Distribution and origin of higher diamondoids in the ultra-deep Paleozoic condensates of the Shunbei oilfield in the Tarim Basin, NW China

Abstract

Higher diamondoids in the condensates of the Shunbei oilfield were analyzed using GC–MS. The findings indicate that the condensates in the Shunbei oilfield were subjected to a superimposed effect of secondary alterations, including biodegradation, hydrothermal alteration, evaporative fractionation, cracking, and TSR. High local concentrations of thiaadamantanes in oil may result from the interaction of H₂S, derived from Cambrian TSR, with Ordovician hydrocarbons, influenced by hydrothermal activities. Comprehensive analysis shows that TSR is local and limited in Ordovician reservoirs in the Shunbei oilfield and has no significant impact on oil chemistry. Comparative studies indicate that the diamondoid concentrations in condensates from the Shunbei oilfield have not been significantly impacted by biodegradation and evaporative fractionation. Parameters related to diamantanes can effectively characterize their geochemical characteristics and secondary alteration. A multi-parameter correlation heat map suggests that higher diamondoids in the condensates of the Shunbei oilfield are not due to high thermal evolution but have been altered by hydrothermal activity. The hydrothermal activity promotes the formation of higher diamondoids in the condensate. The higher diamondoids that are formed by hydrothermal activity offer a new perspective for studying hydrothermal processes. Additionally, this aids in studying the organic-inorganic interactions of ultra-deep organic fluids with their mineralogical and aqueous environments.