CO2 Storage and EOR Resource Assessment of the Cypress Sandstone Residual Oil Zone Play in the Illinois Basin

Nathan Webb, Nate Grigsby, Scott Frailey

Research output: Chapter in Book/Report/Conference proceedingConference contribution


Residual Oil Zones (ROZs) are volumes of rock in which oil accumulated and was later naturally displaced by encroaching water, leaving residual oil saturation in the range of 20 to 40%. Carbon dioxide enhanced oil recovery (CO2-EOR) is used to recover oil from known thick carbonate ROZs in the Permian Basin. In other basins, ROZs are not recognized or are ill-defined. Due to the technical and economic limitations of recovering oil at residual saturation to water, there have been few attempts at ROZ production. Therefore, a challenge in ROZ resource assessment is identifying ROZs and quantifying the oil in place. In addition to the economic benefit of oil production via CO2-EOR, ROZs have the potential for associated CO2 storage. In the Illinois Basin, an ROZ play has been identified in the Mississippian Cypress Sandstone. The results of the resource assessment of the Cypress ROZ play will be presented. The Cypress Sandstone includes a fairway of thick (up to 175 ft (53m)) multistory fluvial sandstones that are laterally continuous and vertically amalgamated with high porosity and permeability. Regional correlation and mapping using log data from 4,500 wells provided the basis for a net-sandstone isopach map. Conventional core analysis and porosity log data from ~2,000 wells were combined with the isopach map to generate an isoporosity map. Based on these maps, the thick Cypress fairway covers ~1.9 million acres and contains ~1 million acre-ft (1 billion m3) of pore volume. To identify the oil saturation distribution in the fairway, the Illinois State Geological Survey oil field database was searched for ROZ indicators (e.g. oil shows, core saturation). The Cypress ROZ play was delineated by 18,000 wells with ROZ indicators. To quantify the oil in place, 200 well logs were analyzed, providing thickness and saturation data to constrain the locations of ROZs prospects within the play. Using the median residual oil saturation from well log analysis of 23%, an estimated 1.7 billion barrels of oil in place is contained within the prospects, of which up to 168 million barrels is estimated to be recoverable using a continuous CO2 flood EOR factor of 9.9% assuming miscible conditions. Associated CO2 storage in the ROZs, not accounting for main pay zones or underlying brine aquifers, is estimated to be 7.6 billion tonnes, assuming 45 tonnes/1000 barrels. This resource assessment demonstrates the potential of the Cypress ROZ play for CO2-EOR and associated storage.
Original languageEnglish (US)
Title of host publication2019 AAPG Annual Convention and Exhibition, San Antonio, Texas
StatePublished - 2019


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