Organic Matter Enrichment Environmental Analysis On Shale Of Longmaxi Formation In East Sichuan Wuling Mountain Area

Eastern Sichuan Wuling Mountain area developed a series of thick organic-rich dark shale in paleozoic era,especially the shale in the Lower Silurian Longmaxi Formation.The shale in the Longmaxi Formation in the Eastern Sichuan Wuling Mountain area is characterized by good organic matter type and high maturity.However,the shale lacks studies on organic sources,types and controlling factors,and the study on the sedimentary environment model of organic rich shales is weak.Therefore,this paper aims to use organic and inorganic geochemical methods to study the enrichment of organic matter in the shale of the Longmaxi Formation in the Eastern Sichuan Wuling Mountain area,and to establish a model for the enrichment of organic matter in the study area.On the basis of petrology,the study of sedimentary characteristics lays the foundation for the establishment of organic matter enrichment model.The lithology of the Eastern Sichuan Wuling Mountain area is dominated by dark mud shale,with siltstone and muddy siltstone sandwiched locally.There are three sedimentary facies including gentle slope,deep sea shelf and shallow sea shelf,and six sedimentary subfacies including deep gentle slope,shallow gentle slope,muddy shelf,sand-shale shelf,sandy shelf and storm current in the study area.The deep sea shelf facies is the dominant sedimentary facies of dark shale development.In order to analysis the vertical distribution of organic matter content,a sequence stratigraphic framework was established in the Niujiaojian section and the Hongyanxi section,and 2 third sequences and 4 fourth sequences were divided respectively in Longmaxi Formation.In the evolution of the section,the content of organic matter in the transgression period increased obviously,and the organic matter content reached the local high value during transgression period of the MSC2 sequences and MSC3sequences in the deep sea shelf facies.The key to the analysis of organic matter enrichment environment is to explore the source and evolution characteristics of organic matter.The organic matter sources of the shale in the Longmaxi Formation in the Eastern Sichuan Wuling Mountain area were algae and tiny amounts of protozoa,photosynthetic bacteria and other biological input through the analysis of relationship between bitumen A and TOC,the characteristics of the group components,the n-alkanes characteristics,Pr/nC17 and Ph/nC18 characteristics,steranes and terpanes characteristics.Moreover,the organic matter types of the shale in the Longmaxi Formation in the eastern Sichuan Wuling Mountain area were sapropel,humic-sapropel types and very few sapropel-humic types through the analysis of microscopic composition characteristics,kerogen?13C,HI,Tmax and other parameters.Finally,the paleogeographic evolution characteristics of the study area were determined by using the correlativity among the trace elements,rare earth elements,CIA(chemical weathering index)and organic matter.The characteristics of aerial gravity and magnetic data and ICV(compositional variability index)are used to confirm the paleo-tectonic of the study area is relatively stable.The palaeoclimate in the study area is determined to be warm and humid by using the parameters of Sr/Cu,?REE,(La/Yb)_N and CIA.The paleoproductivity evolution characteristics of the study area were determined by using the correlativity among Baxs,P element characteristics and organic matter.The paleo-sedimentation rate evolution characteristics of the study area were determined by using the correlativity between(La/Yb)_N and organic matter.Using the correlation among Pr/Ph,V/(V+Ni)and organic matter to determine the ancient water environment as the main control factor of organic matter enrichment.