Light hydrocarbons and diamondoids of light oils in deep reservoirs of Shuntuoguole Low Uplift, Tarim Basin: Implication for the evaluation on thermal maturity, secondary alteration and source characteristics

Abstract

The Shuntuoguole Low Uplift in Tarim Basin is particular because it provides insights into geochemistry of light oils at ultra-deep Ordovician oil reservoirs (7200–8200 m). The light hydrocarbons and diamondoids of 16 oil samples from Shunbei No. I Fault Zone (F1) and V Fault Zone (F5) were analyzed to investigate their thermal maturity, source characteristics and secondary alteration. The oils show high similarity in the characteristics of light hydrocarbons and diamondoids. The light hydrocarbons are characterized by high contents of alkanes and low contents of cycloalkanes and aromatics, and the diamondoids show a domination of adamantanes and their alkylated isomers compared diamantanes. Thermal maturity indicators derived from diamondoids and C 5 –C 7 light hydrocarbons indicate that the oils are highly mature, and the corresponding equivalent vitrinite reflectance ( R c) determined by light hydrocarbons and methyldiadamantane index are in the ranges of 1.2%–1.5%. Signatures of biodegradation and thermochemical sulfate reduction were not observed in the oils. The geochemical information from the light hydrocarbons and distribution patterns of molar concentration of n -alkanes indicate that the oils from F1 experienced noticeable evaporative fractionation, and the oils from F5 did not. This was supposed to be the primarily reason for the physical difference between oils from the both fault zones. The parameters derived from the light hydrocarbons and diamondoids indicate the oils are from same source rocks, which are associated with marine materials with type II kerogen, such as algae and bacteria, and both of Cambrian–Lower Ordovician (Є-O 1 ) and Middle–Upper Ordovician (O 2–3 ) source rocks in the deep Lower Palaeozoic strata contributed to the oils. • Light hydrocarbons and diamondoids of the deep Ordovician oils were revealed. • The oil maturity was calibrated at equivalent vitrinite reflectance values of 1.2%–1.5%. • Both contributions of the Є-O 1 and O 2+3 source rocks to the oils were suggested. • No or negligible biodegradation and TSR alteration were suggested. • Noticeable evaporative fractionation was observed in the oils from No. 1 Fault.