(457a) Assessing the Potential of the St. Peter Sandstone and Everton Formation for CO2 Storage in Illinois: A Comparison with the Mt. Simon Sandstone

The Illinois Basin (ILB), covering most of Illinois, southwestern Indiana, and western Kentucky, exhibits substantial potential for CO₂ storage. Among the ILB geologic formations, the Mt. Simon Sandstone stands out as a prominent storage resource estimated ranging from 12 billion to 172 billion metric tons. The Illinois Basin-Decatur Project (IBDP) is a noteworthy large-scale injection project targeting the Mt. Simon Sandstone. This injection project, which commenced in November 2011 and was completed in November 2014, entailed the injection of 1 million metric tons of CO₂ into the formation. The successful completion of the IBDP provided localized validation of the capacity, injectivity, and containment of the Mt. Simon Sandstone for CO₂ storage.

While the Mt. Simon Sandstone has been recognized as a viable option for CO₂ storage, other geological formations within the Illinois Basin (ILB), such as the Ordovician St. Peter Sandstone and the Everton Formation, have also emerged as promising candidates for CO₂ storage. Specifically, these formations have been identified as potential options in several counties in southern Illinois, where the thickness of the Mt. Simon Sandstone is limited or absent.

A comprehensive regional study evaluated the storage resource and locations for CO₂ injection into the St. Peter Sandstone and Everton Formation. This study used petrophysical log data and a limited number of core samples and thin sections. Subsequently, a stratigraphic framework was developed by integrating lithological and petrophysical data to assess reservoir characteristics and trends regionally. Furthermore, a regional geocellular model was constructed using this data, with porosity and permeability data populated stochastically within the 3-D grids using regional reservoir property trends. This model enabled the identification of areas where the St. Peter Sandstone and Everton Formation are present and where sandstone intervals display favorable reservoir quality in terms of porosity, permeability, and lateral thickness. Additionally, this study conducted a comparative analysis between the St. Peter Sandstone and Everton Formation, evaluating their reservoir quality and lateral extension in comparison to the Mt. Simon Sandstone.

The evaluation of the petrophysical logs and core intervals indicate that the St. Peter Sandstone is predominantly characterized by its fine to medium, well-sorted pure quartz sandstone composition, which is notably devoid of clay minerals. The St. Peter Sandstone demonstrates an average porosity of 12% and an average permeability of 100 millidarcies in several counties in southwestern and southern Illinois. Furthermore, the Everton Formation locally exists in southwestern and southern Illinois and has two distinct intervals: the upper dolomite and the lower fine to medium grained, well-sorted quartz sandstone intervals. The Everton sandstone interval in southwest Illinois has reservoir properties similar the St. Peter Sandstone. The thickness map and porosity and permeability analysis of the sandstone deposits in southwestern Illinois reveal that the St. Peter Sandstone and the Everton Formation exhibit an average thickness of approximately 180 feet. In contrast, in southern Illinois these formations have a greater average thickness of over 530 feet. However, it is noteworthy that despite the greater thickness of these formations in southeastern and southern tip Illinois, the average porosity measures less than 5%, while the permeability is less than 10 millidarcies.

The thickness map of the Mt. Simon Sandstone in central and northeastern Illinois reveals a thickness variation, ranging from 600 to 2,600 feet. The lower portion of this sandstone is primarily composed of fine to coarse grained sandstone, with an average porosity of 18% and an average permeability of 200 millidarcies. Given its favorable reservoir properties, this lower portion has been identified as a suitable target for CO₂ injection. In designated areas, the thickness of the lower portion generally ranges from 200 to 500 feet.

Comparing these sandstones reveals that the St. Peter Sandstone demonstrates lower reservoir quality and thickness relative to the Mt. Simon Sandstone. However, in several counties in southwestern Illinois and southern Illinois, the St. Peter Sandstone can be considered a viable target for CO₂ storage, particularly in areas where the Mt. Simon Sandstone is absent. The Everton Formation exhibits similar reservoir quality to the St. Peter Sandstone, but its extension is limited to a few counties in southwestern Illinois.