The Main Controlling Factors And Depotional Model Of Organic Matter Accumulation In The Wufeng–Longmaxi Formations In The Sichuan Basin

There was significant changes in the biological,climatic and environmental aspects of the earth from Late Ordovician to Early Silurian,and many major geological events occurred during this period.During the Ordovician – Silurian period,the Sichuan basin and its peripheral area deposited theWufeng formation and Longmaxi formation,the shale was the highest layer in the organic carbon content which located in upper Wufeng formation and lower formation the black shale of the rich organic matter,shale gas content in the same peak.Therefore,these two sets of Marine hydrocarbon source rocks are the key strata of shale gas exploration and development in China.But there is still larger issues at the main controlling factors and depositional model of the organic matter enrichment,in order to solve this problem,this article the organic carbon and elemental chemical analysis were carried out in the Tianba section and Qiliao section of Sichuan basin,discussed the sedimentary environment of rich organic matter layer of shale from upper Wufeng formation to lower Longmaxi formation(terrigenous input,REDOX conditions and productivity),obtained the following results:(1)The shale could be divided into four different layers according to the organic carbon content(TOC).The lower Wufeng formation was low organic matter layer,the sedimentary environment was oxidized-oxygen deficient environment;The upper Wufeng formation was high organic matter layer,the sedimentary environment was anoxic environment;The Guanyinqiao layer was low organic matter layer,the sedimentary environment was oxidized-oxygen deficient environment;The lower Longmaxi formation was high organic matter layer,the sedimentary environment was anoxic environment.The ancient productivity indicators(P,Babio and Sibio)indicated that there was highly productive during the sedimentary periods of the Wufeng formation-Longmaxi formation.(2)The accumulation of organic matter is not a single model in the lower shale of the Wufeng formation-Longmaxi formation in Sichuan basin,and in the same shale layer could have different sedimentary patterns in different regions.The sea level was in a slow descent stage in Wufeng formation early sedimentary stage,but ascending ocean current resulted in a oxygen enrichment environment,that was bad for organic matter preservation.The sea level was at its lowest level in Wufeng formation late sedimentary stage,and the ancient uplift acted as a barrie,the water's intersection weakened between basin and ocean,so an anoxia environment appeared in seabed,so enrichment of organic matter increased.In Guanyinqiao layer sedimentary stage,the sea level was in a slow descent stage,and because Qiliao section was located in deep water depression of eastern Sichuan,water's intersection became weaker between basin and ocean,So the water was more anoxic than the Tianba section,that was better for organic matter preservation.When the shale deposited in lower Longmaxi formation.The global sea level rose rapidly,The Sichuan basin became a semi-stagnant sea basin,Tianba section was located in the north margin of the Yangtze plate,it contacted with the ocean directly,the sea level rise leaded to an anoxia appeared on the bottom of sea.Qiliao section was located in the inland of eastern Sichuan,the ancient uplift blocked the water's effective communication between basin and ocean,some water was still stuck in Qiliao Section during this period,some nutrients are recycled,organic matter was better preserved in this anoxia condition.