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Crude oil has been discovered in the Paleogene and Neogene units of the Weixinan Sag in the Beibu Gulf Basin. To determine the source and accumulation mode of this crude oil, 12 crude oil samples and 27 source rock samples were collected and an extensive organic geochemical analysis was conducted on them. Based on the geological conditions and the analytical results, the types, origins and accumulation patterns of crude oil in the study area were elucidated. Except for a shallowly‐buried and biodegraded crude oil deposit in Neogene rocks, the crude oil samples in the study area were normal. All of the crude oils were derived from lacustrine source rocks. According to biomarker compositions, the crude oils could be divided into two families, A and B, distinctions that were reinforced by differences in carbon isotope composition and spatial distribution. Oil‐source correlation analysis based on biomarkers revealed that Family A oils were derived from the mature oil shale at the bottom of the second member of the Liushagang Formation, while the Family B oils formed in the mature shale of the Liushagang Formation. The Family A oils, generated by oil shale, mainly migrated laterally along sand bodies and were then redistributed in adjacent reservoirs above and below the oil shale layer, as well as in shallow layers at high structural positions, occupying a wide distribution range. The Family B oils were generated by other shale units before migrating vertically along faults to form reservoirs nearby, resulting in a narrow distribution range. [ABSTRACT FROM AUTHOR]