HYDROGEOLOGY AND CO2 STORAGE CAPACITY OF THE ST. PETER SANDSTONE IN THE FOREST CITY BASIN, MID-CONTINENT, USA

The purpose of this research is to assess the potential of the St. Peter Sandstone in the Forest City Basin as a site for future geologic carbon sequestration. The St. Peter Sandstone is the deepest major aquifer in the Forest City Basin, a shallow, Paleozoic, intracratonic basin that straddles Missouri, Kansas, Iowa, and Nebraska and is close to several major coal-burning power plants. Using borehole data published by the geological surveys of these four states, a detailed three-dimensional stratigraphic model has been constructed. This model could be used to estimate the volume of pore space in the St. Peter Sandstone that lies below a depth of 750 m, the approximate threshold below which CO₂ exists as a supercritical phase under hydrostatic pressure, given the average regional geothermal gradient of 25°C/km. Although much of the St. Peter Sandstone in the Forest City basin lies above a depth of 750 m, about 67,000 km² of the St. Peter lie below this threshold near the depocenter of the basin. For an average porosity of 11% and typical CO₂ storage efficiency factors ranging from 0.4 to 5.5%, where sandstones typically lie toward the higher end of this range, then about 3 to 45 million tons of CO₂ could possibly be sequestered in the St. Peter Sandstone in the Forest City Basin. Total dissolved solids (TDS) content of groundwater in this region ranges from about 10,000 to 50,000 mg/L, which is high enough for the St. Peter to be used for waste disposal. Preliminary groundwater modeling carried out for the basin in the present study shows the basin depocenter to lie near a broad topographic low that causes groundwater flow to stagnate. These hydrogeologic conditions suggest that the St. Peter Sandstone in the Forest City basin is likely to have modest CO₂ sequestration capacity but that nonetheless is high enough to merit further investigation.