Evolution of the Jurassic sedimentary environment in northern margin of Qaidam Basin and its significance in petroleum geology

The Lower Jurassic lithology at the Dameigou section of Qaidam Basin is characterized by fine-siltstone interbedded with carbargilite and shale.The carbargilite and shale of lake-swamp facies from the braided delta plain are more developed in Members One and Two of the Dameigou Formation,and most of them contain fossil conchostracans,bivalves and plants of a quiet-water environment.The sedimentary environment is a system of braided delta and shore-shallow lake,and three middle-period cycles constitute a long period fluctuation cycle of datum.Thick sandstones and conglomerates of fluvial-alluvial facies are more developed with fossil aspects mostly of ostracodes and plants in the Middle Jurassic,and the sedimentary environment is a system of fan-delta and shore-shallow lake with semideep lacustrine facies developed in the middle and upper parts,and a long period fluctuation cycle of datum is constituted by nine middle period ones.Fossils and sedimentary facies reveal the eastward transportation of the depocenter.Structural movement and climate influence the sedimentary environment greatly,and the former is a control factor.The braided delta plain under wet climate in Early Jurassic is the important hydrocarbon-generating site,but the semiarid climate in Middle Jurassic benefit a generation of high quality source rocks.