Sedimentary Characteristics And Filling Sequence Of Doushantuo Formation,key Phase Areas In The Periphery Of Sichuan Basin

Doushantuo Formation in the middle upper Yangtze area is the first set of cap rocks in the interglacial period of the stable craton depression.It is the target bed for shale gas exploration and the hydrocarbon supply section of Dengying Formation.Its sedimentary filling evolution is of great significance to basic geology and production research.In this paper,by using the methods of field geological observation,microscopic observation and description,we have classified the lithofacies,analyzed the sedimentary facies and divided the sequence units of Doushantuo Formation in Xiaoyang section of Northeast Sichuan depression,Yangjiaping section of Western Hubei platform and Huanglianba section of southeast margin of Yangtze plate,and established the sedimentary filling evolution model of each sequence unit in the study area,The conclusions are as follows:(1)There are seven lithofacies in Zhenba Xiaoyang section: turbidite facies,tempestite facies,shelf gravity flow lithofacies,intermittent sandy laminar mudstone lithofacies,shallow water mixed lithofacies,deep water mixed lithofacies and suspended mudstone lithofacies,indicating slope,gentle slope,deep water shelf facies,shallow water shelf facies,mixed tidal flat facies and mixed shelf facies;The Yangjiaping section can be divided into six lithofacies: granular limestone,granular marlstone,laminar mud powder crystal dolomite limestone,storm mud powder crystal limestone,marlstone and siliceous limestone,indicating shelf,subtidal flat and subtidal beach facies deposition;The Huanglianba section can be divided into three types of lithofacies: dark laminar micritic limestone,micro scoured micritic limestone/dolomite and silty mudstone,indicating shelf facies deposition.(2)There are three complete sequences SQ1,SQ2,SQ3 and transgressive system tract tst4 of sq4 in Doushantuo Formation of middle upper Yangtze area.Sequence SQ1 in Xiaoyang section includes progradational turbidite facies of lowstand system tract,retrograded low-density turbidite facies of transgressive system tract,and aggradated gentle slope mudstone deposition of highstand system tract;In Yangjiaping section,transgressive system tract tidal flat shelf facies and highstand system tract tidal flat facies are developed;Huanglianba section includes transgressive system tract and highstand system tract shelf facies.In sequence SQ2,turbidite of lowstand system tract,gentle slope facies of transgressive system tract and deep water shelf facies of highstand system tract are developed in Xiaoyang section;In Yangjiaping section,transgressive system tract shelf facies and highstand system tract subtidal sediments are developed;Transgressive system tract and highstand system tract shelf facies are developed in Huanglianba section.In SQ3,deep shelf facies of transgressive system tract and shallow shelf facies of highstand system tract are developed in Zhenba Xiaoyang section;Transgressive system tract and highstand system tract are developed in Yangjiaping section;There are transgressive system tract and highstand system tract clastic shelf facies in Huanglianba section.In sq4,the mixed tidal flat mixed shelf facies of transgressive system tract is developed in Xiaoyang section of Zhenba;Subtidal flat deposits of transgressive system tract are developed in Yangjiaping section;The shelf facies of transgressive system tract is developed in Huanglianba section.(3)The west side of Northeast Sichuan depression has a steep slope and is close to the provenance of Hannan paleocontinent.It has experienced the filling evolution model of "slope filling stable shelf deposition shallow water platform rudimentary formation".The east side of the depression has a gentle slope,and the filling model is characterized by relatively stable carbonate gentle slope;The western Hubei trough western Hubei platform experienced an evolution process of "shelf sedimentary filling gentle slope platform";The passive continental margin in the southeastern margin of the Yangtze basin is characterized by stable sedimentary filling.