The origin of the unusually high dibenzothiophene (DBT) concentrations in Lower Ordovician oils from the Tazhong Uplift, Tarim Basin was studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The most abundant sulfur compounds in the oils are S₁ species with doublebond equivalent (DBE) values of 1–19 and 11–48 carbon atoms. The range of the number of carbon atoms in the sulfur compounds detected by the FT-ICR MS (S₁ species with DBE=9) is about ten times larger than that for sulfur compounds detected by GC/MS (DBTs). This suggests that FT-ICR MS is a much better approach than GC/MS for characterization of DBTs in crude oils. The abundance of S₁ species with DBE=1–8 decreased with increasing thermal maturity, while the abundance of S₁ species with DBE=9 (primarily DBTs) increased. Therefore, thermal maturity is an important factor in the formation of oils with high DBT concentrations. Unusually high abundances of S₁ species with low DBE values (1–8), which include sulfide, thiophene and benzothiophene, were observed in several oils, especially the TZ83 (O₁) oil with high or very high thermal maturity. Thermochemical sulfate reduction (TSR) was thought to be the reason for the high abundance of these low DBE compounds in deep reservoirs, and thermochemical sulfate reduction could affect the distribution and composition of DBTs in the oils. According to the results of FT-ICR MS analysis, there are no signs that TSR is occurring or has occurred recently for most of the Lower Ordovician oils.