The formation environment and developmental models of argillaceous dolomite in the Xingouzui Formation, the Jianghan Basin

Abstract

Shale oil resources are found in the argillaceous dolomite in the Xingouzui Formation of the Jianghan Basin, revealing a new field for petroleum exploration. Studying the environment in which argillaceous dolomite forms reveals details regarding the formation and distribution of shale oil in the basin. By combining a detailed analysis of the basic geological data from petroleum exploration with methods based on organic petrology and geochemistry, the formation environment and developmental models of argillaceous dolomite in the Yajiao–Xingou Uplift and Chentuokou Sag are discussed. The results are as follows: the average TOC values in the argillaceous dolomite exceed 1.0% in the Yajiao–Xingou Uplift and Chentuokou Sag, reaching better source rock standards. The type of organic matter of the argillaceous dolomite from the Chentuokou Sag is obviously better than that in the Yajiao–Xingou Uplift. The maturity of organic matter in the source rocks is lower. Tricyclic terpanes, pregnane and homopregnane, which originated from algae, are detected in the argillaceous dolomite in the Chentuokou Sag, while these compounds are not found in Yajiao–Xingou Uplift. In addition, the alginite and sporinite are respectively detected in the two areas described above. Therefore, the lake productivity is higher in the Chentuokou Sag relative to the Yajiao–Xingou Uplift. The Pr/Ph, Fe2+/Fe3+ and gammacerane/C30 hopane data reveal that argillaceous dolomite developed in an anoxic environment with better water column stratification. The anoxic environment is the major factor controlling the formation of argillaceous dolomite. The central belt of the Chentuokou Sag was where the lake productivity was higher and preservation conditions of organic matter were excellent, making it the most favorable zone for forming argillaceous dolomite that tends to generate oil.