Further evaluation of the Re-Os systematics of crude oil: Implications for Re-Os geochronology of petroleum systems

Abstract

Here we further demonstrate asphaltene to be the main carrier of Re and Os in crude oil and additionally show that generally the less soluble fractions of asphaltene contain higher concentrations of Re and Os, with the 187Os/188Os and 187Re/188Os values showing different trends between oils. These observations are considered as evidence of the existence of different Re and Os carriers in crude oil. The multiple heteroatomic ligands and porphyrins proposed as Re and Os hosts in crude oil are likely present as free molecules, initially absorbed and occluded in the asphaltene aggregates and co-precipitate with asphaltene in response to the addition of n-alkanes. The ReOs elemental and isotopic systematic behaviour revealed herein are interpreted to be the result of the involvement of Re and Os carriers in the aggregation and precipitation of asphaltene, and the chemical make-up of each particular crude oil. Asphaltene and maltene separated by a series of different alkanes show that asphaltene yields decrease from n-pentane to n-heptane and then tend to stay stable until n-decane. The 187Os/188Os values of the series of asphaltenes are extremely similar for each oil sample, inhibiting the possibility of determining any ReOs dates with the asphaltenes of a single crude oil. In contrast, the maltene fractions exhibit ReOs isotopic variation. Nevertheless, only the maltenes of one oil defined a reasonable isochron with an age similar to that of source rock. As such, this study highlights the need for a greater understanding in order to determine whether, and in what cases, fractions separated from a single crude oil by different alkanes can provide meaningful ReOs ages.