Differential enrichment of middle-low maturity lacustrine shale oil in the late Eocene Shahejie Formation, Bohai Bay Basin

The Tertiary lacustrine shales in China display middle-low maturity but have significant resources potential for shale oil compared to North American marine shales. The accumulation mechanism of hydrocarbons in shales of middle-low maturity is of importance for the evaluation of shale oil resources and exploration. This study employs a comprehensive range of geological and geochemical methods to analyze the aggregation characteristics of hydrocarbons in the middle-low maturity lacustrine shales of the late Eocene first member of the Shahejie Formation (EOS shales) in the Bohai Bay Basin. Based on this analysis, the study proposes an aggregation pattern for hydrocarbons specifically in middle-low maturity lacustrine shales. The EOS shales are dominated by felsic minerals followed by clay and carbonate, and enriched Type II1 and Type II2 organic matter with an average TOC content of 2.03%. The presence of halite exhibits a positive correlation with the organic matter content, suggesting its important role in the preservation and concentration of organic material. The content of ankerite is negatively correlated with Delta S1 and positively correlated with OSI, indicating that ankerite promotes hydrocarbon enrichment. The free hydrocarbon index( Delta S1)was used to evaluate the hydrocarbon migration in EOS shales, and show differential enrichment in vertical profile as verified by multiple indicators. The mineral, lithology and lithofacies affect the enrichment of hydrocarbons. In the differential enrichment model, the hydrocarbons of Group I enriched from outside, and the hydrocarbons of Group II were self-generated, while the hydrocarbons of Group III expelled outward. This study clarifies the differential enrichment characteristics of hydrocarbons in middle-low maturity lacustrine shales, and proposed the enrichment model, which provides powerful support for the in-depth exploration and effective development of lacustrine shale oil.