Sulfur rich petroleum derived from lacustrine carbonate source rocks in Bohai Bay Basin, East China

Crude oils produced from the Zhanhua depression, Bohai Bay Basin are high in sulfur and differ from most lacustrine oils in this basin and elsewhere. Physical properties, biomarker ratios and aromatic hydrocarbon assemblages reveal that they are derived from carbonate source rock horizons. Representative source rock samples with abundant organic matter (TOC ⩾ 1% and S2 ⩾ 5 mg HC/g rock) were selected for further correlation, and their high bitumen contents indicated their indigenousness to saline or carbonate depositional environments. The results of oil–source rock correlation suggest that these sulfur rich heavy oils originate from Es3–4 carbonates and marls limited to the Bonan sag. Maturity assessments with burial history showed that they were expelled at the early oil generation window. High levels of 20S-5α,14α,17α- and 5α,14β,17β-steranes, together with low values for the Ts/(Ts + Tm) ratio and two methylated aromatic ratios, MDR′ and MPI 1, were observed in these oils. Sulfur, associated with evaporite and carbonate environments, is a significant factor not only in controlling the occurrence of biomarker isomers but also for petroleum formation at abnormally low thermal stress levels.