The Depositional System Characteristic Of Middle Permian Maokou Formation In Northwestern Sichuan Basin

In recent years,high quality reservoirs have been found in the Middle Permian Makou Formation in northwestern Sichuan Basin,and some wells obtained industrial air flow,and made significant breakthroughs.Because of its unique palaeogeographic pattern,the Permian in Sichuan Basin has carried out a lot of research work,and a series of research achievements and knowledge have been achieved,which has greatly promoted the development of sedimentary palaeogeography and oil and gas exploration in Sichuan Basin,however,the research datas show that the study of Middle Permian Maokou Formation with huge thickness is lagging behind,it hinders the understanding of the Permian depositional pattern of the upper Yangtze Region.In view of the significance and problems of the above research,the Middle Permian Maokou Formation in northwestern Sichuan Basin is taken as the research object,based on the theory of sedimentology and lithofacies palaeogeography,combined with field geological profiles,drilling,logging,seismic and test analysis data,the characteristics of lithologic combination,strata thickness and lateral variation are studied in detail,the stratigraphic division and contrast scheme of the Middle Permian Maokou Formation in northwest Sichuan Basin is clearly defined.On the basis of stratigraphic research,sedimentary facies markers,geophysical marks and geochemical markers are identified,the characteristics of sedimentary facies,subfacies and microfacies are discussed in depth,based on the analysis of typical outcrop sedimentary facies and the analysis of well sedimentary facies,the types and distribution characteristics of sedimentary systems have been established.Comprehensive stratigraphic and sedimentary facies characteristics,segment as a unit,the lithofacies palaeogeographic map of the Middle Permian Makou Formation in northwestern Sichuan Basin is plotted by compression method and dominant facies method,and the sedimentary model is established.It is considered that the Makou Formation in northwest Sichuan Basin can be divided into 4 members,influenced by the Tung-wu movement,the top of Maokou Formation is not complete,Yanting-Yingshan-Kaijiang in the south of the research area are completely eroded.Maokou Formation is mainly composed of open platform deposits,platform margin deposits and continental shelf deposits,it belongs to carbonate platform sedimentary system of marine sedimentary system group.During the Member 1 of Maokou Formation deposition period,large scale transgression occurred,two large continental shelf deposits were developed in the study area,which laid the rudiment of Kaijiang-Liangping shelf and Deyang-wusheng shelf;on this basis,the sea level of the 2 and 3 members of Makou formation continued to decline,the scale of the shelf is obviously narrowed,this period is mainly manifested in the formation of the shoal;the 4 member of Makou Formation is eroded in the study area,and Kaijiang-LiangPing shelf is reactivated at the same time.The study reveals the palaeogeographic pattern and evolution rule of Maokou Formation in different periods.