Sedimentary Environment Of Lower Longmaxi Formation In Sichuan Basin And Periphery

Organic-rich mudstones in Wufeng-Longmaxi Formation,spreading widely on Sichuan basin and periphery,are paid main attention on in the development of shale gas industry.The sedimentary environment and factors that influenced the organic matter accumulation affected the pattern of organic matter accumulation and formation of shale gas that of Longmaxi Formation can favor further exploration and development of shale gas in Sichuan basin and periphery.According to preceding works,the lower Longmaxi Formation is significant and stable shale gas reservoir in Sichuan basin and periphery.This paper sets lower Longmaxi Formation as study focus,and based on paleogeography characteristics,Sichuan basin and periphery are divided into three sedimentary regions—the north region,the middle region and the south region.What’s more,the lower Longmaxi Formation is separated into two parts—the Unit1 and Unit2,which helps the studies of sedimenry environment and main organic matter enrichment factors.Wuxi1-well is selected to sample and researches is carried out about whole rock geochemistry,total organic carbon content and minerals to established profiles of the minerals contents,geochemical,and paleoenvironment proxies(detrital inputs proxies:Al contents and Ti/Al values;paleoproductivity proxies: Siex contents,P contents and(Ni+Cu+Zn)/Al values;redox proxies: U/Th,V/Cr,Ni/Co,V/(V+Ni),TOC/P,Mo[EF]and U[EF])that indicate the Unit2 of north basin has more clay minerals,same degree of redox and lower degree of watermass restriction.U-Pb ages and Hf isotope characteristic of volcanic rocks from bottom Unit2 of Bailu section,Wuxi region,imply volcanic activities in north sedimentary region led to high contents of clay minerals,weaker upwellings and anoxic watermass of this region during Aeronian.Combining with geological features of sections such as Daheba in Nanchuan,Moshigou in Leibo,Pengye1-well,Longye1-well and so on,the minerals contents and proxies of detrital inputs,paleoproductivity and redox reflect there are different sedimentary patterns for Unit1 and Unit2: during Unit1 interval,watermass in three regions was anoxic-euxinic and upwelling center was at the middle region;during Unit2 interval,with falling of sea level,watermass in south region became oxic and water oxygenation was proceeding northwardly,and the upwelling center moved to the north region accompanying with lower restriction degree of watermass.The main machanisms of organic matter accumulation were affected by differences of sedimentary environment.In north region,main mechanisms were redox condition for Unit1 and redox condition and detrital inputs for Unit2;in middle region,they were redox condition and paleoproductivity for Unit1 and redox conditio for Unit2;in south regoin,they were paleoproductivity for Unit1 and redox condition for Unit2.