A Comparative Inorganic Geochemical Analysis Between the Marcellus and Utica Shales Based on XRF

Abstract

A comparative chemostratigraphic analysis of a Marcellus Shale core from southwestern Pennsylvania and a Utica Shale core recovered from eastern New York using handheld XRF technology reveals significant differences in the concentration of elements that serve as proxies of detrital flux and redox conditions. Perhaps the most noticeable differences between the Marcellus and Utica is reflected in the abundances of Al, a robust proxy for clay content, and redox-sensitive elements, U and Mo, both of which are especially useful to the analysis of oxygen-deficient marine systems. Though enrichment of Mo and U in marine deposits can be ascribed to the authigenic uptake from seawater enhanced by oxygen deficient conditions, authigenic enrichment mechanisms of both elements differ from each other. The Marcellus succession illustrates a general increase of Al upsection from the TST through the RST deposits. No such trends are observed in the Utica Shale as Al remains generally consistent throughout the most organic-rich intervals. Overall, Al is higher in the Marcellus relative to the Utica suggesting higher clay content in the former. Impressed upon the generally increasing clastic input of the Marcellus are marked redox variations indicated by U and Mo enrichment that tell of increasingly reducing environmental conditions. These data reflect sediment accumulation in an “unrestricted marine” environment setting in which the supply of Mo to the water column was renewed at a rate that exceeded its rate of sequestration in sulfidic sediment. The concentration of redox proxies in the Utica Shale core is much less than one might expect of an organic-rich black shale. Both Mo and U values are suppressed throughout much of the core and only minimally enriched within the organic-rich sections. Chemostratigraphic analysis of the Utica core suggests that the organic-rich deposits accumulated under anoxic to intermittently euxinic conditions that would have favored the authigenic uptake of U and Mo. However, the depleted nature of the most organic-rich deposits of the Utica Shale reflect major differences between Ordovician and Devonian worlds, possibly the result of global anoxia and consequent drawdown of the global U and Mo inventory and lack of an established land plant root system that would have favored the development of clay soil profiles.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015