The southern gentle slope of the Zhanhua Sag is an important deep heavy oil accumulation zone in the Jiyang Depression, Bohai Bay Basin. Heavy oils are mainly developed in the lower third member (Es3L) and upper fourth member (Es4U) of the Paleogene Shahejie Formation, but their source, migration pathways, and charging history remain poorly constrained. This study integrates biomarker analysis, alkylcarbazole migration parameters, fluid inclusion petrography and homogenization temperatures, and burial history modeling to clarify the origin and accumulation mechanism of these heavy oils. The results show that extraheavy to ultraheavy oils in the Es3L reservoirs of the Sanhecun area were mainly derived from Es4U saline lacustrine gypsiferous mudstones in the Bonan subsag. In contrast, the Es3L heavy oils in the Luojia and Kenxi areas have a mixed-source origin dominated by Bonan Es3L calcareous shales, with minor contributions from Bonan Es4U and Gunan Es3L source rocks. The Es4U oils in the Luojia area represent a mixture of low-maturity oil from Bonan Es4U and mature oil from Bonan Es3L. Alkylcarbazole ratios, including 1,8-/2,7-DMC and 1,8-/1,4-DMC, reveal 15 fault-sandbody-unconformity coupled migration pathways. These pathways indicate that oils from the Bonan subsag migrated laterally and vertically into the Es3L and Es4U reservoirs in Sanhecun, Luojia, and Kenxi, whereas oils from the Gunan Es3L source mainly migrated along the Gunan Fault into the Kenxi area. Fluid inclusion homogenization temperatures show two main peaks at 75 ∼ 85 °C and 100 ∼ 115 °C, indicating two hydrocarbon charging stages. The first stage occurred during the middle to late Dongying Formation period, when Bonan Es4U source rocks expelled low-maturity oil that charged the Luojia Es4U and Sanhecun Es3L reservoirs. The second stage occurred during the Guantao–Minghuazhen Formation period, when mature oils from Bonan and Gunan Es3L source rocks charged the Luojia and Kenxi reservoirs and mixed with earlier low-maturity oils. In Sanhecun, limited later mature-oil charging and subsequent biodegradation resulted in the formation of present extraheavy to ultraheavy oils. This study establishes a source-migration-accumulation model for deep heavy oils on the southern slope of the Zhanhua Sag and provides useful guidance for heavy oil exploration in faulted lacustrine basins.
Wang et al. (Thu,) studied this question.