HIGH-RESOLUTION FACIES VARIABILITY IN THE UNION SPRINGS FORMATION OF THE NORTHERN APPALACHIAN BASIN

Sedimentary geologists turn to facies analysis to understand paleodepositional change across space and time. In the Marcellus shale, variability in the distribution of facies, as defined by compositional and textural parameters, is relevant to its success as a natural gas reservoir and to furthering the understanding of depositional environments within the Appalachian basin during the middle Devonian. This study characterizes the spatial heterogeneity of depositional event-scale physical properties in the Union Springs Formation of the northern basin margin.

The microfacies of three complete Union Springs columns spaced 60, 600, and 38,600 meters from a previous reference column are described using hand sample analysis, thin section petrography, and scanning electron microscopy. Features that distinguish the microfacies include mineral composition, bedding character, and faunal assemblage. Variability of these properties is described both across and within facies, and the distribution and arrangement of these facies are tracked across the studied stratigraphic sections.

Fourteen microfacies have been identified and placed into three broadly defined lithofacies: a limestone facies comprised of wackestones and packstones with thin organic-rich interbeds, a carbonaceous mudstone facies often abundant in calcite, and a concretionary mudstone facies defined by bedding-displacive calcite concretions. The positions of these facies and their spatial relationships reveal variable depositional conditions across the distal rim of the basin, inclusive of bottom-water oxygenation, tractive sediment transport that was inconsistent throughout space and time, and other features important to reconstructing the paleodepositional environment. Spatially variable early diagenetic conditions resulted in significant thicknesses of concretionary horizons as well as varying degrees of post-compaction cementation. An appreciation for these primary variations should lead to improved understanding of the geomechanical and geochemical properties of the formation in the modern day.