ADDRESSING THE CHALLENGE OF KNOX INTERVAL ROCKS IN THE ILLINOIS BASIN

Recent investigations of the Cambrian and Ordovician Knox strata in the Illinois Basin have characterized the reservoir quality of carbonate and clastic intervals, refined regional stratigraphy, and provided estimates of carbon-storage resources based on volumetric analysis, using multiple geologic models. Knox interval lithofacies include quartz-rich sandstone and several dolostone lithofacies: stromatolite/thrombolite boundstone, skeletal/peloidal grainstone, mudstone, intraclast breccia/conglomerate, and paleokarst breccia. Current work on the chemostratigraphy of the Knox aims to bolster regional interpolation and improve storage resource estimates. Petrophysical analysis of 136 core samples indicates a wide range in reservoir quality characteristics. Quartz-rich sandstones have the best reservoir properties of any lithofacies, averaging 9 percent porosity and 194 millidarcys permeability. Dolostone lithofacies varied in mean porosity from 3 to 6 percent and mean permeability from < 1 to 14 millidarcys, varying significantly within a given lithofacies. A particularly promising target for carbon storage is a sandstone unit in the Shakopee Dolomite in southern Indiana and western Kentucky, which has a maximum thickness of more than 100 m. This sandstone-rich interval appears to correlate with the New Richmond Sandstone of Illinois and is expected to have the highest porosity and permeability values. The entire Knox interval is characterized by a high degree of local variability in reservoir quality, complicating our ability to accurately model potential CO₂ storage reservoirs.

Multiple geologic models were developed in conjunction with probability-based storage efficiency factors to generate carbon-storage resource estimates of the Knox interval. Results demonstrate how the range in uncertainty varies as a function of data availability and quality, as well as the underlying assumptions used in the different models. The observed variability in results was evaluated in the context of methodological and data constraints, leading to the conclusion that storage resource estimates from simpler approaches are likely conservative, whereas an approach based on net porosity calculations may overestimate the resource.