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Residency of Rhenium and Osmium Within Heavy Crude Oil


The Re-Os geochronometer is an emerging tool for the study of oil formation and migration processes, and a new technology for petroleum exploration. Very little is known, however, about the residency of Re and Os within asphaltene and maltene sub-fractions of crude oil. This information is crucial if we are to have full understanding of petroleum systems. In this work, a heavy crude oil was separated into soluble (maltene) and insoluble (asphaltene) fractions using n-heptane as the asphaltene precipitating agent. The asphaltenes were separated sequentially into sub-fractions using two different solvent pairs (heptane-DCM and acetone-toluene), and the bulk maltene was separated into saturate, aromatic, and resin (SAR) fractions using open column chromatography. Each asphaltene and maltene sub-fraction was analyzed for Re and Os. The asphaltene sub-fractions and bulk samples were analyzed for a suite of trace metals by ICP-MS. Our results show that Re and Os concentrations co-vary between the asphaltene sub-fractions, and that both elements are found mostly in the highly polar sub-fractions; significant Re and Os are also present in the aromatic and resin fractions of the maltenes. However, each asphaltene and maltene sub-fraction has a distinct isotopic composition, and these sub-fractions are not isochronous. As expected, Re correlates with 187Os, but significantly, 187Os correlates almost perfectly with bulk and common 192Os. This suggests that radiogenically produced Os, once formed, migrates away from its parent Re site and joins common Os in a more Os-accommodating site. Re correlates strongly with Mo and Cd and, to a lesser extent, U and Ag in the asphaltene sub-fractions; as Re and Os track each other, this suggests Mo, Cd, U, and Ag occupy similar sites. We suggest that progressive asphaltene precipitation during migration and mixing of oils can change the resultant oil's isotopic ratios. This is key to interpretation of Re-Os data for tar mats and live oils, whether the data form an isochron or scatter. By combining data from source rocks, oils, and asphaltenes generated along the migration pathway, we are constructing temporal histories for whole petroleum systems.