Source rock palaeoenvironments and controls on the distribution of dibenzothiophenes in lacustrine crude oils, Bohai Bay Basin, eastern China

Abstract

The applicability of sulphur aromatic parameters, based on ratios of dibenzothiophene (DBT) and its alkylated homologues, as indicators of oil maturity has been investigated in 27 oils from the Damintun and Zhuanhua Depressions, Bohai Bay Basin. The oils were characterised by bulk composition and environmentally-sensitive aliphatic biomarkers and were derived from a wide range of lacustrine source depositional environments ranging from highly reducing hypersaline (sulphur-rich oils) to terrestrial input-dominated freshwater (very high wax oils) with maturities ranging from immature to probable mid-mature. Dibenzothiophene-based maturity parameters are not well-correlated with sterane isomerization ratios, and immature oils, in particular, could not be discriminated by these aromatic maturity indicators. In contrast, DBT-based parameters are strongly-correlated with pristane to phytane and Ts/Tm ratios. High methyldibenzothiophene ratios (MDR) are associated with fresh water source deposition whilst low MDRs are found in oils of hypersaline source affinity. Unusually high relative concentrations of 2+3-MDBT were observed in some of the immature oils inferred to be sourced by lacustrine organic laminites. The data provide further evidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils and that source depositional environments can exert a dominant influence when their range of variation is large. Absence of DBT in some biodegraded oils is tentatively attributed to its aqueous solubility during associated water washing.