Abstract In response to rising concerns about atmospheric carbon dioxide (CO 2 ) levels and likely regulations on emissions, investigations into geologic carbon storage options across the United States are underway. In the Midwest, Cambrian sandstones are major targets for potential geologic carbon storage. In some localities, the overlying Cambrian–Ordovician Knox Group is also being investigated as a possible target for primary and secondary storage of CO 2 . The thick dolomitic succession contains intervals that may function as both reservoirs and seals. Gas storage fields in Knox carbonates in Kentucky and Indiana demonstrate that methane can be safely stored in paleotopographic highs along the Knox unconformity surface. Numerous injection wells have also been completed in the Knox Group for brine disposal. More significantly, at least seven class 1 injection wells have used the Knox as all or part of a storage reservoir for industrial wastes. Many of these wells have injected millions of gallons of liquid waste annually into Knox reservoirs. The relative scale of these injection operations can be used to estimate the types and sizes of potential reservoirs within the Knox succession in the Midwest. Specific data on the Knox interval relative to its carbon storage and confining potential are currently being collected from wells drilled as part of U.S. Department of Energy administered carbon storage projects, as well as state-administered carbon storage programs. In this chapter, initial results of carbon storage tests are summarized from the Battelle 1 Duke Energy well, Kentucky Geological Survey 1 Blan well, Battelle-American Electric Power (AEP) 1 Mountaineer well, and Battelle-Ohio Geological Survey 1 CO 2 well. The AEP Mountaineer Power Plant will host the nation’s first commercially integrated carbon capture and geologic storage project, and the storage reservoirs will be in the Knox Group. Because the Knox Group is widespread at depth across much of the Midwest, it will be an important part of sequestration programs as confining interval and reservoir.

Abstract In the eastern mid-continent United States, the Cambrian Mt. Simon Sandstone is a likely deep-saline reservoir target for CO 2 sequestration. The overlying Cambrian–Ordovician Knox carbonate section will be an important part of the confining interval for the Mt. Simon, as much of the Knox is dominated by dense (<0.01 md), well-cemented dolomites with little or no permeability. The Knox, however, does contain discrete zones of porosity and permeability and is locally an important oil and gas producer, as well as gas storage unit. The Knox needs to be considered in any sequestration project in the region because in some localities carbonate and sandstone zones within the unit have better reservoir characteristics than the underlying Mt. Simon or overlying St. Peter Sandstone. An example of such a locality is the DuPont waste-injection site at Louisville, Kentucky, where a thick Mt. Simon section was tested and then abandoned in favor of a fractured, vuggy dolomite facies in the overlying lower Knox with an injectivity rate as high as 568 liters per minute (150 gallons per minute). Thick, dense carbonates of the Knox enveloped the reservoir effectively sealing the porous and permeable zones within the same stratigraphic unit. This is not an exceptional circumstance because several deep tests of Cambrian–Ordovician clastics in the region have encountered tight sandstone in the target horizon but vuggy and fracture porosity in overlying Knox carbonates. Analyses of known Knox enhanced oil recovery operations, waste-injection wells, and gas storage fields illustrate that liquids and gases can be effectively and safely retained within Knox reservoirs. However, porous and permeable zones within the units that constitute the local reservoirs are discontinuous and heterogeneous, and data describing the detailed characteristics of these reservoirs are sparse. More deep subsurface data are needed to better characterize the Knox and similar carbonates in other regions for their use as potential carbon sequestration reservoirs. Some of these data are currently being collected through the U.S. Department of Energy’s Carbon Sequestration Regional Partnership programs.