Re-Os elemental and isotopic systematics in crude oils

Analysis of 12 worldwide oil samples show that Re and Os abundances are positively correlated with the asphaltene content of oil. Light oils with < 1% asphaltene content have basically no measurable Re or Os. Within oil, Re and Os are present dominantly in the asphaltene fraction (> 83%), with < 14% Re and Os found in the maltene fraction, this distribution is similar to other trace metals such as V and Mo. Rhenium and Os could be present in oil as metalloporphyrin complexes, but given their abundance in the asphaltene component they are also likely bound by heteroatomic ligands. The (187)Re/(188)Os and (187)Os/(188)Os values in asphaltene calculated at the estimated time of oil generation (Os(i)) are similar to those of the whole oil, as expected from the elemental results. This suggests that the asphaltene fraction can be used to approximate the Re-Os isotopic compositions of the whole oil. Os isotopic compositions in oils show a considerable range, from (187)Os/(188)Os of 1.9-6.0, and they correlate positively with the age of the proposed source rock. Re/Os ratios also show a large range and overlap the Re/Os ratios found in typical oil source rocks such as organic rich shale. (c) 2006 Elsevier Inc. All rights reserved.