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Shale Geochemistry

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.

Refining Geochemical Proxies

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.    

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