Geochemical Characteristics and Hydrocarbon Expulsion Efficiency of Different Types of Shale: Taking Chang 7 Member and Shanxi Formation in Ordos Basin, China, as Examples

With the global expansion of shale oil and gas exploration, analyzing the geochemical characteristics of different types of shale has seized extensive attention. Taking the Chang 7 Member and Shanxi Formation in the Ordos Basin as examples, this study investigated the geochemical and hydrocarbon expulsion characteristics of different types of shale. The results show that the Chang 7 shale is mainly type I. The total organic carbon (TOC) distribution of the Chang 7 shale in the study area was between 0.72% and 27.5%, with an average of 7.46%, and the vitrinite reflectance (Ro) ranged from 0.45% to 1.28%. It is a good source rock in immature to mature stages. The Shanxi shale is mainly type III. The TOC distribution of the Shanxi shale was between 0.6% and 58.5%, with an average of 8.92%, and the Ro distribution was between 0.42% and 2.39%, indicating good immaturity over mature source rock. The average hydrocarbon expulsion efficiencies of the Chang 7 shale calculated using the hydrocarbon generation potential method and the original hydrocarbon generation potential method were 82.49% and 94.81%, respectively. Simultaneously, a correlation analysis of hydrocarbon expulsion efficiency with organic matter abundance, maturity, and type found that when TOC was less than 5%, hydrocarbon expulsion efficiency increased with increased organic matter abundance and maturity. When the TOC exceeds 5%, the hydrocarbon expulsion efficiency is mainly related to maturity. The hydrocarbon expulsion efficiency of the sapropelic shale was much higher than that of the humic shale. This study is significant for evaluating shale gas and shale oil resources in similar areas.