Effect of evaporative fractionation on the distribution and composition of diamondoids in crude oils: a case study of crude oils from Yangtake structure, Kuqa Depression, Tarim Basim

Abstract

Two sets of crude oil samples from the Yangtake structure in the Kuqa Depression of Tarim Basin were analyzed with the aid of GC and GC-MS to evaluate the effect of evaporative fractionation on the distribution and composition of diamondoid hydrocarbon in crude oils. The light hydrocarbon signatures of oils from wells YT 5 and YT 101 indicate secondary products of evaporative fractionation for the condensates from the upper reservoirs, and the oils from the lower reservoirs are residuals. Moreover, the distribution and composition of steranes and terpanes in two sets of crude oils appeared to be comparable, suggesting that these oils shared a same source. Evaporative fractionation has imperceptible effect on the distribution and relative abundance of alkyl admantanes and diamantanes, but their concentrations in secondary condensates and residual oils appeared to be significantly varied. For example, the concentration of diamondoids in secondary condensates is much higher than that in residual oils, and the increasing extent of concentration for alkyl admantanes in secondary condensates is much higher than that in residual oils. Therefore, it should be very cautious to use the concentration of diamondoids to determine the degree of maturity of crude oils if evaporative fractionation occurred. However, the effect caused by evaporative fractionation to the maturity parameters such as MAI and MDI is relatively minor and they can still be used as indicators to evaluate maturity of crude oils even with the alteration of evaporative fractionation.

Keywords

• alkyl adamantanes
• alkyl diamantanes
• fractionation
• secondary condensates
• residual oils
• yangtake structure
• kuqa depression