanTimeDepth640 600 560 520 480 440 400 360 320 280 240 200 160 120 80 40 0 (Ma)(m)|IP60CC2000.05%E1.0%R40C1.5%R1.7%R25%F30%R3. 8%Fig. 5 Thermal history of well Dingshan 1, Dingshan stucure, Southeast Sichuanprocess (a detailed discussion is presented in section 4.3). trap was further developed. The Weiyuan area was locatedThe current Sinian gas pools are only gas-bearing structures, on the south slope of the Ziyang paleo-trap. A large amountwhere a majority of the natural gas may have migrated to the of hydrocarbon generation occurred in the CambrianWeiyuan and Ziyang areas with a higher tectonic position source rocks of the Weiyuan area (Weiyuan 118 well) (R。after the uplift and exsolution (Fig. 3, Fig.6).was approximately 0.65%). In the Permian-Triassic, the4.2.2 Weiyuan structurehydrocarbons were captured by the Ziyang-Weiyuan paleo-The Weiyuan gas field and Ziyang gas-bearing area trap, leading to the formation of the late paleo-reservoirs (Fig.entered a subsidence stage in the Cambrian -Late Silurian. A 3). In the Early-Mid Jurassic, the Cambrian source rocks ofLeshan-Longnisi paleo-uplift occurred, and the Ziyang and the Weiyuan structure evolved to the stage of condensate andWeiyuan areas were located in higher positions (Fig. 4, Fig. wet gas formation (R。was approximately 1.3%). Until the late8). The Lower Cambrian source rocks of south and southeast Mid-Jurassic, a large amount of dry gas began to be generatedSichuan were mature with hydrocarbon expulsion. The paleo- in the Cambrian source rocks (R。was approximately 2.0%).uplift provided a favorable area for hydrocarbon migration, However, the Sinian reservoirs had a roof buried at a depthleading to the formation of paleo-oil pools in the Weiyuan- of 4,'中国煤化工ofupto 160°C sinceZiyang area (Cui et al, 2008) Similar to the paleo-oil poolsin the E: of oil to gas occurredthe Anpingdian-Gaoshiti structure, these paleo-oil pools were in lar|Y片C N M H Gretaceous, which wasdamaged slightly at the end of the Silurian and biodegraded generated in the paleo-reservoirs. The kerogen-cracking gasbitumen was formed (Fig.7).and oil-cracking gas formed almost at the same time, andOverall subsidence occurred in the Ziyang and Weiyuan they mixed with each other. Through an analysis of apatiteareas since the Permian, and the Ziyang- Weiyuan paleo- fssion track data (Table I, Fig. 4) and inclusion temperaturePelSei:(2012)9:129-14035measurement data, it was shown (Tang et al, 2005) that the the organic mineral inclusions that flled the Sinian pores,Sinian reservoirs had a roof buried at a depth greater than holes, and fractures in the Weiyuan-Ziyang area, Southwest7,000 m and a paleo-temperature above 210 °C. Crude oil Sichuan, during different stages showed that at least threein the paleo-reservoirs was almost completely cracked to phases of hydrocarbon migration occurred in this area,natural gas, leading to the formation of bitumen. Few changes and the paleo-temperatures range 110-130 。C, 150-160occurred to the morphology and location of the Ziyang- °C, and 170-230。C, respectively (Tang et al, 2003; 2005).Weiyuan paleo-trap during this period.According to the reconstructed evolutionary history of theThe characteristics and homogenization temperatures of paleo-temperature, Stage-I hydrocarbon migration occurredTheNWhasss DephThe phase ofSiructural(m) |Anpingdian- Tongxianchanghydrocarbon pool evolution| roluton_A _Shiqiaowan Gaoshi tvctureDongchansi A10002000During the Hmalayanmovement, the uplfted strata3000T.+Twere denuded. the dsslvedgas exsolved from formation4000|8water and dssipated. and thegas-bearing structure formed.|号5000|垦6000Zdn70000By the end ol theCretaceous. the sourcegenerated dry gas. the40000crude ol almost crackedT.+T20 gaswithan abnormacoffcient is between|21.8-2.4, the dssoved gas. pools formed.7000B000Eary-Middle Jurassic, thesource rock of the Cambrianandwet gas Middie-LateJurassic, the dry gas generaledMiddle Jurassic part of crude|paleo-gas pools formedgraduallyZd|美T:+T:By the end of the Tassic,the source rock subsidedpalo-oland g8 poolsformed.By the end of he Siurian, thepools were dameaped. By the中国煤化工om BahepeTYHCNMH(Siansoureroer ator[0o]●●区]▲▲十中Gas poolsHi poolsWaterAsphaltOil migationGas migrationFig. 6 Evolution of oil and gas pols, Anpingdian-Gaoshiti structure36_PetSe.(2012)9:129-140in the Indo-Chinese Epoch as the oil-generation peak of the source rocks of the Cambrian Niutitang Formation had asource rocks. Stage-lI hydrocarbon migration occurred in buried depth of 2,100 m, where oil generation began inthe Yanshanian, when some liquid hydrocarbons started to organic matter (R, was approximately 0.5%). Until the Latecrack, and organic matter reached a high-maturity stage. Silurian, the Cambrian source rocks generated oil in largeStage-Ill hydrocarbon migration occurred in the Yanshanian- amounts (R。was 0.7%), and the paleo-petroleum reservoirsHimalayan (Fig. 4, Fig. 8).formed. At the end of the Silurian, because of the CaledonianHimalayan tectonism played a decisive role in the uplift, hydrocarbon generation stopped, and the paleo-formation of the Weiyuan gas field. The Weiyuan structure on reservoirs were destroyed, because the strata were upliftedthe south slope of the Leshan-Longnisi Paleo-uplift reached and denuded (Yuan, 2008) (Figs. 5, 7, and 9).a structural high taking the place of Ziyang structure, and theIn the Permian, continuous burial occurred to thefinal seting and positioning of the traps were established. A Cambrian source rocks with renewed hydrocarbon generation.portion of the natural gas migrated from the Ziyang paleo-trap In the Late Permian, the top of the Cambrian source rocksto the Weiyuan structure, and more natural gas was trapped in had an oil generation peak (R。 was 1.3%). During the Early-the Sinian trap of the Weiyuan structure (Dai and Wang, 999; Mid Jurassic, the source rocks evolved to the dry gas stageLiu et al, 2008b; Yuan et al, 2009). In addition, because ofan (R。 was 2.0%). The roof of the Sinian reservoirs had auplif with great amplitude, the reduction of the temperature buried depth of approximately 4,650 m and a correspondingand pressure in the uplift process resulted in the exsolution paleo-temperature of 160 °。C in the Late Triassic Early-Midof natural gas in the Sinian paleo dissolved-gas reservoirs of Jurassic, when gas cracked from previously charged oil inWeiyuan. The solubility in formation water was reduced from the reservoirs of the Sinian Dengying Formation (Z,dn). The7.57 m' /m' before the Himalayan to the present value of2.42 early-charged oil in the paleo-reservoirs had become a majorm/m'. The exsolved dissolved-gas reserves were 493 x 10* source of natural gas in this area. Until the Late Jurassic,m', which is close to the current proven gas reserves of400 x the roof of the Sinian reservoirs had a buried depth of 6,70010* m' located in the Weiyuan gas field (Liu et al, 2000)(Fig. m and a paleo-temperature of 210。C, when oil almost4, Fig. 8).completely cracked to gas (Fig. 5). During the Triassic-4.2.3 Dingshan structureLate Jurassic, the traps of the Dingshan structure were inHydrocarbon gencration occurred in the Cambrian source a successive development stage, with changes in the traprocks of the Dingshan structure in the Early Silurian. The morphology (Fig. 9).nC18(nC20|nC21nC21nC22| nC231C230C25nC27C2nc31C26hC32WellAnping 1nC16Well Gaoke 1Z.dn*Z_dn*nC16|PrnC34_nC3nC15JncC35nC25nC14nC:nC26:17Well Wei 117Zdn'nC19nC28xC18nC18||nC19|nC20中国煤化工Well Dingshan1nC13|| nC21CNMHC_Z_dn'1nC23Fig.7 Saturated hydrocarbon chromaograms ofbitumneu samples in Sinian reservoirs of typical structures, Sichuan BasinPet Sei:(2012)9:129-14037Since 82 Ma, uplifts with great amplitude occurred in the the ground surface. These faults and fractures constituted aDingshan structure in three phases, i.e., rapid uplift, slow rapid dispersive channel for natural gas, which completelyuplift, and rapid uplift, where the tctonism was significantly destroyed the natural gas reservoirs in the Dingshan structurestronger than that in central and southwest Sichuan. Asa (Fig. 5, Fig. 9). The water type in the present Sinianresult, the Permian strata were exposed in well Dingshan formation was NaHCO, indicating that the Sinian reservoirsI. Because of the uplift, the temperature of the surrounding in Dingshan I well were in an open system for a long term.rocks decreased, leading to a break in the balanced system4.3 Exsolution effects of dissolved gas onand the exsolution of natural gas dissolved in water.Because of the strong Himalayan tectonism, faults were hydrocarbon accumulation in Sinian reservoirswell developed in the Dingshan structure, even connected toIt should be noted that during the formation and evolutionanear Depth DengingguanNWThe phase ofWeiryuanB| hyrocarbon pool evolution1000000During the Himalayan3000strata were denuded, thedissolved gas exsolved40and the gas pools weretormed.60007000102000By the end of therock of the Cambrian4000generated dry gas. the5000crude oll almost all crackedto gas with an abnormalpresure: and the gas2| 7000formation water.80009000乙0By the end of the Jurassic,the source rock of theCambran generatedcondensate oil and wetthecrudeoicnackedtogas. and paleogas poolsgradually formed.020By the end of the Thassic,the source rock subsidedand entered the peak olpaleoll and gas poolsformed.黑| 500By the end of the Siurian,the Silurtan strata uplitedand denuded. The earlydamaged, and the bitumenformed owing totheBy the end号|中国煤化工. the paleo-_hydrocartbonZ.dnMYHCNMHGyloio_」source rock[ool●●▲▲Gas pools Ol poolsWaterAsphalt Ol migration Gas migrationFig. 8 Evolution of oil and gas pools, Weiyuan structure38Petsei:(2012)9:129-140process of the Sinian reservoirs, the formation of dissolved- temperature and high pressure, a large amount of natural gasgas pools and Himalayan tectonism were closely related from kerogen and the cracking of oil dissolved in formationto the formation of the present gas reservoirs. The Sichuan water led to the generation of dissolved-gas accumulations.Basin experienced a process of continuous burial and high- The dissolved-gas events may be universal in the Siniantemperature evolution during the Permian-Late Cretaceous. geological history of the Sichuan Basin. The existingIn the Early-Mid Jurassic-Late Cretaceous, the crude oil evidences of paleo dissolved-gas pools were discovered byin the Sinian reservoirs was gradually cracked to gas, and the authors in the fluid inclusions of the Sinian drill cores.this process led to volume expansion and abnormally highThe organic inclusions in the quartz and calcite at the lastpressure in the Sinian paleo-reservoirs. Under the high charge stage of the Sinian reservoirs in well Anping I, Central_SE、The phase ofWell Dingshan 1C' hydrocarbon pool evolution000200-During the Himalayanstrata were denuded,Thedissovedgas5 4000.water and dissipated,and the gas pools filed.5000.62000By the end oftheTCretaceous, the sourcePCnoratedrvasanthe cnude oll almostallcrackedto g8s wth60000Einto hefomatinateroo.rock ofthe Cambriangenerated condensate里6000pools gradually formed.By the endof theTriassic, the source rock。3000--had subsided and enteredthe poak oil generatbonand g8s pools were fomed.e 50000By the end of the Silurian. thedenuded. The eary paleoil300中国煤化Inpo pYC N MH GauourCamonan souro ruck00]中Gas polsol poolsWaterAsphalto1 migration Gas migrationFig. 9 Evolution of oil and gas pools, Dingshan structurePetsSc.(2012)9:129-14039Sichuan Basin, mainly consisted of dissolved methane were formed in the Weiyuan, Ziyang. and Anpingdian-inclusions, which was also proven by the peak intensity andGaoshiti structural belts of Central Sichuan. Influenced byarea distribution of laser Raman spectra. PVTsim simulation the strongest transformation and destruction, the Dingshantechnology was used to obtain the paleo-pressure of the structure in Southeast Sichuan has almost been completelyfluid inclusions (Yuan et al, 2008). Corresponding to thedestroyed.peak temperature of 260- 290。C for the Sinian inclusions inCentral Sichuan, the minimum fuid pressure was 1,200- 1,600 5 Conclusionsbar and the paleo-pore fluid pressure ceoffcient was 1.84-2.40, leading to overpressured fluids. The conversion of oil1) It has been proven in the analysis above that the sourceto gas resulted in a volume expansion, which was the main rocks of the Sinian reservoirs in the Sichuan Basin were thefactor for the formation of abnormally high pressure. Undershales of the Lower Cambrian, which had a high abundancethese termperature and pressure conditions, around 50- 90 m’of organic matter, great thickness, and strong bydrocarbondissolved-gas existed in every I m' water (Price, 1979). As generation capacity. However, the current proven level ofestimated by the same method, approximately 10 30 m' gasSinian hydrocarbon resources is low, and only the Weiyuandissolved in every 1 m' water in the Cambrian reservoirs, gas field has been found, which promises considerableSoutheast Sichuan (Yuan, 2008). Because no core samples exploration space and potential. Favorable conditionsfrom the Weiyuan area are available, no direct evidence couldexist in the source-reservoir-seal assemblage of the Sinianbe given to verify the existence of dissolved gas reservoirs traps, Sichuan Basin. As viewed from the development andin the Sinian Weiyuan gas field. However, it was proven in evolution history of the traps in typical stuctures, traps wereprevious studies that the pressure coficient could reach 2.05formed prior to the oil generation peak of the source rocks,in the cracking of oil to gas in the Sinian Weiyuan gas field which is beneficial for the accumulation and preservation of(Sun et al, 2007), and dissolved-gas events ocurred (Yuan,hydrocarbon.2008; Yuan et al, 2009). After the formation of the dissolved-2) The thermal evolutionary history of the Cambriangas reservoirs, the formation water in the reservoirs provided source rocks constrained by the apatite fssion track data, asthe natural gas with its preservative carrier. Natural gaswell as the trap evolutionary history of typical structures,always existed in a water soluble form until the occurrencerevealed that Sinian natural gas pools underwent anof the uplift from the Himalayan movement, which could evolutionary accumulation process, i.e, paleo-oil poolseffectively prevent the destruction of the gas pools caused by→paleo-gas pools - →current-gas pools (adjusting andthe tectonic movement and other factors during this period,reforming). The exsolution of dissolved-gas caused byand this provided the safeguard for the final setting of the tectonic uplift played an important role in hydrocarbonSinian gas reservoirs.accumulation since the Late Cretaceous. The formation ofInfuenced by the Himalayan tectonism, an overall upliftthe Weiyuan gas field, Ziyang gas pools, and Anpingdian-occurred in the Sichuan Basin, except for the West Sichuan Gaoshiti gas-bearing structure was closely related to theDepression, since the Late Cretaceous. Because of theprocesses mentioned above.decreased temperature of the carly-formed dissolved gas3) The formation and evolution of the reservoirs showedpools, the original balance of gas reservoirs was broken, and that the early-existing reservoirs located in areas that wereexsolution of a large amount of natural gas occurred whichsubjected to strong tectonic deformation and reformationhad been dissolved in water, accompanied by the partial loss were easily damaged (such as the Dingshan structure).and transfer of natural gas. The establishment of the Sinian However, the Leshan-Longnisi Paleo-uplift area was lessgas reservoirs was in their final stting. In fact, the previousinfluenced by the later tectonism, with no strong deformationresearch results also verify that the formation of some giant and no development of deep and large faults, which isgas fields was related to the exsolution and accumulation of beneficial for natural gas accumulation and preservation. Thispreviously dissolved-gas (Cramer et al, 2002; Litlke et al，area shall be a target area for further exploration, and oil and1999; Qin et al, 2006; 2007; Li et al, 2003; Chen et al, 1997),gas exploration could have a breakthrough in this area.including the Hetianhe gas field in the Tarim Basin and theUrengoy gas field in the West Siberia Basin.AcknowledgementsThe formnation and evolution of the Sinian reservoirs in theThis paper is funded by the National Key Basic Researchtypical structures of the Sichuan Basin mainly underwent theProgrars (973) "Deep hydrocarbon accumulation mechanismfollowing stages: 1) Paleo-oil pool formation and destructionof China's marine carbonate strata”(2005CB422106) andin the late Silurian; 2) Renewed hydrocarbon generation of"The structure evolution in middle and upper Yangtzethe Cambrian source rocks and hydrocarbon recharge in theregion and multi-period hydrocarbon accumulation in lowerEarly Permian-Early-Mid Triassic; 3) Cracking of oil togas and dissolved-gas formation in the Late Triassic-Mid-中国煤化工Late Jurasis; 4) Phasic uplif, exsolution of natural gas, RefeMHCNMHGfinal stting and positioning of gas reservoirs from the LateCretaceous until now. 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