The Mississippian Thick Cypress Sandstone: A Nonconventional CO2-EOR Target in the Illinois Basin

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The Mississippian Cypress Sandstone is the most prolific oil reservoir in the Illinois Basin (ILB). A fairway of thick Cypress Sandstone that can exceed 46 m was deposited in an incised valley system in the ILB. In some places, these thick sandstones contain oil reservoirs in the top with the potential for an underlying residual oil zone (ROZ). These reservoirs typically have low primary recovery due to excessive water coning and are an under produced resource in the ILB. Nonconventional carbon dioxide (CO2) enhanced oil recovery (EOR) in such reservoirs provides economic incentive to mitigate CO2 emissions by storing rather than recycling CO2 compared to conventional CO2-EOR programs. This research focuses on quantifying the thick Cypress Sandstone CO2-EOR and storage resource in the ILB. Reservoir characterization using core and analogous outcrop data and analysis of open-hole geophysical logs of varying types and ages to identify the presence of ROZs are underway. If ROZs are present, the mechanism for their emplacement and the geologic characteristics of the formation must be understood so the distribution of ROZs can be predicted. Regional geologic characterization indicates that the thick Cypress Sandstone occurs as multistory fluvial sandstone bodies that become estuarine upward and were deposited in an accommodation limited setting. Marine incursions punctuate the fluvial sandstones with implications for reservoir quality, as marine beds create laterally continuous baffles to vertical fluid flow. Areas with the highest potential for CO2-EOR and storage occur where multiple sandstone bodies amalgamate and limit the thickness of the intervening marine strata while facilitating enhanced stratigraphic trapping due to differential compaction over the thick sandstones. Open-hole log analyses are being assessed to determine whether ROZs are present with early results showing oil saturation trends within the thick Cypress Sandstone indicative of ROZs. These methods are being refined to provide more confidence of the presence ROZs, with efforts being taken to collect cased-hole pulsed neutron logs and new core to validate open-hole log analyses. Methods developed here could be applied broadly, as analogous thick sandstones with oil reservoirs and potential ROZs are common in other Carboniferous strata in the ILB and likely in similar settings. Such regional resource assessments may provide industry with information needed to initiate CO2-EOR in the ILB.
Original languageEnglish (US)
StatePublished - 2016


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