Aromatic sulfur compounds as maturity indicators for petroleums from the Buzuluk depression, Russia

This study demonstrated that phase state and compositional variations of hydrocarbons in the Buzuluk depression, Russia, are related to maturity levels of Devonian source rocks. Maturity levels of organic matter of source rocks estimated by Rock-Eval Tmax and vitrinite reflectance (Ro%) are in the range from ‘oil window’ to ‘overmature’ and increase from east to west in the depression. The volume of the gas-condensate deposits increases in the same direction. Geochemical analyses using GC with dual FID/FPD and GC-MS show the variation in distributions of aromatic sulfur compounds in oils and condensates with different maturity levels. Very mature petroleums are characterized by very low concentrations of benzothiophenes (BTs) and abundant methyldibenzothiphenes (MDBTs). Maturity parameters such as the 4-MDBT/1-MDBT and newly proposed dimethyldibenzothiophenes (DMDBT) ratios of sulfur aromatics correlate well with other maturity indicators based on hydrocarbon distributions in gasoline-range and saturate fractions of petroleum. Sulfur aromatic parameters work well over a wide range of catagenesis, showing no reversal at the advanced levels of thermal evolution and can efficiently discriminate very mature petroleums.