Abstract:

Unusually high dibenzothiophenes (DBTs) oils were detected in the Tazhong Uplift, Tarim Basin, with the maximum absolute abundance up to 26,859 μg/g and relative abundance among the aromatic hydrocarbons recognizable up to 58.2%. The oil is usually situated in the Lower Ordovician in the No1 Fault Belt and the TZ4 and TZ1-6 wellblocks. An integrated geochemical approach was utilized in characterizing the oils. The results show that the type of source rock has an significant controlling on the distribution and abundance of DBTs. Normal DBTs were detected in pure dark mudstones and shales, whereas super high DBTs were observed in the source rocks with high content of sulphate. An increasing trend of the DBTs content was observed for the source rock and the related oils within normal oil window in the Dongying Depression. However, in the case of the Tazhong oils with high thermal maturity, there is an increasing and then decreasing trend with a breakpoint around 5,500 m, suggesting strong controlling of thermal evolution. It was also observed that biodegradation and waterwashing have an effect on DBTs in a varying degree, with an decreasing trend of DBTs concentration with increasing biodegradation. But it is unlikely that this kind of secondary alteration is one of the reasons responsible for the abnormal oils in the Tazhong region. It was detected that there is a positive relation between DBTs, H2S content (in associated gas), mercaptan and long chain alkylthiolanes in the high DBTs oils, which is recognized as productions of TSR. We believe there are several factors having an effect on the DBTs in the Tazhong oils. Thermal chemical alteration and TSR are suggested to be the essential factors responsible for the high DBTs in partial Lower Ordovician oils, and little possibility of a kind of special source rocks have caused the abnormal oils, although it could not be excluded at present. As to the oils in the Carboniferous in the TZ4 wellblock etc, we suggest that mixing of later charged deep oils with high DBT concentrations with earlier oils in situ is the most likely mechanism responsible for the abnormal DBT concentrations.

Key words: Tazhong Uplift, dibenzothiophenes, sourcerock type, thermal chemical alteration, thermochemical sulphate reduction (TSR), biodegradation