Behavior of Re and Os during contact between an aqueous solution and oil: Consequences for the application of the Re-Os geochronometer to petroleum

摘要

Several recent studies have raised the exciting possibility that oils can be dated using the Re-Os radioisotope system. However the exact nature of the events dated by this technique often remains unclear. Geochronologic interpretation of Re-Os data for oils is hampered by our limited knowledge of how these metals behave in petroleum systems. In particular, it is difficult to understand how isotopic homogenization, an essential prerequisite for the development of an isochronal relationship, can be achieved at the scale of a petroleum basin. The mechanisms capable of fractionating the Re/Os ratio in a suite of oils are also poorly understood. For this reason, we have performed an experimental study aimed at investigating the behavior of Re and Os during a particularly widespread phenomenon in petroleum systems, the interaction of formation waters with oils during migration. Contact experiments between natural oils and aqueous solutions enriched in Re and/or Os were carried out for varying lengths of time (6 h to 5 months), at different temperatures (25-150 degrees C), over a wide range of metal concentrations in the enriched solution (0.001-100 mu g/g for Re; 1 and 10 ng/g for Os). In addition, the effect of oil composition on Re-Os exchange at the water-oil interface was examined by testing two oils with very different properties.