The Geochemical Characteristics Of Source Rock In The Middle Jurassic Yangye Formation Of The Kashi Sag,southwestern Tarim Basin

The Kashi sag,which is one of the secondary tectonic units,is located in the south-western depression,and the area is about 2.65 x10⁴km².It developed three sets of hydrocarbon source rocks including the Carboniferous,Permian and Jurassic,especially the source rock in Middle Jurassic Yangye Formation,which is one of the most favorable source rocks.So in this paper,it regarded the source rock of Yangye Formation as the research object,and a comprehensive study about the geochemical characteristics was conducted.The paper was guided by the theory of sedimentology,organic and inorganic geochemistry.A-nd basic on the source rock samples from the outcrop section and well drilling data and the organic geochemical analysis results,a system and comprehensive study which included the OM(organic matter)abundance,type,maturity,biomarker compounds,sedimentary and the main factors was conducted.The results suggested:First,the organic geochemical characteristics of the source rock showed that the source rock in which the OM abundance was higher,the type of OM was mainly type III,and it had entered high-mature stage,was in the good level in the Tuyon Basin;And the source rock of the Wuqia region which had the highest OM abundance and its type of OM was mainly type II₁,II₂,III,and it had entered low mature to mature stage was in the middle to good level,even the good to very good level;And the source rock in the front of the West Kunlun Mountains was in the non to bad level,and its OM abundance was lower,the type of OM was mainly type III,and it had entered mature to high mature;And the source rock had a higher OM abundance,and its type of OM was mainly type II₁?II₂?III,and it also had entered low mature to high mature,which suggested the middle to good level in the Yingjisha region.The potential of the source rock in the Yangye Formation was ranked Wuqia region,then Yingjisha region,Tuyon Basin and West Kunlun Mountains in different regions;And it was ranked Yangye which was the highest profile,and then the Yingdi well-1,Yundi well-1,Kuzigongsu,Kushanhe,Tongyouluke,and then Yigeziya,Qiemogan,Gaizhihe in different profiles.Second,the characteristics of the biomarker compounds suggested that the parameters of the biomarker compound were effective;And the characteristics which included the alkanes distribution patterns,the main carbon of the alkanes,the weight ratio,the Pr/Ph ratio and the gammacerane index showed synthetically that the source rock was formed in the environment which was in weak reductibility to oxidizability and fresh to brackish water,and the OM of the source rock was inputted by the lower aquatic organisms and terrigenous higher plants.The maturity parameters,such as the OEP,C₃₁22S/(S+R),indicated that the source rock had entered the mature stage at least.Third,the characteristics of the element geochemistry which includes the trace and rare element showed that the source rock were controlled by the active and passive continental margin and the continetal island arc,and its parent rock were from the Calcium sedimentary rocks,alkaline basalt and granite.The source rock in the Yangye Formation were formed in these environment which was the semi-arid to semi-humid climate,and the fresh to brackish water,and the weak reductibility to oxidizability.It was in favor of the development of the biological and provided the favorable transforming conditions for the peat and sapropel.The source rock in Yangye Formation were together controlled by the palaeotectonic background,the ancient climate,the salinity,the primary productivity and the preservation conditions and so on.