Vanadium isotope composition of crude oil: effects of source, maturation and biodegradation
Yongjun Gao, John F. Casey, Luis M. Bernardo, Weihang Yang, K. K. (Adry) Bissada, 2018. "Vanadium isotope composition of crude oil: effects of source, maturation and biodegradation", From Source to Seep: Geochemical Applications in Hydrocarbon Systems, M. Lawson, M.J. Formolo, J.M. Eiler
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We present a study of vanadium (V) isotope compositions for 17 crude oils spanning a wide range of concentrations and formation ages. Bulk organic geochemical and biomarker compositions are investigated for 11 co-genetic crude oils from the Barbados oil field. About 2‰ V isotope fractionation was observed and they are primarily correlated with V/Ni ratios and most likely reflect the depositional environment of the petroleum source rocks. Factors such as the lithology of the source rocks, Eh and pH of the depositional environment, and possibly the V isotope composition of seawater could all play a role in the V isotope composition. The Eh and pH conditions determine the speciation and coordination of V ions in fluids, whereas the lithology of the source rock defines the competing phases for available V ions in solution. The V isotopes are significantly modified by maturation and biodegradation. The V isotope fractionation during biodegradation has most probably resulted from the microbial activity-induced changes in the species and coordination geometries of V ions in fluids. The progressive decrease of δ51V with the increase of maturation might suggest a preferential loss of 51V during de-metalation and/or a preferential incorporation of 50V in the newly formed V-organometallic compounds.
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Hydrocarbon systems, by nature, are a complex interplay of elements that must be spatially and temporally aligned to result in the generation and preservation of subsurface hydrocarbon accumulations. To meet the increasing challenges of discovering hydrocarbon resources, it is essential that we advance our understanding of these systems through new geochemical approaches and analytical developments. Such development requires that academic- and industry-led research efforts converge in ways that are unique to the geosciences.
The aim of this volume is to bring together a multidisciplinary geochemical community from industry and academia working in hydrocarbon systems to publish recent advances and state-of-the-art approaches to resolve the many remaining questions in hydrocarbon systems analysis. From Source to Seep presents geochemical and isotopic studies that are grouped into three themes: (1) source-rock identification and the temperature/timing of hydrocarbon generation; (2) mechanisms and time-scales associated with hydrocarbon migration, trapping, storage and alteration; and (3) the impact of fluid flow on reservoir properties.