Study On Petroleum Geological Conditions Of Lei 4~3 Sub-south-Maantang Sub-southern In Southwestern Sichuan

As one of the earliest stratas that have been explored in Sichuan Basin(one of the first places in the world to discover and utilize nature gas),Leikoupo Formation has a proven reserve of 582.93 × 108m3 and a controlled reserve of 67 × 108m3,including the typical Wolonghe,Moxi,Zhongba and Longgang gas reservoirs.The target interval of this study is from the 3rd sub-member of the 4th Member,Leikoupo Formation(referred to here as Lei43 sub-Member)to the Ma'antang sub-Member,distributed in southwest Sichuan Basin.Guided by the principle of carbonate sedimentology,petroleum geology and carbonate reservoir geology,its sedimentary facies,reservoir characteristics,hydrocarbon accumulations and the favorable zones for further exploration are investigated and analyzed in this study basing on outcrops,well data as well as associated laboratory data.Stratigraphic study shows that the top of the 4th Member,Leikoupo Formatiion is eroded and the relationship between it and the overlying Ma'antang sub-Membeer is parallel unconformity in the southwestern Sichuan area.Tianjingshan Formation is only distributed in the Longmenshan nappe tectonic belt but not developed in the inner part of the basin.The remnant of Lei43 sub-Member in the study area is thickest in the central part and thins to the northwest,southwest and southeast.Sedimentary-facies study shows that Lei43 sub-Member in the study area are dominated by beach facies,attached with some tidal plat deposits.And the sedimentary facies of the first layer of Ma'antang sub-Member are littoral facies,hybrid platform facies and organic reef facies of platform margins from east to the west of the basin.To the westmost part of the basin,the fore-platform slope and shallow shelf facies are supposed to be dominated due to the existence of Paratethys Sea.Reservoir study shows the lithology of the Lei43 sub-Member are fine-grain dolomite and oolite dolomite and that the types of reservoir space are intercrystalline pores,intergranular dissolution pores,intergranular pores,and intragranular pores.The distribution of the beach facies and the hypergene dissolution are the major controlling factors during the formation of reservoir of Lei43 sub-Member.The lithology of the first layer of Ma'antang sub-Member are arenite limestone?dolomite and dolomitic limestone,corresponding to the types of reservoir space of intergranular dissolved pores,intragranular dissolved pores,intercrystalline pores,and microcracks,which are dominantly influenced by dolomitization.Hydrocarbon-accumulation study shows that the types of gas reservoirs in Lei43 sub-Member are mainly composed of structural traps and structure-lithology traps.Near the denudation area of Lei4-3 sub-Member this interval tapers out updip,which provides the possibility of the formation of structure-strata traps.Different fault styles and stratigraphic contaction act as an important role in optimizing the links of source rock and reservoirs,which in turn forms diverse accumulation characteristics.There are several favorable accumulation model for the target gas reservoir:(1)the faults have a throw larger than the thickness of the target interval,which results in the direct contact or the communication through fault conduit of the source rock from Xujiahe Formation and the study interval;(2)the existence of the discordogenic faults that link the study interval to the underlying hydrocarbon source of Permian and Cambrian act as conduits of hydrocarbon accumulation.