Geochemical signatures of the source rocks from the Liushagang Formation in the Wushi depression,Beibuwan Basin

Based on the analysis of organic geochemical data of source rocks, drilling and seismic data, the present paper focuses on the geochemical signatures of the source rocks from the Liushagang Formation in the Wushi depression, Beibuwan Basin, including organic matter abundance, type, maturity, parent rock source, sedimentary environment and biomarkers. The analytical results show that the third member of the Liushagang Formation was deposited in a reduced lake environment with higher organic matter abundances and organic matter types of II₁-II₂. The second member of the Liushagang Formation was deposited in a slightly reduced to reduced environment with the maximum organic matter abundances and organic matter types of II₁-II₂, indicating the excellent source rocks of the oil shales in this member. The first member of the Liushagang Formation was deposited in an oxidation-prone shallow lake environment with minimum organic matter abundances and organic matter types of II₂-III. According to the thermal evolutionary profile of the source rocks, four stages for the thermal evolutionary processes are recognized, including immature, low mature, mature and high mature stages. The threshold depth is 2900 m for the generation of hydrocarbons, and maximum depth is 3600 m for the generation peak of hydrocarbons in the source rocks. The low gammacerane indices have reflected the freshwater environments for the deposition of the Liushagang Formation. The low aquatic plants made greater contributions to the sources of the parent rocks indicated by abundant 4-methyl steranes.