The stable isotope composition of vanadium, nickel, and molybdenum in crude oils

Abstract

Crude oils often have high concentrations of transition metals including vanadium (V), nickel (Ni), iron (Fe), and to a lesser extent molybdenum (Mo). Determining the conditions under which these metals enter into crude oil is of interest for the understanding of biogeochemical cycles and the pathways leading to oil formation. This study presents the first high precision measurements of V, Ni, and Mo stable isotopes determined for a set of globally distributed crude oils as a first examination of the magnitude of potential stable isotope fractionation. Vanadium stable isotope compositions are presented for crude oils formed from different source rocks spanning a range of geologic ages (Paleozoic–Tertiary) and are complemented by Ni and Mo stable isotope compositions on a subset of crude oils produced from lacustrine source rocks in the Campos Basin, Brazil. The crude oils span a wide range of V and Mo isotope compositions, and display more restricted Ni stable isotope signatures. Overall, the stable isotope composition of all three systems overlaps with previously determined values for igneous and inorganic sedimentary materials. Comparisons between vanadium concentration and stable isotope composition yield distinct clusters associated with crude oils predominantly derived from terrestrial/lacustrine or marine/carbonate source rocks. The Ni stable isotope signatures of studied crude oils are similar to that of carbonaceous shales. The Mo stable isotope signatures of the lacustrine sourced crude oils are similar to what is observed for rivers. This indicates trace metal stable isotopic composition of crude oils are unlikely to result from mass dependent fractionation associated with the generation, expulsion, and migration of petroleum and are instead primarily inherited from the initial sedimentary organic matter or metal bearing fluids present during metalation. In contrast, although Mo stable isotopes also span a wide range of values, they do not appear to be correlated with source rock lithology, perhaps suggestive of a greater role for secondary processes. Finally, both V and Ni stable isotope compositions co-vary with V/(V+Ni), a commonly used parameter in determining crude oil grade. Since V/(V+Ni) ratios reflect redox conditions, the correlation implies that stable V and Ni isotope compositions may also respond to the redox conditions of the depositional environment. The contrasting behavior of these three isotope systems in this initial dataset provides fundamental guidance for future investigation to fully exploit the potential of these new isotopic tracers. These isotope tracers can be developed to determine a broad range of factors fundamental to the formation and preservation of petroleum source rocks that spans from provenance evaluation to paleoredox chemistry.