川东北地区陡山沱组页岩地球化学特征与古环境分析OA北大核心CSTPCD

The shale in Doushantuo Formation is the main source rock of the Ediacaran in Sichuan Basin and the major hydrocarbon source layer of the ancient marine strata for natural gas exploration in recent years.Forty-six samples were collected from two outcrops(Baibaoxi and Shiping)in Chengkou county in northeastern Sichuan Basin.We analyzed the change of the sedimentary environment of the Member Ⅱ and Ⅳ of Doushantuo Formation in terms of their organic geochemical characteristics(total organic carbon,rock pyrolysis,kerogen carbon isotopes)and element geochemical characteristics(major elements,trace elements and rare earth elements).Results show that the shale of Doushantuo Formation in northeastern Sichuan Basin exhibit a high abundance of organic matter,with TOC content ranging from 0.19％to 20.38％(mean value is 4.76％).The dominant type of organic matter is type Ⅰ kero-gen,and it has reached over-maturity stage.High 8Eu anomalies and abnormally richness in V,Mn,Mo,Ba and U elements of the black shale revealed strong hydrothermal action during this period.The distri-bution pattern of rare earth elements and various redox indexes indicated that the redox fluctuates re-peatedly under the conditions of anaerobic-anoxic-sulfidic environment during the depositional period of the Member Ⅱ.The water environment of Member Ⅳ changed from anoxic to anaerobic-sulfidic environ-ment.In general,the water connectivity of the basin is strong,the depositional rate is relative high.The nutrients carried by upwelling from hydrothermal sources proliferate productivity,which promotes the formation of organic shale.