SEQUENCE STRATIGRAPHIC FRAMEWORK AND RESERVOIR CHARACTERIZATION OF THE CAMBRIAN GALESVILLE AND IRONTON SANDSTONES, ILLINOIS BASIN

This study focuses on sequence stratigraphic framework and reservoir characteristics of the Cambrian (Furongian) Galesville and Ironton Sandstones in Illinois using available subsurface data. The Galesville and Ironton Sandstones cover the northern half of the state and consist of porous quartz sandstone, dolomitic sandstone, and sandy dolomite. The Galesville is up to 100 feet (30 m) thick and overlies, with gradational contact, the Eau Claire Formation. The Ironton is over 100 feet (30 m) thick and consists of alternating porous sandstone and dense dolomite or dolomitic sandstone. It conformably overlies the Galesville Sandstone and underlies, with sharp contact, the glauconitic sandstone of the Franconia Formation. The Galesville and Ironton Sandstones thin southward and in south-central Illinois, they grade to carbonates in the upper part of the Bonneterre Formation.

Numerous erosive-based storm deposits, which may include sandy dolomite containing relics of ooids and bioclasts or sandstone with shale clasts and dolomite matrix are recognized particularly in the Ironton Sandstones. The sandstone intervals in Galesville and Ironton are exceptionally mature and display coarsening-upward cycles suggesting deposition in a shoreface setting. The storm beds and interfingering relationship of these units with the upper part of Bonneterre carbonates suggest deposition in a storm dominated mixed siliciclastic-carbonate depositional system. The Galesville-Ironton interval and their basinward carbonate equivalent constitute the uppermost part of Sauk II supersequence. They comprise the highstand package of a depositional sequence in which the gray glauconitic sandy shale in the upper part of Eau Claire represents the transgressive tract capped by coarsening upward shoreface deposits.

The reservoirs in Galesville and Ironton are generally fine to medium grained sandstone. Core analysis data in several wells record reservoir porosity and permeability as high as 27.8% and 4,330 md with average porosity and permeability of 12% and 202 md, respectively. The reservoir intervals of the Galesville and Ironton are encased in impermeable intervals; thus, these units have excellent potential to serve as repositories for waste disposal, anthropogenic CO₂ storage, or other energy storage applications.