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.
In fine-grained sedimentary rocks, the most complete
understanding of a rock’s history is accessed through a combined
sedimentological and geochemical approach. In this endeavor, geologists know
that ‘the present is the key to the past.’ Unfortunately for the development of
geochemical proxies useful in deep-water shale systems, most geochemists have
not had access to much of the ‘present.’ This is mainly due to the simple fact
that geologists and oceanographers inhabit different academic worlds and
different departments. Thus a ‘dirty secret’ is that many geochemical proxies
in common use throughout industry and academia were often calibrated decades
ago on a relatively small number of samples, sometimes with incomplete
environmental data. Many proxies also rely on a single threshold value rather
than more appropriate confidence intervals. Through links to many oceanographic
institutions—including SPODDS long-standing association with MBARI—we are
fortunate to have access to a large library of modern sediment samples that are
directly associated with high-quality in-situ oceanographic data. Many of these
samples are from modern upwelling margins (Oxygen Minimum Zones) or other
hypoxic/anoxic areas—modern analogues of the sediments that form source rocks
and unconventional targets. We are currently analyzing this sediment library
using a variety of methods to better refine interpretations from existing
proxies and to develop new proxies. By analyzing these data with machine learning
algorithms we expect this project to provide the new standard in geochemical
proxies and ultimately a richer understanding of ancient deep-water systems.