GEOLOGICAL CARBON DIOXIDE STORAGE POTENTIAL OF SILURIAN AND DEVONIAN CARBONATES IN THE ILLINOIS BASIN

The U.S. Geological Survey (USGS) was tasked with developing a methodology (Brennan and others, 2010) and conducting a national assessment of the geologic storage potential for carbon dioxide (CO2) within the confines of the United States. Each storage assessment unit (SAU) identified in the assessment is defined by a regional seal formation that is at least 50 ft. thick which acts as a confining unit between strata, as well as a storage formation(s). Specifically, the storage formation must be able to contain CO2 in a supercritical state, so the upper depth limit of a storage formation is at least 3,000 feet below land surface. The lower depth of the storage formation is set at 13,000 feet, below which excessive pressures would be required for injection. In order to avoid sequestering CO2 in potential drinking or agricultural water, the total dissolved solids of the formation must exceed 10,000 parts per million. Within the Illinois Basin, shale of the New Albany Group makes an ideal seal for CO2 sequestration. The shale consists of two general shales – a black organic rich shale and a greenish gray organic poor shale. Collectively, the shale units make up a seal that is 420 feet thick at the deepest part of the basin. Eliminating the area of the New Albany that has tops above 3,000 feet and thickness less than 50 ft.; areas of low salinity; and structural highs where probable leaking occurs around the Rough Creek Graben, the shale seal provides an area of approximately 8,400,000 acres that is potentially suitable for sequestration. The underlying storage formations include the Middle Devonian carbonates, Dutch Creek sandstones, Lower Devonian carbonates, Upper Silurian carbonates, Upper Silurian calcareous siltstones and Lower Silurian carbonates. Isopach analysis of these units gives a mean total SAU thickness of 1,750 feet. Net porous interval thickness averages for the SAU averages around 240 feet, with a median porosity of 8%.