FRACTURE PATTERNS IN MARCELLUS BLACK SHALE ACROSS THE APPALACHIAN BASIN OF NEW YORK STATE

The Devonian Marcellus is a black shale with sporadic thin limestone layers that occurs from New York to West Virginia. This formation exhibits fractures in outcrop that can serve as a model for understanding subsurface fracture pathways for natural gas migration or storage for CO2 sequestration. In this study for CO₂ sequestration, fractures were recorded in the Marcellus Formation and adjacent units in order to characterize the fracture systems, and how these systems vary across the Appalachian Basin in New York State. We measured fracture attributes including spacing, strike, dip, apparent length and height, abutting relationships and sedimentary unit descriptions. Data were collected using a variety of methods including scan lines; scan grids and the abbreviated method. Fractures at the various sites are displayed in modified rose diagrams that display rose diagram petals as a function of fracture frequency, rather than numbers of fractures, and the lower semicircle of the rose diagram is used to display abutting relationships. The abutting relationships are also shown in stick diagrams, which display all abutting relationships in a more conceptual manner.

Most outcrops across the outcrop belt of New York State exhibit one dominant fracture set; this set strikes between 50^o and 90^o. The ENE orientation is the classic set III of Engelder and Geiser (1980) and J1 of Lash and Engelder (2009). The variance away from 60^o is found in central and eastern New York and supports earlier suggestions that a 60^o fracture orientation in the black shales is not ubiquitous across the state (e.g., Jacobi et al., 2007). For example, a dominant fracture strike of > 80⁰ typifies Marcellus in East Berne, Chittenango, and some regions in Marcellus, NY. Studies have suggested that J1 fractures are mostly gas generated, and increase in frequency down-section in the black shale where higher TOC occurs. Preliminary analysis of J1 spacing data show that the assumed higher TOC units that are low in the Marcellus section do not necessarily contain the highest fracture spacing. In some areas, fractures higher up in the section display similar spacing. These anomalies suggest, that fracture generation could depend on a complicated mix of factors including TOC and gas generation, broad tectonic activity, and local stresses related to fault activity.