AAPG Foundation 2019 Grants-in-Aid Projects

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Tracking early Paleozoic marine anoxia and organic-rich mudrock deposition: a multi-proxy non-traditional stable isotope approach

Abstract

Compilations of petroleum resources through geologic time illustrate an emerging correlation between the timing of ocean anoxic events and shale gas reserves. Anoxic depositional environments promote the preservation of organic carbon, and enhanced organic carbon production in turn drives ocean anoxia through microbial respiration, providing fundamental biogeochemical links between anoxia and organic carbon deposition in marine environments. The emergence of stable molybdenum and uranium isotopes as sedimentary geochemical proxies for tracking the global extent of marine anoxia in deep time provides new opportunities for both fundamentally understanding trends in hydrocarbon distribution through time, and developing predictive tools for hydrocarbon exploration. In this study, I propose using a multi-proxy molybdenum and uranium isotope approach to evaluate global marine anoxia during the early Paleozoic. I will measure the δ98Mo and δ238U of anoxic black shale samples collected from the Road River Group, Yukon, Canada, supplemented with graptolitic shales from the University of California Museum of Paleontology, to produce globally supported non-traditional stable isotope datasets for the upper Cambrian to Late Ordovician. I will further apply a novel Monte Carlo isotope mass-balance model to quantitatively constrain the extent of anoxic conditions through this time interval. The Utica Shale, USA, and Goldwyer Shale, Australia, are both prominent shale gas resources deposited during this time period. The quantitative geochemical approach proposed here is intended to test paleo-oceanographic explanations for the occurrence of these hydrocarbon resources, and to provide a predictive tool for evaluating the potential for correlative organic-rich deposits elsewhere.