Origin and environmental implications of dibenzothiophenes in the Huanghekou East Sag and the Laizhouwan Northeast Sag, Bohai Bay Basin

A suite of nine oils from Huanghekou East Sag and Laizhouwan Northeast Sag, Bohai Bay Basin, China, were analysed to investigate the effect of the depositional environment on dibenzothiophenes. Five oils were high-sulfur oils (HSOs, sulfur content >1%) from Huanghekou East Sag while other four oils were low-sulfur oils (LSOs). The major organic matter inputs of all oils were algae, and specific algae might contribute to the high molecular weight alkanes and C29 steranes of HSOs. HSOs mainly originated from carbonate-rich source rocks in more saline environments while LSOs derived from clay-rich source rocks deposited in freshwater conditions, as supported by a series of biomarkers, including phytane, gammacerane, terpanes and their abundance ratios. Maturity parameters show that the samples are all in the low mature to mature stage. The methylated dibenzothiophenes are more abundant in HSOs than those in LSOs. The major sources of methylated dibenzothiophenes are algae and bacteria while the dibenzothiophene is more derived from terrigenous plants, according to the carbon isotope values of individual compounds. Abundant bacterially generated H2S as well as reducing depositional environment result in transformation from algae-originated organic matter to relatively large amount of dibenzothiophenes, further, carbonate-rich source rocks might inhibit the demethylation, leading to the more abundance of methylated dibenzothiophenes in HSOs. Parameters derived from dibenzothiophenes, such as the ratios: methyldibenzothiophenes/methylphenanthrenes, methyldibenzothiophenes/phenanthrenes, dimethyldibenzothiophenes/dimethylphenanthrenes, and dimethyldibenzothiophenes/phenanthrenes are most useful for indicating reducing depositional environments among HSOs and LSOs.