ARE ALL SHALES CREATED EQUAL?: A COMPARATIVE, MULTI-PROXY GEOCHEMICAL STUDY OF PALEOZOIC AND MESOZOIC PRODUCING SHALE BASINS IN NORTH AMERICA

Much of the energy potential in North America is in the form of natural gas that is stored in unconventional shale reservoirs. At present, operators understand how to find, broadly analyze, and produce these hydrocarbon-rich horizons. Finer details about the initial geochemical conditions that give rise to these highly productive intervals remain poorly constrained over variable spatial and time scales.

Here we study a series of Paleozoic and Mesozoic basins in North America using an array of geochemical proxies including total organic carbon (TOC) and δ¹³C_org analysis, iron speciation, and redox-sensitive trace metal abundances, coupled with sedimentology. Together, these proxies for organic matter preservation and provenance, basin restriction, and redox conditions (ie. anoxic, euxinic, or ferruginous) provide a robust, yet nuanced, depositional model for each target zone. Compilation and comparison of each depositional model to others in the series allows for discovery of subtle correlations, or lack thereof, that may have been overlooked as a factor in exploration or production.

Preliminary results from the Williston, Appalachian, Midland, Liard, Peace River, Fort Worth, and Gulf Coast basins suggest that all shales are not created equal. Redox conditions vary greatly from basin to basin, especially in the proportion of sulfidized reactive iron, i.e. whether the overlying water column was ferruginous or euxinic. The variability between basins is especially important with the use of certain proxies, such as molybdenum enrichment, which requires euxinia to correctly interpret basin restriction and paleohydrogeography. The prevalence of ferruginous conditions among the analyzed sections suggests that this proxy is only valid for specific basins. Further, our data shows 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.

Ultimately, creating a more accurate depositional model that can be applied to a single basin or translated from one unit to another is a step towards improving the efficiency and decreasing the environmental footprint of natural gas extraction in the transition to cleaner energy.