Shale oil occurrence state and its controlling factors in saline lacustrine basins: Insights from sequential solvent extraction

Due to the complexity of the lithofacies associated with shale oil in saline lacustrine basins, the differences of shale oil occurrence state and its controlling factors in different lithofacies are not completely clear. This hinders efficient shale oil exploration and development. We investigated the shale oil in the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, China, based mainly on sequential solvent extraction, petrological, organic geochemical, and nuclear magnetic resonance techniques. The fluidity of extractable organic matter decreased from the first extract to the fourth extract, which was caused by the gradual decrease in the contents of saturated and aromatic hydrocarbons, and gradual increase in the contents of NSO compounds and asphaltenes. The contents of free hydrocarbons (the first and second extracts) and adsorbed hydrocarbons (the third and fourth extracts) are very different among the lithofacies. The free hydrocarbon ratios in the siltstones and carbonate rocks are >70% and the main pore throats are >1 μm in size, corresponding to the best sweet spots. The contents of free hydrocarbons in the laminated silty mudstones and shales with bedding fractures are >50%, which are also available. The free hydrocarbons in the siltstones and carbonates are saturated with migrated hydrocarbons, with the contents being more affected by the physical properties of rocks. In contrast, the free hydrocarbons in the mudstones are mainly self-generated and -stored, and their contents are controlled by total organic carbon contents and maturity. For the adsorbed hydrocarbons, the contents in all lithofacies are controlled mainly by the total organic carbon contents. The biomarker parameters record a slight increase in maturity from the fourth to the first extract. The ∑C_22–/∑C₂₃₊ values of n-alkanes in the third extract are the lowest, because carbonate minerals tend to capture long-chain n-alkanes. The sequential solvent extraction method provides new insights into the occurrence state and molecular geochemical characteristics of lacustrine shale oil in different lithofacies. Future exploration should focus on siltstones and carbonate rocks with a relatively high proportion of the first extract (free hydrocarbons occurring in large pores and fractures), and mudstones with a relatively high proportion of the second extract (free to adsorbed hydrocarbons occurring in matrix pores) that are suitable for hydraulic fracturing to aid production.