Controls of organic matter content on shale oil occurrence and distribution: Insights from retained oil composition and pore structure in marine shales

Total organic carbon (TOC) content is a crucial indicator in shale oil exploration due to its close correlation with shale oil content. However, the proportion of movable oil significantly decreases in high-TOC shales. Herein, eight marine shale samples from the same well, with TOC contents ranging from 1.86 % to 13.78 % and vitrinite reflectance values of 0.8–0.9 %, were analyzed to investigate the effect of TOC content on the occurrence and distribution of retained oils. Sequential extraction with various solvent mixtures revealed that extractable organic matter (EOM), as extracted by n-hexane/toluene (9:1v/v), primarily comprises saturated and aromatic hydrocarbons, which are chemically similar to the oils released by Rock-Eval pyrolysis before 300 °C (S₁ peak). Therefore, the EOM is an effective way to evaluate the free oil content in shales. By contrast, the EOMs extracted by dichloromethane/methanol (93:7 v/v) and tetrahydrofuran/acetone/methanol (50:25:25 v/v/v) are mainly composed of resins and asphaltenes that correspond to the oils released during Rock-Eval pyrolysis above 300 °C, indicating their predominant occurrence as adsorbed oil. After sequential extraction, the specific surface area and pore volume of shale samples increase by an average 369 % and 254 %, respectively. Pore structure analysis reveals that organic matter (OM) content significantly affects the occurrence space of retained oil. In the case of early oil window maturity, excessive OM can lead to a low free oil ratio, low storage space, high adsorption capacity, and high threshold pore diameter of movable oil, indicating that excessive OM is unfavorable for the enrichment of movable oil. Therefore, there may be an upper TOC limit for shale oil sweet spots. For our samples, the free oil content significantly decreases when TOC exceeds 10 wt%. This threshold may vary for different shales depending on thermal maturity, kerogen type, and pore structure. Shale dominated by Type I/II kerogen typically exhibit a lower optimal TOC threshold at the main oil generation stage (Cf. Type III kerogen), further emphasizing the importance of identifying these thresholds during exploration.