RE-OS SYSTEMATICS AT THE WATER-OIL INTERFACE FROM AN EXPERIMENTAL PERSPECTIVE

Single crude oils have been successfully dated using the whole crude oil and its maltene and asphaltene fractions to create isochrons [1]. Interpretation of Re-Os ages for oils is coupled to geologic, biostratigraphic and other geochronologic information, trace metals, and stable isotopes for petroleum systems [1,2]. Oil-water interaction is a key process for transferring Re and Os into oil [3,4], whereas Re-Os partitioning during maturation may be minimal [5]. Re-Os chemistry controlling these processes, however, remains poorly understood.

Here we present results from experimental studies, in which aqueous fluids were reacted with different types of crude oils with variable reaction times, water to oil ratios, and Re-Os concentrations in the doped waters. The experiments were carried out in batch-type glass vessels using a HPA. After completion of each experiment, the two phases were separated and each was analyzed by NTIMS. The three different types of crude oils used in our experiments, had distinct Re-Os concentrations and Os isotopic signatures. These were overprinted by the aqueous fluid, with the Os isotopic composition of the oil rapidly approaching that of the fluid. The crude oil chemistry appears to control the rate of Re and Os transfer. The lightest oil shows quick equilibration with Os, but slow and incomplete equilibration with Re. This leads to a new and homogeneous ¹⁸⁷Os/¹⁸⁸Os isotopic composition in the oil, identical to that of the aqueous fluid. Importantly, with increasing experimental run time, ¹⁸⁷Re/¹⁸⁸Os ratios imparted to the oil approach the ¹⁸⁷Re/¹⁸⁸Os ratio of the starting aqueous fluid. Crude oils with higher asphaltene contents show complete equilibration of Re and Os isotopic compositions. Partial overprint experiments show that the crude oil and its maltene and asphaltene fractions keep their isochronous relationship during interaction with the aqueous fluid, thereby preserving age information. However, the Os isotopic compositions change and form a mixing line between the original unreacted oil and the starting aqueous fluid.

[1] Georgiev et al. (2016), GCA 179, 53–75. [2] Stein & Hannah (2014), Springer Encyclopedia of Dating Methods, 2^nd Ed., 87-118. [3] Mahdaoui et al. (2015), GCA 158, 1-21. [4] Finlay et al. (2010), Geology 38, 979–982. [5] Rooney et al. (2012), GCA 77, 275–291.