Abstract

Shale oil exploration is a hot topic of global oil and gas exploration. China has great potential for shale oil resources. At present, the discovered shale oil resources in China generally exist in lacustrine shale sediments with salt interlayer. The geochemical characteristics of shales formed in salt water environment (saline-shales) and shales formed in fresh water environment (freshwater-shales) is vital for the shale oil exploration in China, and is useful for the evaluation of hydrocarbon potential in saline sediments.

The s saline-shales have higher organic matters enrichments than the freshwater-shales. Meanwhile, the hydrocarbon potential in freshwater-shales is higher, while saline-shales have higher shale oil potential in the saline-shales. The high thermal conductivity of salt sediments facilitate the hydrocarbon generation. The organic matters in freshwater-shales are mainly I/II₁ types and dominated by sapropelic substance, dominantly origin from the aquatic organism. While, the organic matters in saline-sahles are dominated sapropelinite and liptinite with II₁/II₂ types, which derive from the mix source of aquatic algae and terrestrial higher plants. The biomarkers shows that the organic matters in saline-shales deposited under strong reducing environment, while freshwater-shales were under generally reductive to weak oxidation environment. The depositional environment of shale sediments was affected by the climate. The Nanpu depression had higher the mean annual temperature (MAT) and mean annual precipitation (MAP) than Dongpu depression during middle Eocene. The warm and humid climate in Nanpu depression facilitated the weathering of parent intermediate igneous rocks, and led to the runoff and enrichment of elements during middle Eocene. The cooler and drier condition Dongpu depression led more weathering of felsic igneous provenance. The difference of provenance between Nanpu and Dongpu depression mainly were affected by the tectonic background. The Nanpu depression had active continental margin tectonic setting, and Dongpu depression was occanic island margin tectonic setting during middle Eocene.