Effects of evaporative fractionation on diamondoid hydrocarbons in condensates from the Xihu Sag, East China Sea Shelf Basin

Abstract Evaporative fractionation is a complex phenomenon associated with the separation of gas from oil in the subsurface, generally involving migration fractionation and gas invasion. The concentrations and distributions of diamondoid hydrocarbons in condensate samples with different type and extent of evaporative fractionation from the Xihu Sag, East China Sea Shelf Basin were investigated to determine the effects of evaporative fractionation on diamondoid hydrocarbons. The results suggest that both migration fractionation and gas invasion can affect the concentrations and distributions of diamondoid hydrocarbons. Residual oils after migration fractionation are generally characterized by relatively high concentrations of diamondoid hydrocarbons and low methyladamantanes/methyldiamantanes (MAs/MDs) ratios. However, maturity parameters based on diamondoid hydrocarbon isomers do not vary significantly from other typical maturity parameters, suggesting that they can still be used to assess maturity. Gas invasion causes the concentrations of diamondoid hydrocarbons in oil samples to rise generally, along with methyladamantanes/methyldiamantanes (MAs/MDs) and adamantane/diamantane (A/D) ratios, indicating that the effects of gas invasion on diamondoid hydrocarbons in oils are different from those of migration fractionation. In addition, gas invasion causes some changes in the diamondoid derived maturity parameters. Thus, the effects of gas invasion should be considered when using diamondoid hydrocarbons to evaluate the thermal maturity of crude oil. Diamondoid hydrocarbon concentrations and distributions in petroleum can be affected by evaporative fractionation, suggesting that it is possible to determine evaporative fractionation according to the diamondoid hydrocarbon composition. It can be proposed that the decreasing values of MAs/MDs ratios and increase of diamondoid concentration are indicators of migration fractionation, and increasing values of A/D, MAs/MDs and diamondoid concentration might suggest the introduction of additional gas.