Geochemical Characteristics And Sedimentary Environment Of Triassic Yanchang Formation(Ch-7 Member)Oil Shale At Southeast Of Ordos Basin,China.

Triassic Yanchang Fm(Ch-7)oil shale is widely distributed in Ordos Basin,while Tongchuan is one of the typical outcrop areas of distribution.The objective of this study is to determine the level of maturity of source rock,kerogene type,on the based of rock-eval pyrolysis analysis,and charactristics of Organic content of study area.On the basis of inorganic element contents,the depositional environment and some tectonic setting in which the Oil shale of the study area is deposited,is deterenined.Ten sample were analysed for Rock-Eval pyrolysis.The plots,TOC vs S2(mg HC/g TOC)and Tmax vs Hydrogen Index(HI),indicated that the kerogene type in the study area is type-?(most concentration).On the basis of graph between TOC and HI indicated that the Oil Shale of the study areea is a good oil source rock.Sixteen selected samples were analyzed by using ICP-MS at the Northwest Institute for Non-ferrous Metal Research for the abundances of some trace elements contents such as V,Ni,Cr,Ba,Sr,U and Th and REE(Rare Earth Elements)(REE)contents like La,Yb,Eu and Ce.Paleo-redox indices such as Th/U,V/(V+Ni),V/Cr and ?U indicate that the depositional environment of Yanchang Formation is reducing environment.Paleosalinity is determined on the basis of Boron(B),Illite,Montmorillonite and kaolinite contents and the results show that the study area belongs to brackish to semi saline water sedimentary environment.The negative values of Ceanomaly and Euanomaly indicate that the study area belongs to hydrothermal sedimentary environment in redox situation.Petrological evidence of source rocks and positive anomalies of S,Cu,Mo and U also supported the existence of hydrothermal activity at the bottom of the lake.All the values of Th/U are less than 1;it means the deposition of the study area occurred under hot water.La/Yb-REE and La/Yb-Ce/Yb graphs show that mostly samples fall into basalt and other into overlapping areas.So in the expansion period of Ch-7,the basement fracture activities are responsible of hot fluids at the basement of the lake.In the study area sedimentary period of Ch-7,hydrothermal activities in the lake bottom exist and played a key role in accelerating the extensive development of oil shale.The positive correlation between La and Th(r = 0.76)indicate that,the behavior of these two elements remained cooperatively throughout the depositional process,so the predictions on basis of La and Th plot is more reliable.According to La and Th plot the study area belongs to Continental Island Arc.