THE MISSISSIPPIAN CYPRESS SANDSTONE: GEOLOGIC CHARACTERISTICS OF A POTENTIAL EOR TARGET IN THE ILLINOIS BASIN

The Chesterian (Upper Mississippian) Cypress Sandstone has produced over 1 billion bbl of oil in the Illinois Basin (ILB) in the past century and is a candidate for carbon dioxide enhanced oil recovery (CO₂ EOR) and geologic storage. Detailed reservoir characterization research, supported by US Department of Energy and Illinois Department of Commerce and Economic Opportunity grants, was conducted in Lawrence and Mattoon Oil Fields in Illinois to assess the suitability of Cypress reservoirs for CO₂ EOR and storage. Mapping reservoir bodies, identifying flow units, and understanding permeability barriers and potential thief zones with respect to production and injection wells are critical to the success of CO₂ EOR and storage projects.

Much of the oil produced from the Cypress has come from highly compartmentalized, stacked sandstone bars that formed in a low accommodation shelf setting, where a high tidal range deposited sediments into elongate, shore-normal bars. Mapping in Lawrence and Mattoon Fields has identified pervasive very fine- to fine-grained sandstone bars up to 3 m thick, less than a kilometer wide, and up to a few kilometers long. Other sandstone facies are also present in the Cypress as intraformational sequence boundaries separate the bar facies from lithologically similar, but genetically distinct, fluvial and deltaic sandstone facies within the Cypress in different areas of the ILB. Thus, detailed mapping of recognizable facies geometries is necessary to distinguish depositional environments from one another.

Sandstone reservoirs were mapped using geophysical logs. Cores were described and petrographic analyses were conducted to better understand porosity and permeability trends in the region and to characterize permeability barriers and define flow units. Diagenetic alterations that impacted porosity and permeability include development of quartz overgrowths, sutured quartz grains, dissolution of feldspar grains, formation of clay mineral coatings on grains, and calcite cementation. This detailed characterization of the Cypress will assist future simulation modeling and injection of CO₂, and may encourage similar CO₂ EOR and storage projects in similar geologic settings around the world.