Application of light hydrocarbons (C5–C7) in Paleozoic marine petroleum of the Tarim Basin, NW China

•The Paleozoic marine oils have distinct light hydrocarbons compositions.•Different light hydrocarbons compositions are determined by source rock facies.•Abnormally high K1 oil values probably result from TSR alteration.

The Cambrian–Lower Ordovician (Є-O1) and Middle–Upper Ordovician (O2–3) sourced oils from the Tarim Basin were distinguished based on their C5–C7 light hydrocarbons compositions. Relative to Є-O1 sourced oils, O2–3 sourced oils are characterized by relatively lower abundance of cycloalkanes, higher n-heptane/methylcyclohexane values (F<1.0) and aromatics content (i.e., benzene and toluene). The difference in light hydrocarbons compositions between the Є-O1 and O2–3 sourced oils is mainly determined by their source facies, rather than by secondary alterations. For most analyzed samples, Mango's parameter, K1 values, are close to 1.0. The ZS1C condensate, derived from Є-O1 rocks, has a distinct light hydrocarbons composition. The most obvious signature of the ZS1C condensate is abnormally high toluene content and abnormally high K1 value. Previous work has confirmed that the ZS1C condensate was altered by intense thermochemical sulfate reduction (TSR) and possesses a distinct geochemical composition, such as high dibenzothiophenes (DBTs) concentrations and high δ34S values of dibenzothiophenes. The abnormally high toluene abundance and K1 values also probably result from TSR alteration. Furthermore, the oils affected by TSR alteration from the east of Tazhong center anticline have been confirmed to possess abnormally high K1 values (>1.0). It seems that TSR alteration has a controlling effect on the kinetics course of C7 light hydrocarbon formation, and the abnormally high K1 values can be used as an indicator of TSR alteration in the Tarim Basin.