The lower Paleozoic Sauk sequence of northern Ohio represents a carbonate and clastic sedimentary sequence deposited during the late Late Cambrian Saint Croixan and early Early Ordovician Canadian. It is bounded below by the unconformity on the Precambrian basement and above by the Knox unconformity. The Sauk sequence was deposited throughout the study area of northern Ohio, which includes the western portion of the Appalachian basin, the Ohio-Indiana platform to the west, and the southernmost portion of the Michigan basin northwest of the Ohio-Indiana platform. The major lithostratigraphic units, all in the subsurface, are the Cambrian Mount Simon Sandstone, Shady Dolomite, Eau Clair Formation, Rome Formation, Conasauga Formation, and Kerbel Formation, and the Cambrian-Ordovician Knox Group.
Stratigraphic interpretations generally show that the Sauk sequence begins with clastic deposition occurring in the north-central and western portions of Ohio and predominantly carbonate and open-marine clastic deposition occurring in eastern and south-central Ohio. The location of the present Cincinnati arch marks a transition zone between the two sedimentary regimes. The final stage of Sauk sequence deposition was marked by a major marine transgression that resulted in deposition of the Knox Dolomite over the entire study area.
Producing reservoirs occur in four stratigraphic-structural settings. The Copper Ridge Dolomite (Knox Group) on the eastern edge of the Ohio-Indiana platform has oil production from glauconitic sandstone reservoirs.
Basinward, the Cambrian dolomitic Rose Run Sandstone Member occurs at the top of the Copper Ridge Dolomite. Mainly gas has been discovered in the Rose Run in secondary reservoirs formed during erosion of the Knox unconformity. Middle Ordovician shales and impermeable limestones are the seals.
The majority of hydrocarbon production from the Knox Dolomite is located in central Ohio’s Morrow County where erosional remnants of vuggy dolomite on the Knox unconformity form paleotopographic highs with Middle Ordovician shale as the seal. Other small pools consist of vuggy dolomite reservoirs with small structural closures.
Although no significant amounts of hydrocarbons have been discovered below the Copper Ridge Dolomite, high porosities and permeabilities in the Mount Simon Sandstone and Eau Claire Formation lend potential to these rocks for liquid waste disposal.