DIAGENESIS AND HYDROTHERMAL ALTERATION IN MIDDLE AND UPPER DEVONIAN SHALES OF THE ROME TROUGH, APPALACHIAN BASIN, USA

The stacked nature of Middle and Upper Devonian organic-rich shales of the Appalachian basin warrants a comparative assessment of the nature and quality of these unconventional reservoirs. This study evaluates variations in mineralogy, petrography, total organic content (TOC), and geophysical log data within the Middle Devonian Marcellus/Skaneateles formations and the West River/Geneseo members of the Upper Devonian Genesee Formation in the EGSP-2 core (API# 3700320980) to gain insight regarding diagenetic and hydrothermal alteration history. The core is located in Allegheny County, Pennsylvania, near the eastern limit of the Rome trough, which is a northeast-southwest trending graben in the Appalachian basin that initially formed during Cambrian rifting and was re-activated throughout the Paleozoic. Lithological descriptions and facies interpretations were prepared using the core and geophysical logs. Bulk mineralogy, TOC, petrographic, and SEM analyses were performed for samples taken at discrete depths throughout the stratigraphic interval.

Lithology generally ranges from carbonate-rich, organic-poor gray shale to quartz-rich, organic-rich black shale. Micro-fossils are ubiquitous throughout the core, often filled with replacement minerals and/or organic matter, and sometimes cross-cut by veins. Vein occurrence is not unique to specific facies, and vein mineralogy is of hydrothermal origin, consisting of plagioclase, sphalerite, jarosite, quartz, dolomite, and calcite, as well as organic matter. These minerals fill micro- to macro-scale fractures that range from horizontal to sub-vertical in orientation, thus eliminating fracture porosity in portions of the study interval. The hydrothermal mineralization also permeates into the matrix surrounding particular veins. The presence of hydrothermal minerals suggests that fluids altered Middle to Upper Devonian organic-rich shales in the Rome trough, as reactivated faults likely facilitated fluid flow within these shales. This alteration has surely impacted reservoir quality and rock mechanical properties. The cross-cutting nature of veins, replacement grains, and complex vein mineralogy suggest that diagenetic alteration of the shale matrix occurred, followed by one or more hydrothermal fluid-flow events through fractures.