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New directions in an establishedgas play: Promising dolomitereservoirs in the Middle TriassicLeikoupo Formation of theSichuan Basin, ChinaDengfaHe,YongshengMa,YingqiangLi, andShunliWu

ABSTRACT

The discovery of carbonate gas fields in the Middle TriassicLeikoupo Formation of theSichuanBasinhas a complex history. Inrecent years, a series of structural fields have been discovered in thewestern Sichuan Basin. Their discovery confirms the immenseexploration potential of the Leikoupo Formation. In this study, weanalyze the characteristics of Leikoupo Formation explorationplays using exploration wells and test data, aiming to providea reference for further discoveries. The Leikoupo Formationrepresents the uppermost unit in the Sichuan marine carbonateplatform succession. During its deposition, the whole basin wascharacterized by a restricted and evaporitic platform. Two classesof reservoirs developed. One is pore–fracture reservoirs, in mar-ginal platform and intraplatform shoals, and another is fracture–vugreservoirs in the karstic weathering crust of the formation-cappingunconformity. Three hydrocarbon accumulation models wereestablished for the Leikoupo Formation based on the spatial andtemporal relationship among the source, reservoir, and cap rocks.Two types of exploration plays are present in the Leikoupo For-mation, that is, shoal (including intraplatform shoal and marginalplatform shoal) dolomite plays and karstic dolomite weatheringcrust plays (including intraplatform shoal karst and marginal plat-form shoal karst). The western Sichuan depression in the karsticslope belt presents immense exploration potential because ofa proximal hydrocarbon supply, charging via an extensive fracture

AUTHORS

Dengfa He ~ Key Laboratory of MarineReservoir Evolution and HydrocarbonAccumulation Mechanism, China Universityof Geosciences, Ministry of Education, HaidianDistrict, Beijing, China; hedengfa282@263.net

DengfaHe is a professor of basin structures andtectonics and petroleum geology at the Schoolof Energy Resources, China University ofGeosciences,Beijing.He receivedhis Ph.D. fromPetroChina Research Institute of PetroleumExploration and Development in 1995. Hefinished his postdoctoral research at theInstitute of Geophysics (now Institute ofGeology andGeophysics), ChineseAcademyofSciences, in 1997. His main research interestsinclude tectonic and depositional environmentof the Sichuan Basin and its petroleumaccumulation. He has published 140 papers ininternational and domestic journals and edited13 memoir volumes and journal special issues.

Yongsheng Ma ~ China PetrochemicalCorporation, Chaoyangmen, ChaoyangDistrict, Beijing, China; mays@sinopec.com

Yongsheng Ma is a professor-level seniorengineer with a doctoral degree andacademician of Chinese Academy ofEngineering. He received his Ph.D. fromChinese Academy of Geological Sciences in1990. He was appointed as chief geologist ofSinopec Corporation in 2013 and wasappointed as vice president of SinopecGroup in 2015. He has conducted petroleumgeology and marine carbonate oil and gasexploration in several Chinese basins.

Yingqiang Li ~ Key Laboratory of MarineReservoir Evolution and HydrocarbonAccumulation Mechanism, China Universityof Geosciences, Ministry of Education,Haidian District, Beijing, China;yingqiangcugb@126.com

Yingqiang Li is a Ph.D. student at Schoolof Energy Resources, China University ofGeosciences, Beijing. He received hisB.Sc. (2012) and M.Sc. (2015) from ChinaUniversity of Geosciences, Beijing. His primaryresearch interests focus on sedimentarygeology and basin structural geology,especially the restoration of tectonic anddepositional environments. He has published

Copyright ©2019. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received December 14, 2016; provisional acceptance February 8, 2017; revised manuscriptreceived July 21, 2017; revised manuscript provisional acceptance September 12, 2017; 2nd revisedmanuscript received November 7, 2017; 2nd revised manuscript provisional acceptance December 4, 2017;3rd revised manuscript received February 26, 2018; final acceptance May 11, 2018.DOI:10.1306/05111816502

AAPG Bulletin, v. 103, no. 1 (January 2019), pp. 1–29 1

network, shoals and karstic reservoir, a good seal rock of terrestrialmudstone, and potential composite hydrocarbon accumulations instratigraphic traps,making it a promising area for future exploration.

INTRODUCTION

The Leikoupo Formation (247.2–242 Ma) is an assemblage ofanhydrite-dominated evaporites and carbonates that representsthe uppermost unit in the Sichuan carbonate platform succession(Lin and Chen, 2008; Li et al., 2012a; Zhu et al., 2014). Based onearlier exploration, the key to the discovery of oil and gas in theLeikoupo Formation is the location of the effective source rocksand the migration pathways connecting those source rocks withreservoirs (Wang et al., 1998; Liu et al., 2011; Feng et al., 2013;Liao et al., 2013). In recent years, exploration wells targeting theLeikoupo Formation made major hydrocarbon discoveries inthe Xinchang–Jinma–Yazihe–Shiyangzhen structural belt alongthe western margin of the Sichuan Basin, with proven gas reservesgreater than 3000 · 108 m3 (10.5945 tcf).

The Leikoupo Formation has gradually become the primaryexploration target in the western and central Sichuan Basin.Previous studies have suggested that additional promising reser-voir intervals exist within the Leikoupo Formation, whereasorganic-rich, lagoonal carbonates within the evaporitic platformof the Leikoupo Formation have excellent source rock potential(Feng et al., 2013; Xu et al., 2013; Huang, 2014; Xie, 2015). Topromote sustainable exploration in the future, it is necessary tounderstand the hydrocarbon accumulation characteristics, espe-cially the distribution and potential of favorable exploration plays.

In this study, the petroleum geology of the Leikoupo For-mation is analyzed using well data available from the SichuanBasin. Various exploration plays (White, 1980, 1988; Miller,1982; Crovelli, 1987; Allen and Allen, 1990; Tong and He, 2001)were investigated with a focus on reservoir characteristics. Finally,favorable areas for further exploration were established based ona framework of spatial–temporal relationships among key hy-drocarbon accumulation elements.

HISTORY OF PETROLEUM EXPLORATION IN THELEIKOUPO FORMATION

Petroleum exploration of the Leikoupo Formation in the SichuanBasin began in the 1970s. In December 1971, the Chuan 19 wellin the Zhongba structure had a blowout in Member 1 of theLeikoupo Formation (Lei-1 Member, T2l1) and produced somelight oil but was soon depleted. In November 1972, Member 3 ofthe Leikoupo Formation (Lei-3 Member, T2l3) in the Zhongba

several papers in international anddomestic journals.

Shunli Wu ~ Key Laboratory of MarineReservoir Evolution and HydrocarbonAccumulation Mechanism, China Universityof Geosciences, Ministry of Education,Haidian District, Beijing, China; wushunli2011@163.com

Shunli Wu is a master’s student of basinstructural geology at School of EnergyResources, China University of Geosciences,Beijing. He received his bachelor’s degree fromHenan Polytechnic University. His researchinterests are in fault-related folding theory andstructural evolution of foreland basins.

ACKNOWLEDGMENTS

We would like to express our appreciation tothe PetroChina Southwest Oil and Gas FieldCompany and Sinopec Southwest Oil andGas Company for providing us with theirabundant drilling and core data. This researchwas financially supported by the NationalNatural Science Foundation of China(projects 41430316 and 40739906) and theNational Science and Technology MajorProject (2011ZX05008-001).

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area of the northwestern Sichuan Basin began toproduce gas flow, indicating that the Leikoupo For-mation is a substantial gas reservoir with reserves of86.3 · 108 m3 (304.8 BCF) (Zhu et al., 2011). Overthe next 12 yr, numerous structures in the LeikoupoFormation of the western Sichuan Basin (e.g.,Sumatou, Youguanding, Laoguanmiao, Daxing, Long-fengchang, and Qinglinkou) were explored, but onlywater was produced (Zhai, 1989; Sun et al., 2017).

In December 1980, the Lei-1 Member gas res-ervoir was discovered in theMoshen 2well, located inthe Moxi structure of the central Sichuan Basin, butit was not put into production because of high sulfurcontent. In May 1987, the Lei-1 Member was con-firmed to have proven reserves of 349.47 · 108 m3

(1.2284 tcf), making it the principal pay zone of theMoxi gas field (Zhai, 1989). In the subsequent twodecades, little progress has been made in the explo-ration of the Leikoupo Formation, except for the dis-covery of some small- to medium-sized gas fields: theLongnusi and Luoduxi structures in the central Si-chuan Basin, the Guanyinchang and Jieshichang struc-tures in the southwestern Sichuan Basin, theWolonghestructure in the eastern Sichuan Basin, and the Yuanbaand Longgang structures in the northeastern SichuanBasin (Figure 1).

From 2007 to 2012, two key exploration wellsin the Xinchang structural belt, western SichuanBasin (Chuanke 1 and Xinshen 1), produced high-yield commercial gas flow from the uppermostLeikoupo Formation, at rates of 86.8 · 104 m3/day(30.65 MMCFGD) and 68 · 104 m3/day (24.01MMCFGD), respectively, suggestive of goodsource–reservoir conditions in the Leikoupo Forma-tion (Song et al., 2013; Xu et al., 2013). In January2014, a well located in the Jinma structure, in thesouthcentral part of the western Sichuan Basin (wellPengzhou 1), produced high-yield commercial gasflow from reservoirs in the upper Member 4 of theLeikoupo Formation (Lei-4 Member, T2l4), belowthe unconformity capping the Leikoupo Forma-tion, at the rate of 121.05 · 104 m3/day (42.75MMCFGD). This discovery marked a breakthroughin exploration of the marine successions of theLongmenshan piedmont structural belt. In July 2015,the Yashen 1 and Yangshen 1 wells in the Yazihe–Shiyangzhen structural belt also produced high-yield commercial gas flow from the Lei-4 Member,at rates of 48.5 · 104 and 60.32 · 104 m3/day (17.13

and 20.30 MMCFGD), respectively, expanding theknown extent of the Leikoupo Formation gas re-source in the piedmont zone. So far, the provenreserves are greater than 3000· 108m3 (10.5945 tcf),implying the potential for a series of future explora-tion plays.

GEOLOGICAL SETTING

The Sichuan Basin in the northwestern part of theYangtze block is located adjacent to the Tethys oceanand the North China block. Its evolution was con-trolled by the Longmenshan intracontinental oro-genic belt along the western margin and the Qinlingorogenic zone along the northern margin (Guo et al.,1996; Meng et al., 2005). A series of major tectonicevents, including the opening and closing of the SouthQinling ocean basin (the northern branch of thepaleo-Tethys ocean) (e.g., Ratschbacher et al., 2003;Nie et al., 2016), the opening and closing of theGanzi–Litang, Jinshajiang, and Lancangjiangbranches of the paleo-Tethys (e.g., Xu et al., 2015),and the opening and closing of the NeotethyanYarlung–Zangbo River ocean basin (e.g., Aitchisonet al., 2003), have formed a set of superimposedbasins. They are composed, with time, of a passivecontinentalmargin succession during the Sinian to theMiddle Triassic (Z–T2), marine-to-continental riftedbasin deposits during the deposition of Member 1 toMember 3 of the Xujiahe Formation of the UpperTriassic (T3x1–T3x3), intracontinental depressionstrata during the Member 4 of the Upper TriassicXujiahe Formation to the Middle Jurassic (T3x4–J2)and foreland basin deposits during the Late Jurassic toQuaternary (J3–Q). The combined thickness of theNeoproterozoic–Middle Triassic marine successionand the Upper Triassic–Quaternary continentalsuccession ranges from 6000 to 12,000 m (19,685 to39,370 ft) (Wang, 2002; He et al., 2011).

The Middle Triassic succession is separated intotwodistinct sedimentary facies zones by theChengkou–Wanxian–Fuling belt. The predominantly clasticBadong Formation is present in the eastern area,whereas the carbonate-dominated Leikoupo Forma-tion consists of the western part of the succession(Bureau of Geology and Mineral Resources of Si-chuan, 1991). The Leikoupo Formation is distributedthroughout the central and westcentral Sichuan Basin

He et al. 3

and represents the last unit deposition of carbonate inthe Sichuan Basin. It pinches out to the south andeast and thickens in the north and west (Bureau ofGeology and Mineral Resources of Sichuan, 1991;Guo et al., 1996).

At the base of the Leikoupo Formation, greenpisolites are widespread, with the thickness of0.5–3 m (1.6–9.8 ft). The lithology transitions up-section into limestone, dolomite interbedded withanhydrite and other evaporites, breccias, sand-stone, andmudstone, with slight lateral variation in

lithology. Bivalves (Eumorphotis [Asoella],Myophoria[Costatoria] goldfussi) and ammonites (Progonocer-alitespulcher), as well as foraminifera (Glomospira sp.)and conodonts (Neospathodus germanicus andNicoraella kockeli) are present (Bureau of Geologyand Mineral Resources of Sichuan, 1991; Guo et al.,1996). The Leikoupo Formation is divided into fourmembers, numbered Lei-1 through Lei-4 vertically.To the west of the Longmenshan Mountains, the topof the Lei-4Member transitions into the TianjingshanFormation, which is a set of thick continental flysch

Figure 1. Distribution of the Middle Triassic Leikoupo Formation strata in the Sichuan Basin. Major structures are numbered as follows: 1,Zhongba; 2, Qinglinkou; 3, Laoguanmiao; 4, Yuanba; 5, Longgang; 6, Longfengchang; 7, Xinchang; 8, Yazihe; 9, Jinma; 10, Shiyangzhen; 11,Sumatou; 12, Daxing; 13, Youguanding; 14, Moxi; 15, Longnusi; 16, Luoduxi; 17, Wolonghe; 18, Jieshichang; 19, Guanyinchang. Mts. =Mountains.

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deposits containing effusive mafic to intermediatevolcanic rocks. Carbonate gravity flow deposits(deposited along the margin of an ocean trough) arefound in the transition zone along the westernmargin of the Yangtze block.

At the end of the Middle Triassic, the Indosinianmovement led to broad tectonic uplift of the SichuanBasin. As seawater regressed from the upper Yangtzeplatform and the Luzhou and Kaijiang paleo upliftswere formed, an approximately 10-m.y. depositionalhiatus resulted. Consequently, Middle Triassic car-bonates were widely eroded and karstified, with re-gional unconformities developed (An, 1962, 1996).The upper part of the Lei-4 Member in the westernSichuan depression was partially eroded, with thethickness of denudation decreasing from east to west(Figure 1). This period of exposure resulted in theformation of karst reservoirs in the uppermostMiddleTriassic strata (Zhong et al., 2011; Tang, 2013; Ninget al., 2015). During the early stages of the LateTriassic, the western Sichuan Basin was compressedbecause of gradual closing of the paleo-Tethys ocean(Zhai et al., 2016). The Upper Triassic MaantangFormation (T3m) is believed to be correlativewith theMember 1 of the Xujiahe Formation (T3x1), wasdeposited in a gently sloping marine ramp formed bythe foreland flexure, and unconformably overlies theLeikoupo and Tianjingshan Formations (Liu et al.,2009a).

CHARACTERISTICS OF LEIKOUPOFORMATION RESERVOIRS

Lithology and Lithofacies

During deposition of the Leikoupo Formation, theSichuan Basin was a partially enclosed epicontinentalseaway with a paleotopographic high in the southeastand a low in the northwest (Feng et al., 1997; Liuand Tong, 2001). The majority of the Sichuan Basinwas a rimmed, restricted, evaporitic platform (Linand Chen, 2008; Li et al., 2012a, b; Lu et al., 2013).Laterally, beaded platform margin shoals occurred,along a northeast trend on the western margin of theSichuan Basin. Lacustrine facies, gypsiferous lagoons,dolomitic flats, and shoal subfacies constitute themultiple sedimentary cycles. The Leikoupo Forma-tion is 200–1200 m (656–3937 ft) thick, with the

greatest thicknesses (generally >800 m [>2625 ft])presented in the western Sichuan depression. Theformation is generally divided into four membersbased on lithologic characteristics (Figure 2).

Lei-1 MemberThe Lei-1 Member (T2l1) is dominated by thickanhydrite and anhydrite-dolomite strata, interbeddedwith silty dolomite, as well as minor halite andpolyhalite. Its base is characterized by a succession ofgrayish-green to grayish-white illitic and siliceousmudstones, commonly known as green pisolites(Bureau ofGeology andMineral Resources of Sichuan,1997). During deposition of the Lei-1 Member, theLongmenshan–Yanyuanmarginal basin existed to thewest of the Longmenshan paleoisland chain, ina slope–bathyal environment; to the east wererestricted evaporite platforms and marginal plat-form shoals, representing slight uplifts and sags(Figure 3A). Tidal flat and lagoonal carbonate rocksinterbedded with evaporites accumulated in theplatform interior. A succession of gray calcisparitedoloarenite, limy doloarenite, arenaceous limy do-lomite, light gray algal dolomite, oolitic dolomite, anddark gray dolomicrite is developed along the north-west margin of the Sichuan Basin, with individualgrainstone packages with thicknesses of 8–30 m(26–98 ft), indicating deposition occurred inmarginalplatform shoals and intershoal environments. In theYilong area, the Lei-1 Member gradually transitionsinto gypsiferous dolomitic flats; the depocenter islocated near the Chuanke 1 well, where the mem-ber is approximately 240 m (~787 ft) thick. TheLei-1 Member thins to northeast and southwest(Figure 3A).

Lei-2 MemberThe Lei-2 Member (T2l2) in the western SichuanBasin can be subdivided into three sections (Xu et al.,2011). The lower section is composed of dark grayalgal calcarenite, algal oncolitic dolomite, calcisparite,arenaceous oolitic dolomite, and oolitic limestonewith interbeds of argillaceous dolomite. The middlesection transitions upward from light gray algal-laminated dolomite and algal arenaceous dolomiteto micritic dolomite and silty dolomite. The uppersection is dominated by dark gray, finely crystal-line doloarenite and algal-laminated dolomite with

He et al. 5

dissolution bird’s-eye pores and geopetal structures.The depositional thickness of individual grainstoneintervals ranges from 12 to 158 m (39 to 518 ft).

The total thickness of the Lei-2 Member in theChuanke 1 well is 381 m (1250 ft), symmetricallydecreasing to 80–120 m (262–394 ft) to the south-west and northeast. Deposition was centered aroundthree local subbasins (Figure 3B). The first is centeredin the Yilong–Dazhou–Nanchong area and has a totalthickness of 100–200 m (328–656 ft). These strataconsist of a lower interval of light gray dolomite withthick interbeds of evaporites and an upper interval

composed of dark gray limestone and argillaceouslimestone with thinner evaporite interbeds. The sec-ond is centered in the Santai–Chengdu area and hasa thickness of 180–260 m (590–853 ft). Strata arecomposed of interbedded argillaceous dolomite,anhydrite–dolomite, and evaporites. The third iscentered in the Leshan–Ziyang–Zigong area, witha thickness of 80–120 m (262–394 ft). It is charac-terized by massive argillaceous dolomite and argilla-ceous limestone strata, interbedded with evaporites.The Lei-2 Member depositional setting inherited theframework of uplifts and sags that characterized the

Figure 2. Generalized stratigraphy and source rocks in the Sichuan Basin (modified from Dai et al., 2009, 2012), with enlarged partshowing the subdivision of the Middle Triassic Leikoupo Formation (Fm.). Ro = vitrinite reflectance; TOC = total organic carbon.

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period of Lei-1 Member deposition; with the gradualregression of seawater, an increasingly restricted andevaporitic platform resulted.

Lei-3 MemberThe Lei-3 Member (T2l3), in the Dujiangyan–Hanwang–Guangyuan belt along the western andnorthwestern margins of the Sichuan Basin, is com-posed of gray calcisparite and doloarenite, arenaceouscalcitic dolomite, light gray algal dolomite, ooliticdolomite, and light gray dolomicrite, with echinoidspine debris and ostracods. The thickness of individ-ual grainstone packages is 45–155 m (148–509 ft),whereas that of oolitic and arenaceous shoals is greaterthan 90 m (295 ft). In the Qionglai–Yaan area, alongthe southwesternmargin of the SichuanBasin, the unitis mainly composed of dark gray argillaceous lime-stone, gray limestone, dolomite, anhydrite–dolomitewith interbedded doloarenite, calcarenite, and algaldolomite. Grainstone packages are generally 14–80 m

(46–262 ft) thick, whereas oolitic or arenaceous shoalshave an average thickness of 40m (131 ft). The largesttransgression of the Middle Triassic occurred duringdeposition of the Lei-3Member, when shallowmarineorganisms (such as ammonites and brachiopods) wereabundant.Nodular limestonesweredeposited in awidearea of the platformmargin. The depocenter shiftedeastward, indicating migration of the intraplatformsag and changing of the regional tectonic environ-ment (Figure 3C).

Lei-4 MemberThe Lei-4Member (T2l4) is eroded to varying degreesin the western Sichuan Basin (Tang, 2013). Thecontact with overlying Upper Triassic strata is mostlydisconformable, but in the Yazihe, Dayuanbao, andHuanglianqiao areas it is conformably overlain by theTianjingshan Formation. The residual Lei-4 Memberin the western Sichuan depression is 350–380 m(1148–1247 ft) thick, with a lower interval consisting

Figure 3. Sedimentary facies during deposition of members different of the Leikoupo Formation in the Sichuan Basin: (A) depositionalenvironments of the Lei-1 Member, (B) depositional environments of the Lei-2 Member, (C) depositional environments of the Lei-3Member, and (D) depositional environments of the Lei-4 Member.

He et al. 7

of massive gray to white anhydrite with interbeddeddolomicrite and an upper interval consisting of lightgray anhydrite–dolomite and dolomicrite with cri-noids and algal lamellae. The Lei-4 Member can bedivided into three submembers based on lithologicalassociations (Figure 4). The lower submember (T2l4

1)is 180–200 m (591–656 ft) thick and is dominatedby thick anhydrite strata interbedded with darkdolomicrite. It is well preserved in most parts ofthe western Sichuan Basin. The middle submember(T2l4

2) is 70–80 m (230–262 ft) thick and consistsmainly of interbedded anhydrite and dolomicrite;beds are of varying thickness and pinch out to theeast. The upper submember (T2l4

3) is 90–120 m(295–394 ft) thick, and it is lithologically dominatedby dolomicrite and finely crystalline dolomite, limydolomite to dolomitic limestone, and algal calcarenite.It is mainly located along the western margin of theSichuanBasin and pinches out to the east (Figure 3D).The Longmenshan paleoisland chain had largely al-ready formed by the time the Lei-4 Member wasdeposited; marginal platform shoals and intershoalsedimentary facies belts are present along the paleo-island front.With ongoing uplifting of the Luzhou andKaijiang structures, denudation accelerated, whereasthe basin became strongly evaporitic because ofregression and the arid climate (Gong et al., 2015).Gypsiferous lake and gypsiferous lagoon depositsexpanded as the depocenter of the gypsiferous lakemigrated westward (Li et al., 2012b).

Types of Leikoupo Formation Reservoirs andTheir Properties

Two types of high-quality reservoirs were developedin the Leikoupo Formation, that is, shoal dolomitereservoirs and paleokarst weathering crust reservoirs.The shoal dolomite reservoirs include both marginalplatform and intraplatform shoal pore–fracture res-ervoirs, characterized by the Lei-1Member and Lei-3Member gas reservoir in theMoxi and Zhongba areas.The paleokarst weathering crust reservoir includesthe unconformity karst (a fracture–vug reservoir),commonly in association with a marginal platform orintraplatform shoal pore–fracture reservoir, charac-terized by the Lei-4 Member gas reservoir in theLonggang area and in the western Sichuan depres-sion as representative examples. In general, the rock

physical properties of the Leikoupo Formation res-ervoir are controlled by sedimentary microfacies,dolomitization, and dissolution (Wu et al., 2011;Song et al., 2013; Tang, 2013; Long et al., 2016).

Shoal Pore–Fracture ReservoirsThe period of Leikoupo Formation deposition cor-responds to an important interval of shoal formationin the upper Yangtze area. Marginal platform shoalsand intraplatform tidal flat shoals were developed inall Leikoupo members and can serve as hydrocarbonreservoirs (Shen et al., 2008; Wang et al., 2009;Li et al., 2011; Tan et al., 2014).

Intraplatform shoal reservoirs were developed inthe Lei-1 through Lei-4 Members, although they arebest developed in the Lei-1 Member (Ding et al.,2012). Commercial gas flow has been produced fromthe Lei-1 Member in many areas, with the Moxistructure gas field as the most typical example. Thereservoirs are lithologically diverse and include mi-crite to powder–crystalline limestone and dolomite,fine powder–crystalline limestone and dolomite,doloarenite, calcisparite, oolitic dolomite, and dolo-micrite. Based on the pore-space classification schemedby Choquette and Pray (1970), we further considereddiagenesis and reservoir quality evolution and identifiedthe reservoir space as mainly composed of intercrys-talline pores as well as solution-enlarged intercrys-talline, interparticle, and intraparticle pores.

Marginal platform shoal reservoirs were foundin the Lei-2, Lei-3, and Lei-4 Members. Large andlaterally continuous arenaceous shoals are distributedthroughout the northern and western Sichuan Basin.A commercial gas reservoir in the Lei-3 Member wasdiscovered in the Zhongba area of the northwesternSichuan Basin, with reservoirs composed of doloar-enite, algal-bound dolomite, powder–crystalline do-lomite, oolitic dolomite, and argillaceous dolomite.Reservoir space consists chiefly of interparticle, in-traparticle, and intercrystalline pores, fractures, andsolution-enlarged pores and fractures, making it a poreand fracture–pore type reservoir.

The Lei-1-1 submember reservoir in the centralSichuan Basin is relatively continuous and corre-latable from well to well. The reservoir is mainlydeveloped in the middle part of the Lei-1-1 sub-member, with a cumulative thickness between 7 and10m (23 and 33 ft) (Figure 5). Tests were conducted

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He et al. 9

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on core samples taken from dozens of wells in thecentral Sichuan Basin and show that the effectiveporosity ranges from 3% to 27.6%, averaging 5.35%.The fine to powder–fine crystalline dolomite anddoloarenite lithofacies show the highest porosity,averaging 4.78% and 5.54%, respectively (Figure 6A).The Lei-1 Member reservoir in the central SichuanBasin is generally characterized by moderate po-rosity and low permeability, with locally moderateto high porosity and permeability. According toevaluation criteria of carbonate reservoirs in Si-chuan Basin (Table 1), it can be classified as a typeII–III reservoir.

The Lei-3Member reservoir, in the Zhongba areaof the northwestern Sichuan Basin, was deposited

in an intertidal–subtidal shoal environment, wheredoloarenite, algal stromatolitic dolomite, and algal-bound dolomite were developed. In these algal do-lomites and doloarenite, clay content is less than2%. Dissolved pores (pinholes) were developed inthe Lei-3 Member dolomite grainstone reservoir,characterized by solution-enlarged interparticle, intra-particle, and intercrystalline pores. Greater than90% of total porosity is from dissolution. Thesedissolution pores are most consistently developed150–200 m (492–656 ft) below the upper surface ofthe Leikoupo Formation (Zeng et al., 2007). TheLei-3 Member gas reservoir has low to moderatereservoir capacity, with an effective porosity of 2.4%–

6.5% (averaging 4.57%) (Figure 6B) and a permeability

Figure 6. Histograms of porosity frequency and average porosity of different rock types for the (A) Lei-1 Member intraplatform shoalreservoir in the Moxi area and (B) Lei-3 Member marginal platform shoal reservoir in the Zhongba area. Porosity data modified after Zenget al. (2007) and Huang et al. (2014) and used with permission of the Journal of Paleogeography and the Journal of Southwest PetroleumUniversity (Science and Technology Edition), respectively.

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of 0.01–1 md. The doloarenite, algal-bound dolomite,powder–crystalline dolomite, and algal stromatoliticdolomite show the highest porosity, ranging from2.16% to 3.6% (Figure 6B). This is mainly a shoal–porereservoir and is classified as type III (Table 1), witha total thickness of 23–120 m (75–394 ft).

Unconformity Karst (Weathering Crust) Fracture–VugReservoirs

Intraplatform Karstic Shoal Reservoirs—The karst reservoirsin the uppermost Leikoupo Formation and the upperpart of the Lei-4 Member are mainly composed of finepowder–crystalline dolomite, micritic algal calcarenite,dolomitic calcarenite, calcisparite, doloarenite, clumpyalgal dolomicrite, and limy dolomite. Reservoir porespaces are mainly composed of solution-enlargedinterparticle, intercrystalline pores, vugs, and fractures.The karstic unconformity fracture–vug reservoir inthe upper Leikoupo Formation (Lei-4 Member) ismainly located 0–9 m (0–30 ft) below the top of theformation, up to a maximum of 50 m (164 ft) belowthe top (Zhou et al., 2010). Its cumulative thicknessis roughly 20–55 m (66–180 ft), with moderatereservoir properties and type II–III reservoirsdominant according to the evaluation criteria ofcarbonate reservoirs in the Sichuan Basin (Table 1).Microfacies and paleokarstification are the key factorscontrolling the formation of fracture–vug typereservoirs (Zhong et al., 2011; Ma et al., 2012; Xinet al., 2012; Yang et al., 2014).

The Lei-4 Member gas reservoirs in the Long-gang area are dolomitic paleokarst reservoirs and arelithologically composed of dolomite grainstone, do-lomicrite, and dolomitic breccia with minor clasticmaterial. Reservoir space is mainly characterized byintercrystalline pores, dissolution pores, dissolution

vugs, dissolution fractures, and structural fractures.Karstic reservoirs are most abundant in the Lei-4Member, approximately 100 m (~328 ft) below theunconformity surface. These are mainly fracture–vugreservoirs, with type III reservoirs predominant(Table 1).

The porosity of the weathering crust dolomitereservoir in the Lei-4 Member of the Longgang area ismainly between 1% and 5%,with amaximumof 9.9%and an average value of 3.2%. Samples with a porositygreater than 3% account for 50% of total samples,whereas those with permeability greater than 0.1 mdaccount for 41% of samples. On the whole, it is a low-porosity and medium-permeability reservoir, withporosity and permeability having an exponential re-lationship (Figure 7A).

Marginal Platform Karstic Shoal Reservoirs—Numerousexploration wells in the western Sichuan depressionhave encountered large-scale weathering crustreservoirs in the uppermost Lei-4 Member. Thesereservoirs formed via subaerial exposure of marginalshoal dolomites during uplifting associated withIndosinian movement and the subsequent weatheringand dissolution of the upper Lei-4 Member (Figure 8)(Zenget al., 2007;Xuet al., 2012; Fenget al., 2013;Songet al., 2013; Meng et al., 2015). These reservoirs arewidely distributed throughout the western SichuanBasin; they are extremely thick, with moderate tohigh reservoir properties, making them the mainexploration target in the marine strata of the westernSichuan Basin.

Well data and thin section study indicate thatthe Leikoupo Formation reservoir is lithologically com-posed of dolomicrite (Figure 9A), stromatolitic powder–crystalline dolomite (Figures 9B, 10A, C, E), and algal

Table 1. Evaluation Criteria of Carbonate Reservoirs in the Sichuan Basin

Reservoir Quality I II III IV

Porosity, % >10 10–5 5–2 <2Permeability, md >1.0 1–0.25 0.25–0.002 <0.002Throat midvalue, mm >1 1–0.2 0.2–0.024 <0.024Pore structure Large pore–medium-coarse

throatLarge pore–medium-coarse

throatIntermediate pore–fine

throatMicropore–microthroat

Intermediate pore–medium-coarse throat

Fine pore–fine throat

12 E&P Note

doloarenite (Figure 10B). They display good continuitywith cumulative thickness of 45–80 m (148–262 ft)(Figure 8). The reservoir spaces foundwithin the karstare diverse, including solution-enlarged intercrystalline(Figure 10A), interparticle (Figure 10B), fenestral (Figure10E) and moldic pores (Figure 10F), dissolution vugs(Figure 10C), structural fractures (Figure 10D), anddissolution fractures. Porosity has been enhanced throughdissolution and dolomitization and dominated by disso-lution vugs, dissolution pores, and dissolution fractures.

Dissolution VugsDissolution vugs are mainly in the anhydrite–halite ofthe lower Lei-4Member and in dolomitic grainstones.

The dolomitic calcarenites found in the Xinshen 1,Yashen 1, andYangshen 1wells are especially enrichedin dissolution vugs, which form a dense honeycombpattern (Figure 9). Statistical data show that thedensity of dissolution vugs in theupper part of the Lei-4 Member in the Xinshen 1 well ranges from 195 to314 vugs per meter (59 to 96 vugs per foot). All vugsare small sizes (2 · 2 to 3 · 4 mm [0.079 · 0.079 to0.118 · 0.157 in.]). They are of subround shape andpartially filled with secondary calcite.

Dissolution PoresDissolution pores are common in these reservoirs,with typical diameters of 0.05–1.0 mm (0.002–0.039

Figure 7. (A) Porosity and permeability of the Lei-4 Member intraplatform karstic shoal reservoir in the Longgang area, Sichuan Basin.(B) Porosity and permeability of the marginal platform karstic shoal reservoir in the uppermost Leikoupo Formation, in the western Sichuandepression.

He et al. 13

Figu

re8.

Fine-scalestratigraphyandrock

propertiestoevaluatethereservoirqualityintheupperm

ostLeikoupoForm

ation(Lei-4-3subm

ember),intheShiyangzhen–Jinma–Yazih

estructuralbeltofthe

westernSichuandepression(sectionlocationshow

ninFigure1,CC

9).DF=dissolvedfractures;DV=dissolvedvugs;Intercry.P=intercrysta

llinepores;Interpar.P

=interparticlepores;Interpar.RP=interparticleresidualpores;Intrapar.P=intraparticle

pores;F=fractures;FP=fenestralpores;MP=moldicp

ores;PEM

=perm

eability(m

d);POR=

porosity(%

);SE-Intercry.P

=solution-enlarged

intercrysta

llinepores;SE-Interpar.P

=solution-enlarged

interparticlepores;SE-Intrapar.P=solution-enlarged

intraparticle

pores;

SF=structuralfractures;S.PEM

=sm

allcoresampleperm

eability(md);S.POR=sm

allcoresam

pleporosity(%);T 2l 42

=MiddleTriassicLei-4-2subm

ember;T 2l 43

=MiddleTriassicLei-4-

3subm

ember.

14 E&P Note

in.) (maximum1.8mm[0.071 in.]). Solution-enlargedintercrystalline pores are mainly found in dolomite,whereas solution-enlarged interparticle pores arefound in algal dolomitic calcarenites (and to someextent in other calcarenites) (Figure 8). Some ofthese dissolution pores are filled with calcite.

FracturesBoth structural fractures and dissolution fracturesare well developed in these reservoirs, with typicaldensities of 13–26 fractures per m (4–8 fracturesper ft). Dissolution fractures are generally 0.01–1.5 mm(0.0004–0.0591 in.) wide, whereas structural fracturesare commonly 40–160 mm (1.575–6.299 in.)long, 0.2–1.2 mm (0.0079–0.0472 in.) wide, andcontinuous to semicontinuous. Some are partiallyfilled with secondary calcite.

Detailed tests were conducted on 82 core sam-ples, taken from 5wells in the western Sichuan Basin.

The results show that the reservoir porosity ranges from0.26% to 20.04%, averaging 3.66%. The samples withporosity greater than 2% have an average porosity of6.29%. The samples with a porosity less than 2% ac-count for 49% of total samples, those with a porosity of2%–5% account for 32% of samples, and those withporosity greater than or equal to 5% account for 19% ofsamples. The permeability ranges from 0.000 to 21.62md, with a peak value of 0.002–0.25 md (Figure 7B),and is strongly heterogeneous. In Figure 7B, theporosity–permeability correlation is good inmedium- tohigh-porosity intervals but poor in low-porosity in-tervals, although some samples are characterized by lowporosity and high permeability. Pore and fracture–poretype reservoirs predominate in the Leikoupo Formationgas reservoir; type II–III reservoirs are most common,with local development of type I (Table 1) reservoirs insome intervals (Figure 8).

EXPLORATION PLAYS IN THE LEIKOUPOFORMATION

Source Rocks for Leikoupo FormationGas Reservoirs

Gas source correlation analysis indicates that the gasin the Leikoupo Formation is mainly derived fromcarbonate source rocks (Figure 11A) (Wang et al.,2003; Feng et al., 2013; Liao et al., 2014) and ar-gillaceous coal-bearing source rocks (Figure 11B) (Daiet al., 1998; Feng et al., 2013) in the upper Permianunits, Middle Triassic Leikoupo Formation carbonatesource rocks (Figure 11C) (Zhang et al., 2007; Luoand Tang, 2012; Xu et al., 2013; Huang, 2014; Xie,2015; Yang, 2016), and the coal-bearing dark gray toblack argillaceous source rocks in the Upper TriassicXujiahe Formation (Figure 11D) (Wang et al., 1989;Zhou et al., 2015).

The upper Permian carbonate source rocks are0–412 m (0–1352 ft) thick, average at approximately100 m (~328 ft) in most of the Sichuan Basin (Tanget al., 2011). This interval thins (generally <50 m[<164 ft]) to the southwest part of the basin(Figure 11A) but thickens to the northeast where itis generally greater than 100m (>328 ft). These sourcerocks contain abundant fossils and organic material,dominated by sapropelic organic matter. Residual totalorganic carbon (TOC) content is generally 0.12%–2.05%

Figure 9. Reservoir characteristics and porosity types in theupper part of the Lei-4 Member, based on cores: (A) the Yashen1 well, Middle Triassic Lei-4-3 submember (T2l4

3), micriticdolomite–dolomicrite, dissolution pores and vugs, 5783.3 m(18,973.1 ft), and (B) the Yangshen 1 well, T2l4

3, stromatoliticpowder–crystalline dolomite, solution-enlarged interparticle andfenestral pores, 6200 m (20,341 ft).

He et al. 15

Figure 10. Thin sections showing reservoir characteristics and porosity types in the upper Lei-4 Member. (A) Solution-enlargedintercrystalline pores, powder–crystalline dolomite, the Qionglai 1 well, 4774 m (15,662 ft). (B) Solution-enlarged interparticlepores, doloarenite, the Xiaoshen 1 well, 5800 m (19,029 ft). (C) Dissolution vug, powder–crystalline dolomite, the Pengzhou 1 well,5817.8–5817.85 m (19,087.3–19,087.4 ft). (D) Structural fractures, dissolution vugs, and bird’s-eye micrite with limy dolomite, calcitecontent of 15%, part of which is micrite or finely crystalline calcite with dissolution pores developed into bird’s-eye structures, the Pengzhou1 well, 5824.86 m (19,110.4 ft). (E) Fenestral pores, stromatolitic texture, fine powder–crystalline dolomite, the Yangshen 1 well, 6222.12 m(20,413.8 ft). (F) Moldic pores (after anhydrite), fine powder–crystalline dolomite with anhydrite, the Yashen 1 well, 5785.56m (18,981.5 ft).

16 E&P Note

with an average of approximately 0.99% and is leastabundant in the central and southern Sichuan Basin (Liuet al., 2012). Organic matter is mainly type I–II1 (Caiet al., 2003; Huang et al., 2014), presenting goodhydrocarbon generation potential.

The coal-bearing argillaceous source interval inthe upper Permian is generally greater than 20 m(>66 ft) thick,with amaximumvalueof 130m(427 ft)in some locations (Figure 11B). These source rocks areabundantly fossiliferous and contain organic matterofmixed terrigenous andmarine origin. Residual TOCof the black mudstones and carbonaceous shales isgenerally 1.0%–5.0% and 5.0%–25%, respectively,and dominated by type III organic matter, withvitrinite reflectance (Ro) ranges from 1.2% to 2.8%(Hu et al., 2013). Moreover, the coaly interval is

approximately 1–4 m (~3–13 ft) in thickness withhigh TOC of 30.9%–73.6% (Wei et al., 2015) and Ro

of 1.9%–3.4% (Hu et al., 2013). So the upper Permiancoal-bearingmeasures are highlymature to overmaturein most parts of the basin (with the exception of areasalong the margins), and gas generation is dominant.

Xu et al. (2013), Huang (2014), and Xie (2015)found that the algae-rich lagoonal carbonates de-posited in evaporite platform environments arepromising source rock potential. The LeikoupoFormation carbonate source rock interval rangesfrom 0 to 550 m (0 to 1804 ft) in thickness, com-monly with a value of 250–350 m (820–1148 ft)(Figure 11C). It is thinnest (generally<100m[<328 ft])in the northern segment of the Longmenshan andaround the Luzhou and Kaijiang paleouplifts and

Figure 11. Distribution of source rocks contributing to the Leikoupo Formation gas reservoir in the Sichuan Basin: (A) isopach map ofupper Permian carbonate source rocks, (B) isopach map of upper Permian argillaceous (coal-bearing) source rocks, (C) isopach map ofLeikoupo Formation carbonate source rocks, and (D) isopach map of Upper Triassic Xujiahe Formation dark gray to black coal-bearingargillaceous source rock (modified from Dai et al., 2009). Formation thicknesses are in meters.

He et al. 17

almost completely eroded in part of the Luzhoupaleouplift. In general, the source rock intervaltends to thin toward its two flanks, away from thethick Qionglai–Santai–Dazhou area. Residual TOCranges from 0.02% to 1.08% but is predominantlyin the range of 0.4%–0.6% (Yang, 2016), showing alimited hydrocarbon potential.

The Upper Triassic Xujiahe Formation is a coal-bearing strata, which can be divided into six members(T3x1–T3x6). The Xu-1, Xu-2, and Xu-3 Members(T3x1, T3x3, and T3x5) contain a dark gray to blackcoal-bearing argillaceous source rock interval, withthickness of 50–1000 m (164–3281 ft) (Wang et al.,2013). In the western Sichuan Basin, this source in-terval is generally greater than 400 m (1312 ft) thick.In the central and northern Sichuan Basin, this ar-gillaceous source rock ranges from 100 to 250m (328to 820 ft) in thickness (Figure 11D). The residualTOC of the T3x1, T3x3, and T3x5 intervals rangesfrom 0.5% to 9.7%, averaging 1.96% (Figure 2), andthe kerogens in these rocks are mainly of types II and III(Dai et al., 2009). It is dominated by humic organicmatter and predominantly generates gas, although oilgeneration occurs regionally. The values of Ro rangefrom 0.72% to 2.10% (Dai et al., 2012; Hu et al.,2013), indicating thatUpperTriassic organicmatter ismature to overmature. These Upper Triassic sourcerocks and the underlying Leikoupo Formation res-ervoirs jointly constitute an upper source and lowerpreservation assemblage.

Hydrocarbon Accumulation Models forLeikoupo Formation Reservoirs

Based on the thermal maturation of source rocksand the distribution of source–reservoir–cap rock as-semblages, three hydrocarbon accumulation modelscan be established for Leikoupo Formation reservoirs.These include a long-range upward migration andaccumulationmodel with the upper Permian (P3l) assource rock and Lei-1-1 submember (T2l1

1) andLei-3Member (T2l3) as reservoir rocks; a self-sourcedmigration and accumulation model, with the Lei-1–Lei-4Members (T2l1-4) as source rocks and Lei-4-3submember (T2l4

3) as reservoir rock; and a verticaldownward migration and accumulation model, withthe Upper Triassic (T3x) as source rock and Lei-4-3submember (T2l4

3) as reservoir rock.

Model 1: Long-Range Upward Migration and Accumulation,with the Upper Permian as Source Rock and the Lei-1-1Submember and Lei-3 Member as ReservoirsIn 2012, the Penglai 18 well in the central SichuanBasin encountered a high-quality porous reservoir inthe Lei-1-1 submember and began to produce high-yield gas flow. This Moxi gas field has proven reservesof 392.15 · 108 m3 (1.3849 tcf ), and its discoveryhas great significance for future exploration of theLeikoupo Formation in the Sichuan Basin.

The Moxi structure is located at the center ofa northeast-dipping slope in the central SichuanBasin. It is a secondary anticlinal structure, which wasformedduring theLateTriassic andmainly developedduring deposition of the Middle Jurassic ShaximiaoFormation (Sun et al., 2009; Yuan et al., 2014). In thefollowing Yanshan and Himalayan periods, the areainherited the Late Triassic paleotectonic framework.Having formed prior to the generation of abundantoil and gas, the Moxi–Longnusi structural trap wasa favorable environment for hydrocarbon accumu-lation (Sun et al., 2011). In the Lei-1-1 reservoir, thegas is derived from a highly mature mixed-gas source(Wang et al., 1998); this is likely to be coal-relatedgas derived from the upper Permian (Longtan For-mation, P3l), as well as a secondary contribution fromLeikoupo Formation source rocks. The reservoir iscomposed of dolomitic shoals, which connect withgas sources via fault systems. In the adjacent Longnusistructure, no hydrocarbon accumulation was formed,because of lack of faults for hydrocarbon migration(Liu et al., 2009b). In summary, the Moxi gas field isa structural trap in a favorable facies belt (Figure 12A).

Additionally, the Zhongba gas accumulation, inthe Lei-3 Member, has an original gas–water con-tact depth of 2871m (9419 ft), a gas-bearing area of13.4 km2 (5.17 mi2), and a gas column thickness of372m (1220 ft), with proven reserves of 86.3 · 108m3

(304.8 BCF). Gas from the Lei-3 Member reservoirin the Zhongba area is highlymature (Qin et al., 2007)and mainly derived from cracked oil generated inupper Permian carbonates and mudstones (Liao et al.,2013). The reservoir is mainly composed of dolomiticshoals. During deposition of the Lei-3 Member, theZhongba area was a marginal platform environment,where algal and arenaceous shoals were widespread(Lin et al., 2007; Zeng et al., 2010). Algal debris,arenite, and fine powder–crystalline dolomite allowfor the development of high-quality reservoirs.

18 E&P Note

Postdepositional dissolution resulted in relativelyhigh porosity and permeability, and pinhole dolomitereservoirs are well developed. Reservoir space takes theform of intergranular pores, intragranular pores, in-teralgal pores, and intra-algal dissolved pores.

The Zhongba structure is located in the toe ofa nappe in the northern segment of the LongmenshanMountains (Zhu et al., 2011). Because of multiplestages of compression, thrusting, and folding, fracturesystems are well developed in these strata. Faults andfractures severed as migration pathways for gas. The

Zhongba structure formed prior to the latest stagesof the Indosinian movement (He et al., 2002), andthis inherited structural high created an anticlinaltrap favorable for hydrocarbon accumulation. TheZhangming fault cutting the southeastern limb ofthe Zhongba anticline is an important factor inhydrocarbon migration (Luo and Tang, 2012). Thispathway allows linkage between the Lei-3 Memberreservoir and Permian source rocks, both of whichare located in its hanging wall. It is a structural trapgas in a favorable facies belt (Figure 12B).

Figure 12. Stratigraphic sections of some typical Leikoupo Formation reservoirs in the Sichuan Basin. (A) Moxi gas pool, long-rangeupward migration and accumulation model, with upper Permian Longtan Formation (P3l) as the source rock and Middle Triassic Lei-1-1submember (T2l1

1) as the reservoir. (B) Zhongba gas pool, long-range upward migration and accumulation model, with P3l as the sourcerock and Middle Triassic Lei-3 Member (T2l3) as the reservoir. (C) Shiyangzhen–Jinma–Yazihe gas pool, self-sourced migration andaccumulation model, with Middle Triassic Lei-1–Lei-4 Members (T2l1-4) as the source rock and Middle Triassic Lei-4-3 submember (T2l4

3) asthe reservoir. (D) Longgang (LG) gas pool, vertical downward migration and accumulation model, with Upper Triassic Xujiahe Formation(T3x) as the source rock and T2l4

3 as the reservoir. T1j = Lower Triassic Jialingjiang Formation; T2l12 =Middle Triassic Lei-1-2 submember;

T2l2 =Middle Triassic Lei-2 Member; T2l31 =Middle Triassic Lei-3-1 submember; T2l32 =Middle Triassic Lei-3-2 submember; T2l41 =MiddleTriassic Lei-4-1 submember; T2l4

2 = Middle Triassic Lei-4-2 submember; T3m = Upper Triassic Maantang Formation.

He et al. 19

Hydrocarbon accumulation in the Moxi andZhongba gas reservoir proves to be a lower source,upper reservoir model, although nearby sourcesrocks may also contribute some hydrocarbons.

Model 2: Self-Sourced Migration and Accumulation, withthe Lei-1–Lei-4 Members as Source Rocks and Lei-4-3Submember as ReservoirThe Lei-4-3 submember gas reservoir in the westernSichuan depression is characterized by a proximalhydrocarbon supply, a fracture network facilitatingcharging, a combined shoal and karstic reservoir, anda seal in the form of terrigenous mudstones. Hydro-carbons accumulated in a stratigraphic trap. The nat-ural gas is derived from mature to highly matureproximal source rocks (Figure 13), which are likely tobe algae-rich lagoonal carbonates in theMiddle Triassicevaporite platform (Xu et al., 2013;Huang, 2014;Xie,2015). Any contribution from upper Permian sourcerocks would need fractures and/or faults for gas mi-gration. However, no large-scale faults are developedin the Shiyangzhen–Jinma–Yazihe–Xinchang struc-tural belt, and evaporates are present in theMiddle toLower Triassic. Therefore, we infer that the principalsource rocks are within the Leikoupo Formationitself.

Karstic and fractured reservoirs and marginalplatform shoal reservoirs are both developed in theupper Leikoupo Formation. The western SichuanBasin is located at the top of the karstic Leikoupoweathering crust slope, and the unconformity cap-ping the Leikoupo Formation is widespread. Depo-sition occurred in shallow water, and marginalplatform shoal grainstones were abundant; reservoirsaremainly composed of doloarenite, algal doloarenite,calcisparite, and algal and bioclastic limestone. Res-ervoir spaces are mainly secondary dissolved vugs anddissolved fractures, with minor primary pores. Thecumulative thickness of these reservoirs is 59–75 m(194–246 ft).

A fracturenetworkapproximately75km(~46.6mi)long and 20 km (12 mi) wide strikes northeastalong the top of the Leikoupo Formation in thecentral western Sichuan depression (Xu et al.,2013). The density of microfractures is 22.33fractures per km2 (57.83 fractures per mi2). Thisnetwork was formed late in the period of In-dosinian tectonism, and it extends upward into the

Xiaotangzi Formation and downward into themiddle part of the Lei-4 Member (Xu et al., 2013;Li et al., 2016). Large-scale relief exists on top ofLeikoupo Formation, forming substantial struc-tural and stratigraphic–lithologic traps; the top ofLeikoupo trap covers a total area of 1198.2 km2

(462.6 mi2).The muddy shales of the Upper Triassic

Maantang–Xiaotangzi Formation (cumulatively30–350 m [98–1148 ft] thick) act as direct cap rocksin this system. The regional indirect cap rocks arethe shales of the Upper Triassic Xujiahe Formation(the Xu-3 and Xu-5 Members) and the red shalesin the Jurassic–Cretaceous succession (cumula-tively 200–1600m [656–5249 ft] thick). Note thatthe formation water is dominantly by CaCl2 type(Zhu et al., 2011; Qin et al., 2018). Anhydrite andhalite are developed in the Middle and Lower Tri-assic. Below this evaporite layer, which is thedominant detachment, only small-scale faults arepresent. In summary, hydrocarbon accumulation inthe western Sichuan Basin proves to be the modelwhere source rocks and reservoir rocks are bythemselves. Underlying source rocks contributehydrocarbons to stratigraphically higher reservoirs,where they accumulate in stratigraphic traps(Figure 12C).

Model 3: Vertical Downward Migration and Accumulation,with the Upper Triassic as Source Rock and Lei-4-3Submember as ReservoirIn 2008, the Longgang 22 well produced high-yieldcommercial gas flow from the Lei-4-3 submember,revealing the characteristics of the gas reservoir inthe weathering crust of the Leikoupo Formation.

The carbon isotopic composition of Lei-4-3Member natural gas in the Longgang area is consistentwith oil type and mixed-source gas from highly maturesources; it is likely predominantly composed of matureto highlymaturemixed source gas,mature coalmeasureoriginated gas, and high-maturity oil type gas (Zhouet al., 2015). Gas source correlation analysis indicatesthat it is likely generatedmainly from source rocks in theUpper Triassic Xujiahe Formation (Zhou et al., 2015).Strata in the upper Lei-4Member are eroded to varyingdegrees, and the karstic weathering crust acts as a fa-vorable reservoir (Xin et al., 2012; Long et al., 2016).The natural gas flows downward along the surface

20 E&P Note

of the Indosinian depositional break and faultsystem; the composite migration pathway is com-posed of both faults and fractures. The Longgangstructure is a buried local structural high in a generallylow-relief area and forms a relatively small structural

trap (Yang et al., 2014). The overlying Xujiahe mud-stone provides good sealing capacity. Hydrocarbonaccumulation in the Longgang gas reservoir generallyproves to be the upper source, lower reservoir model.It is a stratigraphic trap, which formed because of

Figure 13. A summary diagram showing the thermal evolution of organicmatter in source rocks and hydrocarbon accumulation events at theChuanke 1 well in the western Sichuan depression. E-Q = Paleogene to Quaternary; J1 = Lower Jurassic; J1z = Lower Jurassic Ziliujing Formation;J2=Middle Jurassic; J2s=Middle Jurassic Shaximiao Formation; J3=Upper Jurassic; J3s=Upper Jurassic Suining Formation; J3p=Upper JurassicPenglaizhen Formation; K-Q= Cretaceous toQuaternary; K1= Lower Cretaceous; K2=Upper Cretaceous; Ro5 vitrinite reflectance; T15 LowerTriassic; T25Middle Triassic; T35 Upper Triassic; T3x1 =Member 1 of the Upper Triassic Xujiahe Formation (Xu-1 Member); T3x2 =Member 2of the Upper Triassic Xujiahe Formation (Xu-2 Member); T3x3 = Member 3 of the Upper Triassic Xujiahe Formation (Xu-3 Member); T3x4 =Member 4 of the Upper Triassic Xujiahe Formation (Xu-4 Member); T3x5 = Member 5 of the Upper Triassic Xujiahe Formation (Xu-5 Member).

He et al. 21

Figure 14. Classification of Leikoupo Formation (Fm.) exploration plays in the Sichuan Basin.

22 E&P Note

variation in sedimentary facies and Indosinian-ageerosion and pinchout (Figure 12D).

Exploration Plays in the Leikoupo Formation

Two types of exploration plays (each with two sub-divisions) can be identified in the Leikoupo Forma-tion of the Sichuan Basin (Figures 14–16).

Shoal Dolomite Exploration Plays

1. In intraplatform shoal dolomite exploration plays,mudstones and carbonates of the upper Permianact as source rocks (Figure 14). Reservoirs are

type II–III, composed of Lei-1 Member micrite,powder–crystalline and fine powder–crystallinelimestone and dolomite, doloarenite, calcisparite,and oolitic dolomite. The distribution of reservoirsis mainly controlled by dolomitization and earlierdissolution processes (Shen et al., 2008). Evaporiteswithin the Leikoupo Formation are the direct caprock, with the Upper Triassic Xujiahe Formationmudstones serving as an indirect cap rock, im-plying the presence of some thief zones. Anticlinaltraps are predominant, and stratigraphic traps servedas a secondary trapping type. This type of reservoircan form when a single shoal body is sufficientlythick or when multiple shoals are well connectedvertically and laterally continuous. The key to the

Figure 15. Summary map of the Leikoupo Formation shoal dolomite play in the Sichuan Basin. Structural units are labeled as follows: in thewestern Sichuan depression, I1 is the northern segment, I2 is the central segment, and I3 is the southern segment; in the central Sichuan uplift, II1 isthe northern segment, II2 is the central segment, and II3 is the southern segment; in the eastern Sichuan high and steep fold zone, III1 is thenorthern high–steep structural belt, III2 is the central high–steep structural belt, and III3 is the southern folded structural belt. Mts. = Mountains.

He et al. 23

development of these accumulations is the presenceof faults, which establish linkages between sourcerocks and reservoirs. This kind of play is mainlydeveloped in the central part of the central Sichuanuplift (Figure 15), represented by the Lei-1Membergas reservoir in theMoxi area and the Lei-3Membergas reservoir in the Guanyinchang area.

2. In marginal platform shoal dolomite explorationplays, upper Permianmudstones and carbonatesare the primary source rocks, with a secondary con-tribution from the dark shales and coal of theUpper Triassic Xujiahe Formation. The reservoiris mainly composed of algal calcarenite and finepowder–crystalline dolomite in marginal platform

shoal facies. The shales of the Upper TriassicMaantang–Xiaotangzi Formation (50–450 m[164–1476 ft] thick) serve as a cap rock. Anticlinaltraps are predominant, although stratigraphic trapsare also factors. This type of play is mainly devel-oped in the northern part of the western Sichuandepression, along its western margin, with theLei-3 Member gas reservoir in the Zhongba area asa typical example (Figure 15).

Karstic Dolomite Weathering Crust Exploration Plays

1. In intraplatform shoal karstic exploration plays,the dark mudstones of the Upper Triassic Xujiahe

Figure 16. Summary map of the Leikoupo Formation karstic dolomite play in the Sichuan Basin. Structural units are labeled as follows:in the western Sichuan depression, I1 is the northern segment, I2 is the central segment, and I3 is the southern segment; in the centralSichuan uplift, II1 is the northern segment, II2 is the central segment, and II3 is the southern segment; in the eastern Sichuan high and steepfold zone, III1 is the northern high–steep structural belt, III2 is the central high–steep structural belt, and III3 is the southern folded structuralbelt. Mts. = Mountains.

24 E&P Note

Formation serve as source rocks and seal, whereasthe karstic dolomite weathering crust of the upperLei-4 Member acts as the reservoir, controlled bythemicroscalemorphology of thepaleokarst surface.Stratigraphic traps are common, with subordinateanticlinal traps. These plays are mainly distributedthroughout the northern segment of the centralSichuan uplift, represented by the Lei-4 Membergas reservoir in the Longgang area (Figure 16).

2. In marginal platform shoal karstic explorationplays, the source rocks are the algae-rich carbon-ates rock in the Leikoupo Formation. Weatheringand karstification of marginal platform shoalgrainstones produce high-quality reservoirs, withgreat thicknesses and good continuity. Reservoirsare sealed by shales from the overlying Maantangand Xiaotangzi Formations, as well as the mud-stones of theXujiahe Formation. TheLongmenshanthrust belt, along the western margin of the basin,contains several buried structural highs (Meng et al.,2015), which along with the variety of lithofaciesleads to the development of multiple types of traps.Stratigraphic, anticlinal, anticlinal–lithologic,and fault–stratigraphic traps are present. Gasreservoirs of this type are widely distributedalong the western margin of the western Sichuandepression (Figure 16). These plays are the mostimportant targets for future exploration sincethey have the potential to result in large-scale oiland gas accumulations.

DISCUSSION

We comprehensively analyzed the spatial relation-ship between source rocks, reservoirs, and cap rocksbased on available data from exploration plays. Be-cause of the Indosinian movement at the end of theMiddle Triassic, the entire Sichuan Basin was upliftedand subaerially exposed (An, 1962; Mei, 2010). Theuppermost Leikoupo Formation was extensivelyeroded and formed a weathering crust. Reservoirs inthe western Sichuan depression are predominantlylocated in this karst slope belt (Xu et al., 2013), withcombined systems of shoal and karst reservoirs. Thisarrangement is highly favorable for the development ofhigh-quality reservoirs, and this area should be a majorexploration target in the future (Figures 15, 16).

The Leikoupo Formation and several intervals ofthe underlying marine strata and the upper XujiaheFormation have hydrocarbon generation potential.The multiple source rocks are cut and connected bythe piedmont thrust faults. However, hydrocarboncharge to the Lei-4 reservoirs appears to be mainlyfrom proximal sources such as the Lei-3 Member orthe lower Xujiahe Formation, with long-distancemigration from such units as the Permian beinga secondary factor.

Furthermore, multiple folded anticlines (relatedto subsurface faults) are found in the piedmont belt ofthe western Sichuan Basin (Tong, 1990; Li et al.,2006), with high amplitudes favorable for hydro-carbon accumulation. The mudstone of the overlyingMaantang–Xiaotangzi and Xujiahe Formations andthe interlayer anhydrite and halite of the LeikoupoFormation provide good sealing capacity. The west-ern Sichuan depression contains all of the elementsfor the formation of structural traps in favorable faciesbelts, as well as stratigraphic traps, resulting in ex-cellent exploration potential.

CONCLUSIONS

1. Two types of high-quality reservoirs were de-veloped in the Leikoupo Formation, controlled bylithologic associations, sedimentary microfacies,the karstification of paleosurfaces, and laterstructural reworking. The first type is marginalplatform (or intraplatform) shoal pore–fracturereservoirs; the second is reservoirs combininga karstic weathering crust having fracture–vugporosity with a marginal platform (or intraplat-form) shoal pore–fracture reservoir.

2. Three hydrocarbon accumulation models can beestablished for the Leikoupo Formation based onthe spatial and temporal relationship among thesource rock, reservoir, and cap rock. These includemodel 1 (long-range upward migration and accu-mulation, with P3l as the source rock and T2l1

1 andT2l3 as the reservoir rock), model 2 (self-sourcedmigration and accumulation, with T2l1-4 as thesource rock and T2l4

3 as the reservoir rock), andmodel 3 (vertical downward migration and accu-mulation, with T3x as the source rock and T2l4

3 asthe reservoir rock).

He et al. 25

3. Two types of exploration plays (with four sub-divisions) can be established in the LeikoupoFormation reservoir of the Sichuan Basin: (1)shoal dolomite plays (including intraplatformshoals and marginal platform shoals) and (2)karstic dolomite plays (including intraplatformand marginal platform shoal karst plays).

4. The Leikoupo Formation has great potential for oiland gas exploration. The karstic dolomite weath-ering crust and the marginal platform (or intra-platform) shoals of the Leikoupo Formation in thewestern Sichuan Basin are favorable explorationtargets.

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