Geochemical characteristics of crude oils from the Tarim Basin by Fourier transform Ion cyclotron resonance mass spectrometry

Nine marine and two terrestrial oils from the Tarim Basin in Western China were analyzed by Fourier transform ion cyclotron resonance mass spectrometry. Sulfur compounds with 8-47 carbon atoms and double-bond equivalent (DBE) values of 0-21 are abundant in the crude oils. The most abundant sulfur species in Tazhong marine oils are SI species (80.57-85.22%), followed by O1S1 (6.95-14.78%) and S-2(0.71-6.69%) species. The dominant species in Yingmaili terrestrial oils are S-1 (51.41-52.76%), O1S1 (26.83-35.27%) and O2S1 (11.97-21.76%) species; no S-2 species were detected. The results suggest that the sulfur compounds present in oil vary with the oil type. For the S-1 and S-2 species, as the thermal maturity increased, the degree of condensation increased, and the median and range of the number of carbon atoms decreased. Compounds with DBE values of 9, which are most likely dibenzothiophenes, became concentrated as the thermal maturity increased. Therefore, the unusually high abundance of dibenzothiophenes in the Lower Ordovician oils could be related to the thermal maturity. The TZ83 (O-1) oil has an abnormal distribution of Si species, and is characterized by sulfur species with relatively low DBE values (0-7). This abnormal distribution could be caused by thermochemical sulfate reduction, and a relatively high content of H2S in the associated gases and abundant sulfo-diamantane in the oil supported this theory. In conclusion, the thermal maturity, organic facies, paleoenvironment of the source rock, and possibly thermochemical sulfate reduction have a large impact on the sulfur compounds present in the oils. The O1S1/S-1 and S-2/S-1 ratios could be used as indicators of the precursors/paleoenvironment, and C10-19/C20-50DBE9 and DBE1, 3, 6/DBE9 could be used as indicators of thermal maturity. Fourier transform ion cyclotron resonance mass spectrometry is very useful for detecting sulfur compounds, especially those with high molecular weights, in the crude oils. This technique has potential for determining the formation mechanisms of some unusual oils and the geochemical implications of the sulfur compounds they contain.