Origin of petroleum accumulation in the Chaheji-gaojiapu structural belt of the Baxian Sag, Bohai Bay Basin, China: Insights from biomarker and geological analyses

The origin of petroleum accumulations in the Chaheji-gaojiapu structural belt of the Baxian Sag, Bohai Bay Basin was evaluated based on the results of Rock-Eval pyrolysis on 135 samples and a biomarker analysis on 26 source rock samples and 33 oil samples. The four possible source rock intervals have different biomarker assemblages and were deposited in different environments. For example, pristane/phytane, C-27-diasterane/C-27-sterane and C-19/C-23-tricyclic terpane ratios show an increasing trend, but (C-28 + C-29)-tricyclic terpane/[(C-28 + C-29)-tricyclic terpane + Ts] and steranes/hopanes parameters display a decreasing trend among the lower part of the first member (Es-1(L)) and the middle (Es-3(M)) and lower part (Es-3(L)) of the third member of the Eocene Shahejie Formation. This finding indicates that a decreasing anoxic and alkaline environment with an increasing input of terrigenous organic matter developed during the deposition of the Es-1(L), Es-3(M) and Es-3(L) source rocks. However, the biomarker parameters of the forth member (Es-4) of the Eocene Shahejie Formation display highly similar characteristics with the Es-3(L) source rocks, except for the C-29-sterane 20S/(20S + 20R) and C-27-diasterane/C-27-sterane ratios, which show obviously higher than the other source rock values. The Es-3-derived oils are absolutely dominant in the Chaheji-gaojipu structural belt, based on the biomarker analysis of saturated hydrocarbon, with slight or no contributions from the Es-1(L) and Es-4 source rocks because of the low maturation for the Es-1(L) source rock and the absence of vertical migrating channels for the Es-4 source rock. Faults are vital to petroleum migration and accumulation in this area and demonstrate the following double effects: (1) the deep faults cutting off the Es-3 source rocks are the key channels for oil vertical migration; (2) the shallow faults entrap oil and finally form the commercial oil pools. As a result, the fault block reservoir is the major oil-reservoir type in the Chaheji-gaojiapu structural zone, and the lithologic reservoir is another oil-reservoir type in this area. Based on 1D burial and thermal history modelling, the E-3 and E-4 source rocks reached the oil-generation peak at approximately 5 Ma and 20 Ma, respectively, but the Es-1(L) source rock is still in the low-mature stage. Therefore, the process of the large oil migration and accumulation for the Es-3 source rocks occurred during the late Neogene-Quaternary. Through this research, there are two considerable exploration potentials for this area, including condensate reservoirs in the deep strata and low-mature reservoirs in the Es-1 stratum interval.