The Cincinnati Group as a Caprock: Implications for Utica Production and CO2 Sequestration

Hawrylak, Matthew; Daniels, Jeffrey; Cook, Ann; Bair, E. Scott; Welch, Sue; Sheets, Julie; Swift, Alex

Recent hydrocarbon exploration of the Utica Shale in Ohio has raised concerns among the media and public over the impact of hydraulic fracturing on groundwater geochemistry. An overlying poorly-permeable layer would tend to prevent vertical leakage on human timescales. Additionally, a proven caprock could potentially serve to contain sequestered CO2. In this study, we investigated the Cincinnati Group, a Late Ordovician shale that overlies the Utica Shale throughout the Appalachian Basin, which may provide the low permeability and geomechanical strength needed to prevent vertical leakage of fluids. The viability of the Cincinnati Group as a caprock was investigated via petrophysical and geophysical methods, including laboratory evaluation of lithology, permeability, and tensile strength, as well as with seismic and well-log data.

Gamma-ray logs and Ohio Geological Survey maps provided the depth, thickness, and areal extent of the Cincinnati Group; the formation dips and thickens to the southeast, ranging from 500 feet thick in northwest Ohio, to more than 3000 feet in some areas of eastern Ohio and West Virginia. We investigated the mineralogy via Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN), for the purpose of assessing the relationships between lithology, cementation, and permeability. The lower few tens of feet of the Cincinnati Group are carbonate rich (up to 60% carbonates) in some areas; however it quickly becomes clay rich (around 70% clay minerals) upsection. Gas-probe permeametry provided the permeability measurements necessary to determine the resistance to fluid flow; results show permeabilities on the order of 0.1 millidarcies. Geomechanical parameters determined from stress-strain testing and sonic logs help predict the conditions necessary to produce fractures. These results are used in volumetric calculations, to estimate the energy required to propagate a fracture upward to the base of potable groundwater, a distance ranging 800 to 8000 feet across the state. Seismic data from Allen and Scioto counties was used to locate faults and other potential influences on subsurface fluid flow, and thus provide examples of areas that should be avoided.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013