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Subsurface Fracture Characteristics of the Marcellus Shale in the Appalachian Plateau, North-Central Pennsylvania


Subsurface fracture data, collected from core and image logs during recent Marcellus Formation exploration and development activity in north-central Pennsylvania, were utilized to study the genesis of the fractures and their influence on gas production from extremely low permeability shale. The data bridge a gap in published data from outcrop in the Valley and Ridge province of Pennsylvania and the relatively undeformed, Appalachian plateau of New York. We observe one distinct set of vertical veins that strike orthogonal to fold axes, and low angle reverse faults that strike parallel to fold axes. As the trend of the fold axes changes across the study area, the trend of the veins and reverse faults also change to remain perpendicular and parallel to fold axes, maintaining kinematic compatibility with the vergence direction along the Pennsylvania salient. Additional joint sets seen in nearby outcrop are not observed in the subsurface data, and imply that additional deformation seen in outcrop may be related to stress relief during uplift and exposure. Vertical stylolites are observed in carbonate beds, and are typically perpendicular to the veins. The veins often occur in proximity to intervals with high numbers of bedding-plane slip surfaces, and at distinct mechanical stratigraphic positions where the competency contrast between relatively weak shale and stiff limestone is greatest. These observations suggest that bed parallel-shear within detachment zones provides a sufficient driving force required to nucleate vertical veins, similar to axial splitting observed in laboratory testing, which represents a new kinematic solution to the long-standing difference between mechanical predictions and field observations of vein orientations in contractional settings where the minimum stress is vertical. Although these veins are currently oriented orthogonal to the maximum principle horizontal stress, they remain propped open by calcite and quartz, as documented by macroscopic observations in core, gas kicks observed in mud-logs, and observed sonic anisotropy. Homogenization temperatures of fluid inclusions trapped in the veins, combined with 1-D burial and thermal history models, suggest that the pervasive vein set formed during maximum burial at the height of the Alleghenian orogeny (late Permian). The 1-D basin models indicate that a high level of maturity (vitrinite reflectance >2) existed during vein formation.