POROSITY CHARACTERIZATION AND DIAGENETIC FACIES ANALYSIS OF THE MOUNT SIMON SANDSTONE, ILLINOIS BASIN: IMPLICATIONS FOR A REGIONAL CO2 SEQUESTRATION RESERVOIR

The Cambrian Mount Simon Sandstone has been targeted as a primary reservoir for carbon dioxide injection for long-term sequestration. The Mount Simon Sandstone is the basal siliciclastic unit that overlies Precambrian crystalline basement and is conformably overlain by the Eau Claire Formation. Thickness in the reservoir varies from a few hundred feet in the southern Illinois Basin to over 2500 ft to the north and spans the Midwest. The Mount Simon Sandstone’s candidacy as a CO2 sequestration reservoir is due in part to its widespread lateral extent near CO2 point sources, high estimated capacity, and appropriate geologic setting (good seal, lack of faulting, etc.). The Mount Simon Sandstone is a mature quartz to arkosic arenite, although details of the mineralogy, porosity, and diagenetic and depositional influence on reservoir properties have not been previously studied basin-wide or spatially with depth. Prior to this study, increased pressure solution and quartz overgrowth precipitation with depth have been interpreted as the primary porosity reduction mechanism, however, this work demonstrates that there are multiple other factors that influence porosity in this formation. Petrographic image analyses were conducted on 150 thin sections from a range of depths across the Illinois Basin to assess the distribution of effective porosity and characterize pore types. Over ten dominant and supplementary types of pores have been identified within the formation. Shallow depths show depositional influence on porosity and intermediate to good pore connectivity. Deeper samples exemplify diagenetic influence with variable extremes of greater porosity generation and destruction either by dissolution or compaction. Variations in authigenic cements such as iron bearing illitic clays, goethite, hematite, kaolinite, quartz and feldspar overgrowths, calcite, and pyrite also influence porosity. Ten diagenetic facies are interpreted based on authigenic mineraological composition and abundances, degree of dissolution, and lithological differences and were mapped to aid in interpreting diagenetic processes and overall influence on the reservoir. Understanding details of the CO2 reservoir’s composition is essential for predicting mineral reactivity and potential fluid flow pathways.