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Interpreting the depositional and diagenetic processes responsible for variations in uranium content in North American Black Shales

Abstract

While organic-rich shales have become increasingly important in recent years as unconventional petroleum reservoirs, the complex sedimentary and early-diagenetic processes at work in the shale depositional system are still not fully understood. Uranium (U), a redox-sensitive trace element, has long been used as a proxy for depositional redox conditions as well as organic-richness in shales, as interpreted from gamma-ray logs. U as a proxy for organic-richness stems from its typically linear relationship to total organic carbon (TOC) in shale. However, some shales display uncharacteristically low U concentrations despite having high TOC. In this case, how reliable is the gamma-ray log when evaluating shales as potential source rocks? Furthermore, what are the mechanisms responsible for atypical U to TOC ratios? In this study the U to TOC ratios and U isotopic composition of several Devonian shales are examined and results are compared to previous studies of modern analogues. Uranium isotopic composition has recently become a reliable tool used in paleo-redox reconstructions. Comparing the U isotopic inventories and relationships between U and TOC of several Devonian shales with observations of modern processes provides a window into the depositional and post-depositional processes which occurred in North American basins during the Late Devonian. This study leads to an enhanced understanding of the U and TOC relationship, and ultimately of the complex and dynamic shale depositional environment. New insight into these systems will improve predictions of organic matter distribution in shales and assist in locating organic-rich zones that may otherwise be missed by traditional methods.