Maturity thresholds for continental shale oil enrichment based on thermal simulation experiments: a case study of the second member of Funing Formation in Gaoyou Sag, Subei Basin

Continental shale oil is a crucial strategic successor resource for hydrocarbon in China, but it generally faces challenges such as complex geological conditions, strong reservoir heterogeneity, and difficulties in "sweet spot" prediction, constraining its effective development. The second member of Funing Formation (E₁f₂) in the Gaoyou Sag, Subei Basin has substantial shale oil resource potential, but the controlling effect of maturity on its enrichment mechanism remains unclear, making it difficult to support the optimal selection of precise target areas in exploration. Typical low-maturity organic-rich shale samples from the E₁f₂ in Gaoyou Sag were selected. Using high-temperature and high-pressure semi-confined thermal simulation experiments, the evolutionary processes of hydrocarbon generation potential, pore structure, and oil mobility of shale under different maturity levels (R_o) were systematically reconstructed, revealing the controlling mechanisms of shale oil enrichment. The experimental results showed that the hydrocarbon generation process exhibited a typical unimodal evolutionary pattern, with the principal oil window located within R_o=0.75%-1.10%. The peak oil yield reached 25.8 mg/g at R_o=1.10%, contributing approximately 78% of the total liquid hydrocarbons. Correspondingly, the hydrocarbon expulsion efficiency (η) increased significantly to 70.80%, indicating highly efficient hydrocarbon expulsion characteristics. Pore evolution analysis showed that the R_o interval of 0.75%-1.10% was the optimal window for pore development, evidenced by a maximum pore volume of 0.097 cm³/g at R_o=1.10% in thermal simulation and an average peak porosity of 5.8% in geological profiles with mesopores and macropores accounting for over 95% of the total pore volume. Oil mobility enhanced markedly with increasing maturity, as indicated by a decrease in asphaltene content from >80 mg/g to < 50 mg/g and a significant increase in movable hydrocarbon content (S₁) within the R_o range of 0.80%-0.90%. It is comprehensively determined that R_o=0.75%-1.10% (especially 0.90%-1.10%) represents the "optimal maturity window" for shale oil enrichment in the E₁f₂ of Gaoyou Sag. Within this window, shale oil possesses sufficient oil sources, excellent storage space, and high hydrocarbon mobility, providing key quantitative evidence and theoretical support for sweet spot prediction and exploration decision-making of continental shale oil.