Comparative Study Of Jurassic Stratigraphic Sequences And Sedimentary Environments In Altyn And North Qaidam

Altun and the northern margin of Qaidam have always been the key areas of geologists' research,and the study of Jurassic period is more and more in-depth,but the study of paleoenvironment has not formed a complete unified understanding.In this paper,through field investigation,geochemical testing,combined with the comprehensive study of paleontological fossil data and drilling data,the Jurassic stratigraphic sequence and sedimentary Paleoenvironment in Altun and Northern Qaidam are studied in depth,and the consistency or difference of sedimentary environment in Jurassic period in Altun and Northern Qaidam are discussed.The Jurassic developed in the Altun area is the first set of large area sedimentary caprock after Paleozoic tectonic evolution,which exists in the form of fault block mountain.The Jurassic system in the northern margin of Qaidam Basin is widely distributed with continuous sequence,especially in the daciangou area.It is a good base for comprehensive study of Geology and geochemistry.Firstly,according to the comparative study of rock combination characteristics,rock particle size analysis,paleontology combination and other technical methods,it is considered that the rock types in the two areas are relatively similar,most of them are river,Delta and lake facies sedimentary systems,and the sediments are mainly sandstone and mudstone.Particle size analysis shows that the sedimentary particles are mainly transported by rolling and jumping,with a small amount of suspended transportation.The sorting degree is medium.Secondly,the sedimentary paleoenvironment of Jurassic system is analyzed by the method of combining petro geochemical analysis and organic geochemistry,and the geochemical parameters such as Sr/Ba,V/(V+Ni),Mn/Fe,Mg/Ca,C/N and ?13C are used as the discrimination basis.The sedimentary water body of the caishiling formation of the late Jurassic in the Altun area is brackish semi brackish water.The paleosalinity changes in a downward upward cycle,and the depth of the water body is deepening in the whole period.Paleoredox analysis indicates oxidation environment and paleoclimate is characterized by dry heat.In the early Middle Jurassic,the characteristics of paleosalinity and paleobathymetry in the northern margin of Qaidam changed frequently,showing multiple cycles of increase and decrease,and the sedimentary environment was mainly brackish water semi brackish water facies.The paleoclimate features alternate between temperature,humidity and dry heat.In the late Jurassic,the sedimentary environment is still brackish semi salty water,but the salinity of the sedimentary water is large,the depth of the water is gradually deepened,the sedimentary environment is in the condition of oxidation,and the paleoclimate is characterized by dry and hot.Finally,through the analysis of the organic geochemical characteristics of the source rocks,combined with the organic matter abundance,organic matter maturity and other indicators,the hydrocarbon generation potential of the Jurassic source rocks in the two areas is preliminarily evaluated.The analysis shows that the abundance of organic matter in Altun area is low,but the maturity is high,which is over mature stage;the abundance of organic matter in the northern margin of Qaidam area is medium,and the maturity of organic matter is low,which is in the stage of oil generation.