The type, origin and preservation of organic matter of the fine-grain sediments in Triassic Yanhe Profile, Ordos Basin, and their relation to paleoenvironment condition

The Chang 7 oil layer in Yanhe Profile develops comprehensive fine-grained sediments, showing great significance to hydrocarbon exploration. Based on nineteen samples, organic elements, maceral identification, vitrinite reflectance (Ro), biomarkers and inorganic elements were tested. The results show that the fine-grain sediments have relatively high clay minerals and are mainly silicate and clay source rocks, showing good-excellent source rock quality at the bottom with the decreasing quality towards the top. The rock pyrolysis, maceral identification and Ro imply that the organic matter (OM) type is Type II with plankton and bacterials as the main organic origin, entering maturity stage. Biomarkers, major and trace elements manifest that the fine-grain sediments deposited in weak oxidizing-weak reducing environment with fresh water. The diagrams of OEP-CPI and C29steraneββ(ββ+αα)-αααC29sterane 20S(20R + 20S) also reflect the maturity stage of organic matter. The Al/Si ratios are relatively big, implying the clay minerals content is high. This is in accordance with XRD analyses. Trace elements results demonstrate the high paleoproductivity, fresh water, and cold and dry weather. The increasing salinity, anoxic condition, and warm and humid climate are in favor of OM enrichment and preservation. This contribution provides systematic geologic foundation for hydrocarbon exploration and development of fine-grained sediments in Ordos Basin.