Fracture patterns in Marcellus black shale across the Appalachian Basin of New York State
LaGamba, Michael J.
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The Devonian Marcellus Formation is black shale with sporadic thin limestone layers that outcrops 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 CO 2 sequestration. In this study for CO 2 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. I measured fracture attributes including spacing, strike, dip, apparent length and height, abutting relationships and sedimentary unit descriptions. Characteristics of fractures at the various sites are displayed in modified rose diagrams that show rose diagram petals as a function of fracture frequency, rather than numbers of fractures. The lower semicircle of the rose diagram is used to illustrate 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 Marcellus outcrop belt of New York State exhibit one dominant fracture set with strikes between 50° and 90°. The ENE orientation is the classic Set III of Engelder and Geiser (1980) and J1 of Lash and Engelder (2009). Variance away from 60° is found in central and eastern New York and supports earlier suggestions that a 60° 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 greater than 80° typifies Marcellus in East Berne, Chittenango, and some regions near 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 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.