--> Effect of Precipitation of Asphaltenes on Re-Os Isotopic Ratios

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Effect of Precipitation of Asphaltenes on Re-Os Isotopic Ratios

Abstract

The rhenium-osmium (Re-Os) geochronometer has been applied to organic-rich sedimentary rocks, bitumen, and oils of different origin 1, 2, 3. More recently, crude oils and their fractions have been studied, as our understanding of the residency of Re and Os in hydrocarbons may clarify oil generation and migration histories 4, 5. For example, fractionation of Re and Os during precipitation of asphaltenes may be crucial to interpretation of Re-Os geochronology in petroleum systems 4. However, relatively little is known about the placement of Re and Os within the asphaltene fraction4. In this study, a heavy crude oil was separated into asphaltenes and maltenes, and the asphaltenes were separated into sub-fractions based on polarity using heptane-dichloromethane mixtures at specific proportions 4. The Re-Os concentrations and isotopic compositions of seven asphaltene sub-fractions of decreasing polarity, as well as the bulk asphaltenes, maltenes, crude oil, and separated inorganic solids, were determined by HPA digestion and ID-NTIMS analysis 5. In contrast to published data 4, we observe that the intermediate polarity sub-fractions, not the most polar, are the most concentrated in Re and Os. Re concentration in the most polar sub-fraction is 420 ppb; as polarity decreases, Re concentration increases to 1500 ppb in sub-fraction 3, then drops to 220 ppb in the least polar sub-fraction. Os concentration similarly begins at 0.8 ppb in the most polar sub-fraction, rises to 2.5 ppb in sub-fraction 3, and then drops to 0.3 ppb in the least polar sub-fraction. The 187Re/188Os ratios systematically increase from about 5000 to 7000 with decreasing polarity of the sub-fractions, while the 187Os/188Os ratios begin at 6.3, rise to 6.7 in sub-fraction 4, then fall to 6.1 in the least polar sub-fraction. These systematic variations suggest that natural precipitation of asphaltenes is able to fractionate Re and Os within a bulk crude oil. With this discovery, we are one step closer to fully conceptualizing the isotopic consequences of oil maturation and migration, which include possible resetting of the geochronometer or changes as a result of asphaltene precipitation during migration. This work was supported by the Norwegian petroleum industry under the CHRONOS project. 1. Ravizza & Turekian 1989. GCA 53: 3257-3262. 2. Selby et al. 2005. EPSL 235: 1-15. 3. Selby et al. 2007. GCA: 71: 378-386. 4. Mahdaoui et al. 2013. Chem Geol 358: 90-100. 5. Georgiev et al. 2015, in review.