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Abstract: According to observation of cores and thin sections, as well as analyses of fluid pressure features and geochemical indexes, the cause of fluid overpressure and the mechanism of shale oil enrichment in the Lucaogou Formation, Malang Sag, Santanghu Basin, were discussed. The results show that the shale oil of the Lucaogou Formation is generated in the low mature–premature stage of the source rocks and has the features of high density and high viscosity. The low-permeability and thick source rocks lead to high start-up resistance of fluid, which made it difficult for oil and gas to expel, so the retainment of oil and gas in source rocks is the major cause of formation overpressure. Excellent generation potential and well developed reservoir space are the basis for the oil retention; the pressure produced by hydrocarbon generation is not high enough to overcome the resistance of oil migration, which leads to the enrichment of shale oil. The enrichment degree of shale oil is controlled by hydrocarbon generation and storage capacity of lithofacies at various evolution stages. [Copyright &y& Elsevier]