oa Dating Of Petroleum Using The Re-os Geochronometer : Experimental Study

The purpose of this study is to better understand the geochemical behavior of Re and Os in petroleum in the context of the Re-Os radiometric age dating of petroleum. Rhenium (Re) and osmium (Os) are organophile elements. For this reason, the 187Re-187Os radiogenic system is well adapted to date organic-rich rocks such as source rocks. Absolute datation of source rocks is more particularly important for Petroleum Systems knowledge. Recently, it was suggested that the Re-Os system could also be used to date hydrocarbon generation (Selby and Creaser, 2005). However, the meaning of the obtained age is ambiguous. This is mainly due to the lack of knowledge about the geochemical behavior and the speciation of Re and Os in oils. Specifically, using the Re-Os geochronometer requires a complete system reset (corresponding to an age of 0) during the hydrocarbon generation and the system to remain closed through geological times and events. Moreover, additional conditions are necessary as the isotopic homogenization of oils at the scale of a basin and the chemical fractionation of Re from Os to obtain samples with various Re/Os ratios. If one of these steps is not fulfilled, no age can be measured, that why it is essential to develop a better understanding of the geochemical behavior of these elements. Several key events may have a significant influence on Re and Os partitioning such as the formation, primary and secondary migration or alteration of petroleum in reservoirs. Formation was already studied through artificial maturation (Rooney et al., 2012). However, the other aspects have to be investigated. To try to better understand Re-Os chemical behavior in hydrocarbons, we performed experiments to simulate possible events occurring during migration and reservoir storage such as asphaltenes partial deposition (Mahdaoui et al., 2013) and contact with an aquifer (Mahdaoui et al., submitted). After chemical treatment of samples, Re and Os concentrations were determined by isotope dilution. Re and Os isotopic compositions used for concentration calculations were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and by Negative Thermal Ionization Mass Spectrometry (N-TIMS), respectively. Results of our experiments enable us to constrain the processes which control most likely Re and Os content in oil and thus they provide a firm basis to characterise Petroleum System events that are actually dated by the Re-Os geochronometer, e.g.: genesis, migration or reservoir filling. REFERENCES Mahdaoui, F., Reisberg, L., Magali, P., Poirier, Y. Behavior of Re and Os during contact between an aqueous solution and oil. Submitted Mahdaoui, F., Reisberg, L., Michels, R., Hautevelle, Y., Poirier, Y. and Girard, J.-P., 2013. Effect of the progressive precipitation of petroleum asphaltenes on the Re-Os radioisotope system. Chemical Geology, 358, 90-100. Selby, D., Creaser, R.A., 2005. Direct radiometric dating of hydrocarbon deposits using Rhenium-Osmium isotopes. Science, 308, 1293-1295. Rooney, A.D., Selby, D., Lewan, M.D., Lillis, P.G., Houzay, J.P., 2012. Evaluating Re-Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments. Geochimica et Cosmochimica Acta, 77, 275-291.