Analysis of Mudrock Lithofacies and Hydrocarbon-Source Potential of the Middle Ordovician Athens Shale, Alabama Fold and Thrust Belt

Abstract

Lithologic, petrologic, and geochemical studies were performed on the Ordovician Athens Shale, with a focus on a 140-ft-thick core section recovered near Calera, Alabama, supplemented by observations of limited outcrop exposures. Study goals were to (1) characterize the mudrock lithofacies of the Athens Shale, (2) interpret the environmental conditions and processes that influenced Athens Shale deposition, and (3) assess the potential of the Athens Shale as a hydrocarbon-source rock. Four broad lithofacies are recognized in the Calera core interval of the Athens Shale based primarily on the abundance and character of limestone interbeds. Occurring in ascending stratigraphic order, lithofacies A, B, C, and D record a progressive increase in basin slope stability and/or water depth. Lithofacies A, dominated by nodular skeletal limestones, records slope instability and debris-flow deposition. Progressively thinner and finer-grained limestone beds in lithofacies B and C reflect deposition by proximal and distal turbidity currents. Carbonaceous shales, which become more prevalent upward from lithofacies B through lithofacies D reflect pelagic/hemipelagic depositional processes. Quartz silt content and evidence for weak bioturbation increase upward in the shales.

Major and trace element compositions of shale samples are similar to the average black shale, indicating no significant metal enrichment. Geochemical proxies for basin redox conditions yield ambiguous results, but provide some indication of temporal changes in basin oxygenation. Geochemical data also suggest that the Athens Shale muds were derived from a passive-margin, quartzose sedimentary provenance and thus do not reflect an arc-related terrane as expected for Taconic orogenesis. Organic geochemical data from organic carbon and Rock-eval analyses indicate that the Athens Shale contains sufficient organic matter to have served as a hydrocarbon-source rock. However, in the Calera area, the Athens Shale is thermally overmature.