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Redox Depositional Environments of Unconventional Targets

It has long been known that black shales (i.e., the targets in unconventional systems) were
deposited under dysoxic to anoxic conditions. In the modern ocean, essentially
all anoxic environments are characterized by the presence of free sulfide
(euxinia), and this model has been explicitly or implicitly applied to most
ancient shales. Recently, it has been recognized that many black shales were
deposited under ferruginous (free ferrous iron) conditions. This fundamental
feature of depositional redox state likely influences many parameters in the
rock that are ultimately important for production, but these links have yet to
be elucidated. The Sperling research group is conducting detailed case studies
of selected unconventional targets to provide the most nuanced view possible of
the depositional environments of these units. These studies comprise an
integrated analysis of sedimentology, stratigraphy, and multi-proxy
geochemistry (iron speciation chemistry, trace metals, organic carbon contents
and isotopes, and pyrite sulfur isotopes). Studies are currently ongoing in the
Exshaw/Patry, Horn River Group, Montney Formation, Wolfcamp Formation, Cline
Shale, Barnett Shale, Eagle Ford Shale, Bakken Formation, Marcellus Shale, and
Utica Shale.
Moving forward from these detailed studies of individual cores, we are working to
understand how environmental conditions changed across shale basins in time and
space, and the oceanographic factors controlling such changes. We are also
utilizing trace metal isotopes (specifically molybdenum and uranium) as tracers
of the ancient global redox landscape. When these metal isotopic data are
integrated into our modeling framework, we are able to predict intervals of
Earth history when anoxia or euxinia—the conditions necessary for world-class
unconventional targets—will be more widespread. Ultimately this will help
identify under-explored areas of the geological column. Finally, we will
undertake studies designed to mechanistically relate depositional conditions to
parameters important for production.