Source rock characteristics and depositional environment of Lower Sinian Doushantuo Formation in northeastern Sichuan and western Hubei

The dark shale of the Doushantuo Formation is an important source rock of the Neoproterozoic Sinian in northeastern Sichuan and western Hubei. A series of outcrop profiles in this area at Fujiagou, Chenjiawan and Huajipo were observed and sampled. The characteristics and depositional environment of source rocks of the Doushantuo Formation were identified by means of organic geochemical analysis, organic matter type and major-trace element analysis. The source rocks of the Doushantuo Formation are 50-250 m thick, with maximums of Zhenba-Chengkou and Yichang-Wufeng. The abundance of organic matter is relatively high, averaging 2%. The hydrocarbon-generating organic matter is diverse, but mainly phytoplankton. Source rocks have good organic matter types, mainly type I-II₁. Organic matter is in the stage of overmature thermal evolution and generates gas. In this study, the depositional environment of hydrocarbon source rocks was analyzed by using element geochemical indices which show that the source rocks of the Doushantuo Formation were formed in a suboxic to anoxic environment, but differences exist in different areas. Source rocks in the northeastern Sichuan were deposited in a deep water shelf environment with relatively stable energy, low salinity and suboxic to anoxic conditions. Source rocks in the western Hubei were formed in a platform inner sag environment with high energy, low salinity and anoxic conditions.