Oil source and migration process in oblique transfer zone of Fushan Sag, northern South China Sea

Abstract

The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the E₂l₁^L+M and the E₂l₃^U. 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The E₂l₃^U oil, characterized by high maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, shows a close genetic affinity with the E₂l₃^b source rocks, while the E₂l₁^L+M oil, characterized by lower maturity, Pr/Ph ratio and oleanane/C₃₀-hopane ratio, is suggested to be derived from the E₂l₁₊₂^b source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.

Keywords

oblique transfer zone / oil-source correlation / oil loss / oil migration and accumulation model / Fushan Sag

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