Evidence of hydrothermal alteration in Devonian shales from the Eastern Gas Shales Project 2 core of the Rome trough, Appalachian Basin, United States

Abstract

This study represents a comparative assessment of stacked shales within the Middle Devonian Marcellus through Upper Devonian Genesee Formations in the Eastern Gas Shales Project 2 core (API No. 3700320980), Allegheny County, Pennsylvania. Mineralogy, petrography, total organic carbon (TOC), and geophysical logs were initially studied to gain insight regarding depositional controls and structural impacts to shale reservoir integrity, but ultimately, we documented postdepositional processes in these shale reservoirs as well.

The core is located near a northwest-southeast–trending cross-structural discontinuity and is situated within the Rome trough, a northeast-southwest–trending graben in the Appalachian Basin that initially formed during Cambrian rifting. Lithofacies descriptions were prepared using the core and geophysical logs. Bulk mineralogy, TOC, petrographic, and scanning electron microscopy analyses were performed for samples taken at discrete depths throughout the stratigraphic interval.

Lithology ranges from carbonate-rich, organic-poor gray shale to quartz-rich, organic-rich black shale. Vein occurrence is not unique to specific facies, and vein mineralogy is of hydrothermal origin, consisting of calcium plagioclase, sulfides, quartz, gypsum, and carbonates, as well as organic matter and thorium. The hydrothermal mineralization eliminates fracture porosity, permeates into the shale matrix surrounding some veins, and suggests that fluids altered these Devonian shales, with reactivated faults facilitating fluid flow. Alteration has surely impacted reservoir quality, mechanical properties, and/or thermal maturity. The crosscutting nature of veins, replacement grains, and complex vein mineralogy suggest that diagenetic alteration of the shale matrix occurred, followed by multiple hydrothermal fluid-flow events that delivered brines of varying chemistry through rock fractures.