Hydrocarbon generating potential and accumulation contribution of the fourth member of the Shahejie Formation in the Liaodong Bay sub-basin, Bohai Bay basin

Abstract

The Liaodong Bay sub-basin is one of the most petroliferous sub-basins in the Bohai Bay basin. The fourth member of the Eocene Shahejie Formation (E2s4, 50.5–42 Ma) which is an important source rock interval in other sub-basins was rarely studied in the Liaodong Bay sub-basin. The hydrocarbon generating potential, biomarker assemblage and accumulation contribution for discovered oil fields of the E2s4 source rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E2s4 source rocks in the Liaodong Bay sub-basin display relatively high total organic carbon contents (TOC) and Rock-Eval hydrogen indices (HI) and can be considered as a set of excellent oil-prone tendency source rocks. The source rocks interval is characterized mainly by relatively low Pristane/Phytane (Pr/Ph), C19/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic terpane/C26 tricyclic terpane (C24 Tet/C26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC29 steranes ratio (4-MS/ΣC29 ST), C35 22S/C34 22S hopane (C35/C34 SH) and high gammacerane/αβC30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C28 TT + C29 TT)/(C28 TT + C29 TT + Ts)] and were deposited in anoxic to sub-oxic saline-water environment with no or minor terrigenous organic matter and medium dinoflagellate input. The maturity of E2s4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E2s4 source rocks in the south and middle Liaoxi sag displayed opportune maturity within the oil window and were proved by oil-source correlation having important accumulation contribution for discovered oil fields in this area. The E2s4-derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.