EVIDENCE OF HYDROTHERMAL ALTERATION IN DEVONIAN SHALES FROM THE EGSP-2 CORE OF THE ROME TROUGH, APPALACHIAN BASIN, USA

This study represents a comparative assessment of stacked shales within the stratigraphic interval comprising the Middle Devonian Marcellus through Upper Devonian Genesee formations in the EGSP-2 core (API No. 3700320980), Allegheny County, Pennsylvania. Mineralogy, petrography, total organic content (TOC) and geophysical logs were initially studied to gain insight regarding depositional controls and potential structural impacts to shale reservoir integrity, but ultimately documented post-depositional processes in these shale reservoirs.

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 and was reactivated throughout the Paleozoic. Lithological descriptions and facies interpretations 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, sphalerite, jarosite, quartz, gypsum, dolomite and calcite, as well as organic matter. Platinum, silver and thorium were also detected within veins. These minerals fill fractures and eliminate fracture porosity in portions of the study interval. The hydrothermal mineralization also permeates into the shale matrix surrounding some veins.

The presence of hydrothermal minerals implies that fluids altered the Devonian organic-rich shales in the Rome Trough, with reactivated faults facilitating fluid flow within the shales. This alteration is responsible for the sulfide and exotic minerals identified and has surely impacted reservoir quality, rock mechanical properties and/or thermal maturity. The cross-cutting 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.