Reservoir Characteristics And Exploration Prospect Of Triassic Leikoupo Formation-Ma’antang Formation In The Central And Northern Parts Of Western Sichuan Basin

Middle Triassic Leikoupo Formation-Upper Triassic Ma’antang Formationwere the last layers of marine carbonate evolutionary phase in the superimposedSichuan Basin. From the frontal zone of Longmenshan to West Sichuan depressionand then to central Sichuan Basin, the lost strata of Middle Triassic become moreand more. Ma’antang Fm. was sequential deposition in the frontal zone ofLongmenshan and its western areas, but pinched out gradually towards centralSichuan Basin. The burial of this strata set is relatively shallower, and its drillingcost is lower than the deeper stratas, but its exploration degree is also much lower.Among all the drilling wells in western Sichuan, only CK1well and LS1welldrilled meeting Ma’antang Fm. and across Leikoupo Fm.. Only Zhongba gas fieldhas been discovered in Leikoupo Fm. in western Sichuan, CK1well and XS1welldiscovered active industrial gas in Ma’antang Fm..Middle Triassic in western Sichuan Basin can be divided into five segmentsfrom bottom to the top, dolostone is the main rock types of T2l1-T2l4in central andnorthern parts of western Sichuan, limestone is the main rock types of T2l5,Ma’antang Fm. only developed in western Sichuan, its lithofacies are not stablealong Longmenshan because influenced by deposition and tectonic action, it is adeposition set of mudstones and limestones.Restricted platform was the main sedimentary facies among Leikoupo stage inthe central and northern parts of western Sichuan, local T2l3and T2l5were openplatform. From Ma’antang stage, Sichuan Basin came into the ending phase ofmarine carbonate rocks, it was shallow marine facies, marine carbonate rockstransformed into marine clastic rocks gradually, Ma’antang depositon ended finally because of the entire uplift of Longmenshan.Dolomitization and dissolution were the main constructive diagenesis of MiddleTriassic-Ma’antang Fm. in the central and northern parts of western Sichuan. Fourtypes of dissolution have been distinguished, syndiagenetic-early diagenetic stagemeteoric dissolution, epidiagenetic dissolution, shallow burial stage hydrothermaldissolution and middle-deep burial stage dissolution, the first two types were morenormal in northern parts, the after two types were more normal in central parts.Dolomitization developed normally in Leikoupo Fm., but rare in Tianjingshan (T2l5)and Ma’antang limestones. Sedimentary facies and diagenesis types developed orchanged locally, so the reservoirs growed differently in central and northern parts ofwestern Sichuan.Reservoir in the northern parts is mainly located in the third segments ofLeikoupo Fm.(T2l3), the reservoir rocks are chiefly diverse grain dolostones, mainreservoir spaces are inter-and intra-grain dissolution pores, the reservoir is mediumporosity and medium-low permeability. T2l3of LS1well in central parts developedfew pore-fracture type reservoirs which were low porosity but higher permeability.T2l3of CK1well developed few pore type reservoirs which were low porosity andpermeability. The outcrop of Ma’antang Fm. are non-reservoir–low deliverabilityreservoir. The porosity and permeability of Ma’antang Fm. in CK1well are goodlocally, the reservoir rocks are chiefly calcarenites, the main reservoir spaces areintergrain pores, the reservoir is fracture-pore type.The reservoirs of Leikoupo Fm. developed better in northern parts, which aremainly carbonate platform bank facies dissolution types dolostones reservoirs, and itsmain controlled factors were sedimentary facies and diagenesis, few paleo-carstreserveoirs developed and its main controlled factors was paleo-tectonic activities. Inthe central parts, the reservoirs of Leikoupo Fm. developed relatively poor, there isonly few hydrothermal dolostones reservoirs developed in LS1well which controlledmainly by activities of extentional fault. Ma’antang Fm. of CK1well developedshallow marine bank facies limestones intergrain pores types reservoirs, and its maincontrolled factors was sedimentary facies.The source rocks and seal rocks developed well in the central parts of westernSichuan, the preservative and migration conditions are also good, so on the base ofintegrated study of outcrop geology and drilling wells geology, through thecomprehensive research of geology, geophysical and geochemical approaches, thethesis proposed the best developed area of Middle Triassic-Ma’antang Fm. bank facies reservoirs are located in the northern region of Wenxing area–the easternregion of Xinchang area and the nearby region of Majing area, where is the bestexploration prospect area and may obtain oil-gas discovery in the mode of UpperTriassic source rock+the reservoir rocks in the third-fourth segment of LeikoupoFormation and in Upper Triassic Ma’antang (reef) bank facies strata+the mudstoneseal in the Upper Triassic．...