Effects of Saline–Alkaline Conditions on Hydrocarbon Expulsion Efficiency and Chemical Composition Characteristics of Expulsed Oil

Alkaline lacustrine environments, due to their unique coupling mechanism of sedimentation, diagenesis, and hydrocarbon generation, have a significant influence on the generation and migration efficiencies of shale oil. However, the microscopic mechanism by which alkaline fluids regulate the primary migration of hydrocarbons remains unclear. Taking the shale of the Lucaogou Formation in the Junggar Basin as an example, this study integrates extraction experiments, total organic carbon (TOC) determination, gas chromatography–mass spectrometry (GC-MS) analysis, and molecular dynamics simulation to reveal the influence of saline–alkaline solutions on the total amount of hydrocarbons expelled from shale, as well as to systematically investigate the controlling mechanism of saline–alkaline media on the selective expulsion of hydrocarbon molecules from shale. The results show that compared with pure water extraction, shale TOC decreased by 6.78% and 15.00% after salt and alkali extraction, respectively, indicating that salt and alkali can promote hydrocarbon expulsion. The nC20−/nC20+ values of n-alkanes extracted from pure water, salt, and alkali were 0.42, 2.34, and 2.30, respectively, indicating that salt alkali has different effects on the excretion of large- and small-molecule alkanes, with small-molecule alkanes being more sensitive. Molecular simulation further confirms that weakly adsorbed nC15 molecules are more prone to desorption after adding alkali, while nC23 adsorption is almost unaffected, indicating that alkali can promote hydrocarbon expulsion by affecting hydrocarbon adsorption. This mechanism provides a new theoretical explanation for the oil expulsion efficiency of high-quality source rocks and the enrichment patterns of shale oil in alkaline lacustrine environments, and it has important guiding significance for the selection of shale oil exploration targets and the optimization of fracturing methods.