REASSESSMENT OF THE 10,000 MG/L SALINITY BOUNDARY WITHIN POTENTIAL RESERVOIR TARGETS FOR CARBON SEQUESTRATION IN THE ILLINOIS BASIN

The Underground Injection Control program, under the authority of the Safe Drinking Water Act of the United States Environmental Protection Agency, regulates the injection of fluids into subsurface reservoirs. The program establishes a threshold value of 10,000 mg/l as the minimum value for the concentration of total dissolved solids within an aquifer to be considered suitable for the injection of waste fluids. The United States Geological Survey previously estimated the stratigraphic position of the 10,000 mg/l salinity boundary in the northern Illinois Basin. Of relevance for current carbon sequestration studies is the location of the 10,000 mg/l boundary line within the St. Peter Sandstone (Ordovician) and Prairie du Chien Group-Jordan Sandstone (Ordovician-Cambrian) aquifers. The depths of these stratigraphic units in the Illinois Basin vary, but at depths below 800 m, CO₂ occurs as a dense supercritical-phase fluid. Recent studies have measured direct salinity near this salinity boundary line in Macon County, Illinois, and results show a pore-water salinity of 3,900 mg/l sampled at a depth of 1,053 m within the St. Peter Sandstone. The goal of this research is to evaluate, through well log analysis and depth-dependent salinity models, how uncertainty in the salinity threshold value impacts the delineation of suitable sites for carbon sequestration below a depth of 800 m within the Trenton Limestone (Ordovician), St. Peter Sandstone, Knox Supergroup (Ordovician-Cambrian), and the Eau Claire Formation (Cambrian). Well bore data can, typically, provide constraints for the depth of a suitable reservoir and seal system. However, the relative lack of direct salinity measurements and the sensitivity of well log arithmetic transformations (due to limited data) render the demarcation of the salinity boundary to be much less certain. Results indicate that significant changes to the lateral position of salinity boundaries are expected in each of the carbon reservoir targets investigated in this study, more accurately constraining the geographic domain amenable for sequestration.