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Mechanical and Fracture Stratigraphy of the Utica Shale, Eastern New York State

Abstract

The mechanical stratigraphy of the Upper Ordovician Utica Shale is characterized by studying outcrops and core from Montgomery County in eastern New York State. Previous studies of the Utica Shale in New York State have focused on characterizing fracture orientations, distinguishing fracture generations, and establishing a relationship between fracture density and proximity to faults, but fractures in outcrops of the Utica Shale have not been studied in the context of mechanical stratigraphy. The composition, sedimentary texture, strength, and thickness of individual beds within the Flat Creek Shale and Dolgeville Formation are studied to better understand the nature of fracture propagation in relatively thinly-bedded mudrocks. The Flat Creek Shale and Dolgeville Formation are fine-grained siliciclastics containing varying amounts of detrital carbonate grains. There are also multiple bentonites within both members. A fracture bedding termination analysis is conducted at three outcrops of the Flat Creek Shale to identify mechanical interfaces. A Schmidt Hammer is used to measure rock strengths approximately every .2 vertical meters of the outcrop section being studied. Additionally, samples are collected throughout the vertical section to be analyzed by XRD. Utica Core 74 NY-5 is described in terms of bedding thicknesses, lithofacies, and sedimentary texture. Three thin sections are analyzed with petrographic microscopes to identify common textures and mechanical flaws associated with individual lithofacies. The combination of the fracture bedding termination analysis, rock strength measurement, core description, petrographic investigation, and XRD analysis assist with characterizing the mechanical behavior of these rocks. The fracture bedding termination analysis indicates that bentonites are responsible for approximately 51% of identifiable fracture terminations; therefore bentonites act as mechanical barriers to fracture propagation. The bentonites exhibit significantly lower present day rock strength values and are likely primarily composed of clays. While the remaining 49% of vein-filled fracture terminations occur elsewhere, there are few shale mechanical interfaces identified. Our observation that bentonite horizons form significant mechanical barriers to fracture propagation has important implications for modeling subsurface fracture networks because bentonites are widespread in this and other basins, and are easily distinguished on gamma ray logs.