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The Fushan Depression is one of the prolific hydrocarbon‐bearing depressions in the Beibuwan Basin, South China Sea. In this study, a total of 133 mudstone samples, 18 well‐logging curves, and 12 seismic sections were analysed to investigate the geochemical features and spatial distribution of effective source rocks of the Palaeogene Liushagang Formation within a sequence stratigraphic geochemical framework. The results suggest that depositional centres of mudstones northward migrated from the central Huangtong Sag during the Liushagang period. Good hydrocarbon generation potential source rocks containing Type II kerogen occur in the transgressive systems tract (TST)–highstand systems tract (HST) of lower Liushagang sequence (SQls3), the middle Liushagang sequence (SQls2), and the lowstand systems tract (LST)–TST of upper Liushagang sequence (SQls1). The occurrence of effective source rock kitchens, which are mainly controlled by the thermal maturity is distributed in the strip‐shaped area of SQls3 sequence, the whole Huangtong Sag of SQls2 sequence, and the central and northern part of Huangtong Sag of SQls1 sequence, respectively. Fewer hydrocarbon accumulations were found in the southern slope due to the relatively deeper hydrocarbon generation threshold depth and to the limited occurrence of effective source rock kitchens.

The Paleogene sandstone reservoir in the Huangtong Sag of the Fushan Depression is one of the most commercially prolific oil accumulations in the Beibuwan Basin, South China Sea. In this study, twenty-seven crude oil samples from four oilfields in the sag were geochemically analyzed and classified into three oil groups. Oils in the Meitai and Hongguang oilfields show similar geochemical characteristics, with relatively abundant C30 4α-methyl-24-ethylcholestanes and low contents of oleanane and C19+20 tricyclic terpanes (TT), and therefore may be derived from the same source kitchen. Oils from the Yong’an oilfield have a greater proportion of oleanane and C19+20 TT compounds. Oils from the Chaoyang oilfield have intermediate contents of these biomarkers and proportionate values of other related parameters. The unimodal distribution pattern of homogenization temperatures of fluid inclusions found in all the Paleogene reservoirs indicates a single episode of charging. Combining this understanding with the reconstruction of stratigraphy-burial and geothermal histories by 1-D basin modeling, the petroleum charging time was determined to be between 8–2 Ma (the end of the middle Miocene to the early Pliocene). Hydrocarbon migration orientation and charging pathways were traced using molecular indicators (4-/1-methyldibenzothiophene and 1-/4-methodibenzofuran). In the Chaoyang oilfield, the values of these geochemical parameters decreased with decreasing burial depth, indicating vertical oil migration along faults, which are plentiful in the sag. Sandstone bodies with deeper burial depths may therefore be the most promising exploration targets in the Chaoyang area. The Yong’an oils generally migrate from the northwest toward the southeast, suggesting that the source kitchen for the Yong’an oilfield is in the northwest. By similar inference, the hydrocarbon source kitchen for the Hongguang-Meitai area is likely to be on the north side of the Hongguang-Meitai area. By tracing these hydrocarbon charging pathways, the northwest of the Yong’an area and the northern part of the Hongguang-Meitai area are identified as the most promising areas for exploration.

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