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 (E 2 s 4 , 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 E 2 s 4 source rock interval in the Liaodong Bay sub-basin were studied using geological and geochemical data. The E 2 s 4 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), C 19 /C 23 tricyclic terpane (C 19 /C 23  TT), C 24 tetracyclic terpane/C 26 tricyclic terpane (C 24 Tet/C 26 TT), low to medium hopane/sterane (H/S), medium 4-methyl steranes/ΣC 29 steranes ratio (4-MS/ΣC 29 ST), C 35 22S/C 34 22S hopane (C 35 /C 34 SH) and high gammacerane/αβC 30 hopane (G/H), extended tricyclic terpane ratio [ETR=(C 28 TT + C 29 TT)/(C 28 TT + C 29 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 E 2 s 4 source rocks in the Liaozhong and north Liaoxi sags is too high to provide oils for fields. The E 2 s 4 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 E 2 s 4 -derived oils should be taken seriously in the future exploration in the south and middle Liaoxi sag and on adjacent uplifts.