Enrichment Mechanism Of Organic Matter And Depositional Environment Of Longmaxi Black Shale In Southern Formation Of Sichuan Basin

Longmaxi Formation of Sichuan Basin is the key areas in resources exploitation in China.As the main shale gas producing area,the study on the development mechanism of organic-rich shale in Longmaxi Formation is of great scientific significance.The Longmaxi black shale is deposited after the period of the late Ordovician ice age and rich in graptolites.To discuss controlling factors on organic-rich mechanism,from the view of the evolution of paleoclimate,paleontology and paleosedimentary environment will be helpful to further understand the deposition mechanism of black shaleIn this study,we collected black shale samples were collected from the Longmaxi Formation of the N203 well.Vertical variations of?¹³C_org and the abundances of graptolite,organic matter,pyrite,clay minerals,geochemical analyses and their relationships were analyzed in order to research the depositional environment of organic-rich shale.In situ multielement imaging of graptolites was performed by inductively coupled plasma mass spectrometry?LA-ICP-MS?to explore the mechanism of organic matter enrichment in different water environments.The major understandings are as follow:?1?The location of N203 well was shallow shelf sedimentary environment during the Early Silurian,the organic-rich layers was the lower part of Longmaxi Formation,with a total thickness of 30m.The average value of Total Organic Carbon?TOC?was more than 3%,and gradually decreased on upward.Graptolite fossils mostly preserved as a carbonaceous film,which is the characteristic of preserved graptolites in the high evolution degree.Graptolite fossils are mostly slender or short-thick shapes and preserved in disorder superimposition or aggregation.TOC has good correlation with graptolite abundances and the rich layers are the same,which indicated that the development mechanism of organic-rich shale is similar to the formation mechanism of graphite.?2?The results of geochemical analyses of N203 well suggest that the organic-rich layers of Longmaxi Formation was deposit in dysoxic-anoxic environment.TOC has good correlation with REDOX sensitive elements,which indicate dysoxic-anoxic condition is one of the most important controlling factors of the preservation of organic-rich Black Shale.High content and enrichment degree of Mo and the result of In situ multielement imaging of pyritization graptolite may indicate a sulfidic environment during the early Silurian.?3?In situ multielement imaging of graptolites shows that compared with the surrounding rock,Mg,Al,Si,Fe were enriched on the carbonaceous film surface of graptolite,with a enrichment degree ranged from 1.5 to 10.However,Sr/Ba values?1.4².3?of the carbonaceous film type graptolite were lower than that of the surrounding rocks?>5.0?.It means embedding by clay minerals is the main process of graptolite preservation.Sulfidic micro-environment caused by embedding of clay minerals benefited the pyritization of graptolite organism.The flourishing of microbial mat in early stage and sulfate-reducing bacteria in later stage consumed oxygen in the pore water and caused anoxic bottom water,should be the main reason of massive burial of graptolite and organic matter.