Vanadium Isotopic Analysis of Extractable Organic Matter and Bulk Rock Shale in the Eagle Ford Shale



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Crude oils are enriched in a variety of metals (e.g., vanadium, nickel and molybdenum). Advances in instrumentation and research now allow the exploration of more trace elements and unconventional stable isotopic compositions of these metals in oils (e.g., V, Ni, and Mo). Crude oils appear ideal to investigate metal isotope biogeochemistry due to their accessibility and global occurrence. Stable isotopes are subject to mass dependent stable isotope fractionation, which is driven by the differences in chemical and physical properties of the atoms arising from the relative mass difference of a given element. Biodegradation, maturation, valence states/redox conditions can all induce fractionation among vanadium isotopes resulting from the differences in coordination chemistry and bond strength. In this study, a relatively recently developed unconventional stable method of obtaining the precise isotope ratio of 51V/50V was used to analyze for the first time the V isotope ratios of thirteen bulk rock shale samples and seven extractable organic matter (EOM) samples. These samples are from a core drilled from the La Salle county, TX in the Eagle Ford Shale. High concentrations and enrichments of metals are shown in the bulk rock shale where high total organic carbon, TOC, also prevails, which appears to indicate highly euxinic conditions. While the metal concentrations are important, the metal ratios, V/Ni and V/(V+Ni), are also important as they are suggested to be paleoenvironmental indicators. The bulk rock shale and EOM samples show very high V/(V+Ni) ratios (~0.9), which is indicative of very anoxic or euxinic (sulfidic) conditions. However, the V/Ni ratio has variations in the bulk rock shale, EOM and the oil. The 51V/50V of the bulk rock shale and EOM samples shows variability and disparity throughout the core. The deepest samples of the bulk rock and EOM show similar values of approximately -0.7‰ +/- 0.2‰, which are close to bulk silicate earth (BSE). However, the EOM samples diverge away from BSE values, and become isotopically lighter up hole. On the other hand, the bulk rock shale samples become isotopically heavier up hole. There lies an array of fractionation processes that could be causing the disparity in isotopic composition between the bulk rock shale and EOM. The fractionation processes include the vanadium speciation and redox conditions at the sea bottom, fractionation during diagenesis, progressive kerogen-crude oil transformation, progressive crude oil expulsion, and catagenesis-related crude oil demetallation.



Vanadium isotopes in shales, Extractable organic matter