Study On Sedimentary Facies Of Fourth Member Of Middle Triassic Leikoupo Formation,Western Sichuan Basin

The Sichuan basin is the largest oil and gas bearing basin in South China,especially the marine carbonate reservoir occupies an important in the oil and gas exploration.However,the Leikoupo formation is the last marine carbonate formation in the Sichuan basin.The discovery of Pengzhou gas field in the western Sichuan basin achieved a major breakthrough on the exploration of deep marine carbonate rocks.For this purpose,this paper mainly focus on the study of sedimentary facies of fourth member of middle Triassic Leikoupo formation in western Sichuan Basin.On the basis of former researches,drilling and field profile observation statistics,and the identification of depositional facies marks,also,combining with the structure and sedimentary background of the middle Triassic in the upper Yangtze,this paper analyzes the characteristics of the microfacies and dolomite platform of T2l~4 in the Western Sichuan basin.First of all,it is personally accepted that the sedimentary environment of T2l~4 in the western Sichuan basin deposited in the epeiric sea through the analysis of petrology signs and sedimentary tectonics,and the dolomitization is obvious,it well developed many sedimentary facies symbols such as low-angle cross-bedding,wavy bedding,laminar algal,birds-eye structure,desiccation crack,sand ball structure,flower structure and inverse graded bedding.With the combination of the paleogeography and paleoclimate,and the special plate tectonic setting of the upper Yangtze craton by the relative high geomorphology,the sediment-tectonic model of the land surface was established.The comparison of single well/outcrop profile analysis and sedimentary facies profile,revealed the spatial overlapping relationship of sedimentary facies as well as the establishment of sedimentary sequence structure model of tidal flat island.The microfacies distribution of three different lithologic sub-members was prepared based on the three sub-members drawing units of T2l~4.It analyzed the development and distribution characteristics of sedimentary environment,also and the platform structure and sedimentary mode of T2l~4 in western Sichuan basin in detail.According to the characteristics of geomorphic unit,the sub-environment can be divided into intracratonic underwater uplift(tidal flat island),intracratonic depression(lagoon),cratonic margin uplift(tidal flat island)and cratonic margin depression,in addition,based on to the tidal action,the micro environment is able to split into the mud-rich dolomite supratidal flat,upper intertidal algal mats,lower intertidal algal dolarenite,upper subtidal crystalline dolomite and lower subtidal gypsodolomite in further step.Under common control of the low amplitude geomorpholigy and high frequency sea level change,the microfacies formed the potential and wide distributed crystalline dolomite and microbiological dolomite reservoirs.