Are All Shales Created Equal? A Comparative, Multi-Proxy Geochemical Study of Paleozoic and Mesozoic Shale Basins in North America

Abstract

Much of the energy potential in North America is stored in unconventional reservoirs, including shales and more broadly, fine-grained lithologies. These highly productive zones have been evaluated using a range of methods at different scales, but understandably, the focus is on their current properties with little emphasis on their generative environment and processes. Here we aim to characterize the initial depositional geochemical conditions that are, at present, poorly constrained over variable spatial and temporal scales.

We employed an array of geochemical proxies to a series of North American Paleozoic and Mesozoic basins. This included proxies common to industry, such as total organic carbon (TOC) and redox-sensitive trace metal abundances, with others like iron speciation and δ¹³C_org. These data, interpreted within a sedimentological context, can provide a more robust, yet nuanced, depositional model for each unconventional target.

Results from the Williston, Appalachian, Midland, Liard, Peace River, Fort Worth, and Gulf Coast basins suggest that paleoredox conditions varied greatly within and among basins, especially in the proportion of sulfidized reactive iron, i.e. whether the overlying water column was ferruginous (sulfate-limited) or euxinic (iron-limited). This factor is especially critical with respect to the utility of certain proxies, such as molybdenum enrichment, which requires euxinic conditions to accurately record signals of basin restriction and paleohydrogeography. Within the studied basins, the prevalence of ferruginous conditions sections suggests that the Mo proxy is only valid for specific basins and should not be applied as a universal indicator. However, when it occurs, the development of euxinic conditions may be associated with more productive intervals and target zones. Further, our data show signals of changing redox conditions within a single basin depocenter, suggesting that a one-size-fits-all method of reservoir characterization and production may not be the most effective.

Unsurprisingly, not all shales are created equal. Building a more accurate depositional model that can be applicable to a single basin or translated across basins is ultimately a step towards improving efficiency and recovery in the transition to cleaner energy.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018