The Study On Black Shale Mircofacies Of Upper Ordovician And Lower Silurian In South Sichuan Basin

Fine- grained sediments(grains smaller than 62.5um)also commonly named as shale or mudstone are the most widely distributed sedimentary rocks,which record a large amount of information of earth history and contain huge oil and gas resource. They have become the frontier of sedimentary research,but much less work has been done on them compared with other sedimentary rocks.The shale gas exploration in the south part of Sichuan Basin has achieved significant breakthrough,but sedimentology and sequence stratigraphy is still elusive. So the black shale in Upper Ordovician to Lower Silurian,i.e.,Wufeng-Longmaxi Formation in South Sichuan Basin is studied in this thesis to gain insight into microfacies and sequence stratigraphy.First,make detailed description of sedimentary characteristics including total organic carbon(TOC) and mineral content and sedimentary phenomenon,etc through observation in different scale from core to thin section,X-ray diffraction and TOC test. Then classify fine-grained sediments based on their sedimentary characteristics. TOC is considered first,and with the amount of 2wt.% two lithofacies associations can be defined: organic rich and organic poor. Then according to sedimentary pheno menon,organic poor lithofacies association can be further divided into current ripple,wave ripple,bioturbation and biodetritus four secondary lithofacies associations,while organic rich lithofacies association can be divided into massive and current ripple two secondary lithofacies associations. Finally, divide secondary associations in to siliceous,calcareous,argillaceous and hybrid with a criterion that whether a single kind of mineral is more than 50% of rock or not,like organic poor current ripple siliceous fine-grained sediment.With a TOC criterion of 2%,shelf facies in research area can be divided into shallow shelf(TOC<2wt.%) and deep shelf(TOC>2wt.%) two subfacies. Based on various water dynamic and organism activity condition,shallow shelf can be further divided into wave affected microfacies,bottom current affected microfacies,bottom community microfacies and bioturbation microfacies,while deep shelf can be divided into still water microfacies and bottom current affected microfacies.Research interval can be divided in to four third-order sequence based on the increasing-decreasing trend of TOC,combined with vertical lithofacies change and assisted by GR and Th/U curve. Wufeng Formation can be regarded as a third-order sequence(SQ1) including three systems tracts: lowstand systems tract(LST),transgressive systems tract(TST) and highstand systems tract(HST). In the middle of basin,LST is not obvious. The research interval of Lonmaxi Formation can be divided into three third-order sequence(SQ2,SQ3,SQ4),and each sequence contains TST and HST two systems tracts.Lastly,analyze sedimentary evolution under sequence stratigraphy framework. During SQ1 period research area is dominated by deep shelf still water microfacies. In SQ2,deep shelf bottom current affected microfacies is prevail. Sea level decreases in SQ3 and SQ4,and most area is covered by shallow shelf bottom current affected microfacies,although deep shelf bottom current affected microfacies exists in the north part.