Generalized Contours of the Sauk Sequence for Characterization of Saline Aquifers for CO2 Sequestration

The Sauk sequence is the lowest major sequence of rocks (a largely uninterrupted depositional system, bounded below and above by major nonconformities [or periods of nondeposition]) within the Paleozoic strata of North America. The lowermost unit of this sequence is the Mount Simon Sandstone of Middle Cambrian Age. The Mount Simon Sandstone was deposited on a major nonconformity surface of eroded Precambrian Age strata. Within the southern portion of the mapped region, large Cambrian Age rift faults are responsible for dramatic accumulations of sedimentary rocks within the rift systems. Few wells have been drilled to appreciable depths within these rift sequences to allow characterization of their carbon dioxide (CO2) sequestration potential. These areas are blank on the Precambrian and Mount Simon maps.

Mount Simon Sandstone is an excellent candidate reservoir for CO2 sequestration. Favorable sequestration attributes of Mount Simon Sandstone include its relatively high porosity and permeability; its lateral continuity over a multistate area; its thickness; its occurrence at depth beneath thousands of feet of alternating sequences of carbonate, shale, and sandstone strata; and its stratigraphic position directly overlying relatively impermeable Precambrian Age strata. Mount Simon Sandstone underlies one of the most densely populated regions of the United States.

Previous interpretations of Mount Simon Sandstone have shown it to be present over most of Ohio and much of the Appalachian basin. However, current research at the Ohio Geological Survey indicates that Mount Simon Sandstone is not present in much of eastern and central Ohio. The basal stratum in these areas is mostly dolomite with interbeds of sandstone and sandy dolomite. This is the first publicly presented map showing the absence of Mount Simon Sandstone in central and eastern Ohio. More detailed maps on the occurrence and thickness of Mount Simon Sandstone will be made available via the MIDCARB Web site.

The Knox unconformity surface is the top of the Sauk sequence. Rocks of the Knox Group also are extensive, stretching across most of the interior of the North American continent. The Knox Group is largely composed of dolomite and dolomitic sandstone. In eastern and central Ohio, this interval is an important producer of oil and gas and may have potential for value-added sequestration of CO2. The presence of paleokarst-related porosity systems within the Knox Group may prove to be an effective CO2 reservoir in some places throughout the region. The horizon shown here mainly defines the top of the Sauk sequence—a 1,000- to 4,000-foot-thick interval of shale, siltstone, sandstone, dolomite, and limestone—that contains CO2, which could leak from a Mount Simon CO2 sequestration site.