PETROGRAPHY OF THE CHESTERIAN (UPPER MISSISSIPPIAN) CYPRESS SANDSTONE IN THE ILLINOIS BASIN

The Upper Mississippian Cypress Sandstone in the Illinois Basin (ILB) includes a fairway of thick sandstones, interpreted as incised valley fill (IVF) deposits, which contain relatively thin oil reservoirs developed in the tops of the sandstones. These IFV Cypress Sandstones are composed of a range of sedimentary facies, which can be classified as either reservoir or nonreservoir facies based on their petrological properties. Geologic controls on porosity and permeability, including mineralogy, grainsize and sorting, compaction, and cementation, were assessed through petrographic analysis of IVF Cypress Sandstone core samples to classify sedimentary facies according to their reservoir properties.

The IVF Cypress Sandstones are generally composed of quartz with <10% feldspars and rare (1–2%) calcite occurring primarily as cement. Fine-grained, cross-bedded sandstones and ripple-bedded sandstones constitute the reservoir facies. Porosity is generally primary intergranular porosity with some secondary porosity resulting from dissolution of feldspar grains. Clay minerals and quartz overgrowths can occlude porosity and permeability in the cross-bedded sandstones, which have the highest average porosity and permeabilities at 18% and 755 mD, respectively. Ripple-bedded sandstone is much lower quality at 16.6% and 31.9 mD. Very fine grained flaser-, wavy-, and lenticular-bedded sandstones compose the nonreservoir facies and show a much higher degree of compaction than reservoir facies, with quartz overgrowths and clay minerals filling available pore space. This leaves only secondary porosity from the dissolution of feldspar grains and carbonate cements. The relatively low permeabilities of the nonreservoir facies, generally <10 mD, indicate that they may form baffles to fluid movement within the reservoir.

This petrographic study is part of an assessment of nonconventional carbon dioxide enhanced oil recovery (CO₂-EOR) of the IVF Cypress Sandstones in the ILB, which is expected to store additional CO₂ compared to conventional CO₂-EOR. Results of petrographic analysis of the IVF Cypress Sandstone reservoir will be presented and will demonstrate that petrographic analysis, when combined with sedimentary facies analysis, is a powerful tool for characterizing IVF Cypress Sandstone reservoirs.