Research on lower limit of TOC content for hydrocarbon expulsion and shale oil enrichment using carbon mass balance method: a case study of shale of lower submember of the third member of Paleogene Shahejie Formation in Jiyang Depression

Currently, research methods for determining the lower limit of total organic carbon (TOC) content for hydrocarbon expulsion from source rocks and for shale oil enrichment vary, and the determined lower limit values show significant differences. To accurately obtain the relevant lower limits, an organic carbon classification method based on the chemical composition of kerogen was established, dividing organic carbon into generative organic carbon (valid carbon) and non-generative organic carbon (void carbon). Valid carbon referred to the carbon in kerogen containing degradable hydrocarbon groups, while void carbon referred to the carbon in kerogen without degradable hydrocarbon groups. The contents of void carbon and valid carbon could be calculated using a quantitative relationship model between pyrolysis hydrocarbon yield (S2) and TOC content. Based on TOC content testing, pyrolysis analysis, and hydrocarbon generation and expulsion simulation experiment data of mud shale in the lower submember of the third member of the Paleogene Shahejie Formation (hereinafter referred to as lower Es3 member) in the Jiyang Depression, the lower limits of TOC content at different stages of thermal evolution were calculated. The results showed that: (1) The initial void carbon content of the source rocks in the study area was about 0.83%, and it increased with increasing thermal evolution degree. (2) The lower limits of TOC content for hydrocarbon expulsion from source rocks and for shale oil enrichment both decreased with increasing evolution degree. At Ro=0.5%, the lower limits of original TOC (TOCO) and residual TOC (TOCR) contents for hydrocarbon expulsion were 2.39% and 2.20%, respectively, and the lower limits of TOCO and TOCR contents for shale oil enrichment were 2.39% and 2.20%, respectively. At Ro=1.0%, the lower limits of TOCO and TOCR contents for expulsion were 0.99% and 0.91%, respectively, and the lower limits of TOCO and TOCR contents for enriched shale oil were 1.18% and 1.01%, respectively. The research method fully considers the composition and distribution characteristics of hydrocarbon-generating organic matter and emphasizes the important influence of void carbon content on the lower limit of TOC content for hydrocarbon expulsion and shale oil enrichment. The obtained lower limit values are more reasonable and can be applied to the evaluation of the effectiveness of hydrocarbon expulsion and the possibility of shale oil enrichment in source rocks with low TOC content.