Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin: Part 4. Evidence for new exploration horizons in a maturely explored petroleum province

Significant oil reserves have been discovered recently in the Ek1 member of the Paleocene–Eocene Kongdian Formation and the Ordovician “buried hill” reservoirs in the basement of the Niuzhuang South Slope (NSS), Dongying Depression, Bohai Bay Basin. The physical properties and chemical compositions of these oils are different from those in the overlying Eocene–Oligocene Shahejie Formation (Es), displaying high wax (29–32%), low sulfur (0.04–0.25%), and high sterane isomerization ratios [0.46–0.52 for C29-ααα sterane 20S/(20S + 20R)]. In contrast, the Es reservoired oils have relatively low wax but high sulfur contents, with the 20S/(20S + 20R) ratio for C29-ααα steranes in the range of 0.33–0.39. Oil-source correlation results show that the Es reservoired oils were derived from source rocks in the Es4 member of the Shahejie Formation within the conventional oil window, whereas the dominance of C29 steranes observed in oils from the Ek1 and Ordovician reservoirs suggests a contribution from different source(s) for these oils within the deep Es4 interval and the Ek2 member of the Kongdian Formation. The recent commercial oil discovery in the Ek1 and older strata and the confirmation of the presence of organic-rich dark mudstones in the Ek2 member of the Kongdian Formation are significant for further petroleum exploration in the deep strata of the Dongying Depression and other parts of the Bohai Bay Basin.