Geology of deep saline sequestration
Geological sequestration of CO2 by injection into the subsurface is a promising technology under study around the world. Carbon dioxide is captured from the emissions of combustion systems, processed to remove contaminants, transported to a storage site, and injected into geological formations for very long-term storage. Three types of subsurface formations can be used — all of which exist in the Illinois Basin: deep coal formations considered to be uneconomical for mining and development in the foreseeable future, depleted or mature oil and natural gas reservoirs, and saline aquifers that contain non-potable water. Carbon dioxide injection into coal is being tested for methane recovery by replacing the naturally occurring, adsorbed methane with CO2. Injection of CO2 into oil reservoirs has been practiced as an oil recovery method for over 30 years. Deep saline reservoirs (rock units with salt water many times saltier than the ocean) have been used for natural gas storage in the Illinois Basin for decades and are now being considered for CO2 sequestration.
The target saline reservoir for sequestration in the Illinois Basin is the Cambrian Mt. Simon Sandstone. The Mt. Simon is a regional blanket deposit of sandstone that underlies large parts of the Midwest and almost all of the Illinois Basin. In most of the central and southern parts of the Basin, the Mt. Simon is very deep and is well below the depth of potable water aquifers. The Mt. Simon reaches a thickness of about 2,600 feet and is very thick in northern Illinois, east-central Illinois, and west central Indiana. The Mt. Simon Sandstone overall is a very fine- to coarse-grained quartz sandstone with arkosic intervals, particularly near the base of the formation. The sandstone typically has a relatively high porosity (many open voids between the sand-size grains which are currently filled with salt water). The CO2 would be stored in these small pore spaces. High porosity zones within the Mt. Simon are the specific targets for CO2 sequestration. The Eau Claire Formation is the primary seal, or caprock, that prevents upward migration of CO2 from the Mt. Simon Sandstone. The thickness of the Eau Claire ranges from 300 feet in the western and northern parts of the Basin to over 1,000 feet in the southern part of the Basin. This formation is composed of a silty, argillaceous dolomitic sandstone or sandy dolomite in northern Illinois and siltstone or shale in the central part of the Basin. Secondary seals include the Upper Ordovician Maquoketa Shale and the Upper Devonian New Albany Shale, both of which are relatively thick and widespread, fine-grained stratigraphic layers.