The formation mechanism of high dibenzothiophene series concentration in Paleozoic crude oils from Tazhong area, Tarim Basin, China

Dibenzothiophene series is one of the most important compositions in crude oil, which generated under multiple geological and geochemical processes. The relationships between dibenzothiophene series and other biomarkers (C-29 alpha alpha sterane 20R, and C-28 triaromatic sterane 20R), combined with the research of thermochemical sulfate reduction (TSR), biodegradation, geological ages, and oil source in Paleozoic oil in Tazhong area, indicated that there are three control factors for high concentration of dibenzothiophene series. First, both middle-upper Ordovician and lower Ordovician-Cambrian source rocks are marine carbonate sedimentary rocks which could produce abundant organic sulfur compounds including dibenzothiophene series. Second, biodegradation could cause the enrichment of organic sulfur compounds. In addition, sulfate reducing bacteria was able to transfer hydrocarbons and S in SO42- in oil-bearing reservoir water into organic sulfur compound. It might be the main path to produce dibenzothiophene series. Third, sulfur compounds might have been formed by TSR, because S of SO42- in oil-bearing reservoir water could also be transferred to H2S and organic sulfur compounds under high temperature, which might result in increase of dibenzothiophene series in crude oil.