The effect of evaporation on the concentration and distribution of diamondoids in oils

Diamondoids are commonly found in petroleum and sediments and have an inherent resistance to thermal and biological destruction, which means they can provide useful information in situations where conventional biomarkers cannot. Here, we present the results of an investigation of the effects of atmospheric evaporation on the concentration and distribution of low molecular weight diamondoids in four petroleum fractions (gasoline, condensate, diesel and fuel oil). These experiments indicate that both adamantanes and diamantanes evaporate with the other light hydrocarbons from oils and that variations in the concentrations of these compounds during evaporation are controlled by the type of petroleum fraction, the extent of evaporation and the boiling point of the diamondoid compounds within the oil. Evaporation has a significant effect on adamantane concentration ratios, whereas no changes in diamantane concentration ratios occur, suggesting that diamantane-based concentration and distribution indices can be used for the correlation of oils and determination of maturity even if oils have undergone evaporation. Some diamondoid concentration ratios, such as adamantane/1-methyladamantane, 1-methyladamantane/2-methyladamantane, 1-methyladamantane/1-ethyladamantane, 1-methyladamantane/4-methyldiamantane, adamantane/diamantane and 1,3-dimethyladamantane/4,9-dimentyldiamantane, progressively decrease with ongoing evaporation and are independent of petroleum fraction type, indicating that given the original unaltered index value, these indices can be used to deduce the relative extent of oil evaporation. The study also indicates that slight to moderate evaporation of oils leads to an increase in diamantane concentrations that is nearly proportional to the extent of oil evaporation, indicating that these compounds can be used as indices to estimate the extent of oil evaporation.