Abstract
The Cypress Sandstone (Mississippian Chesterian series) is a major hydrocarbon-producing formation in the Illinois basin. Depositional settings consisting of fluvial and deltaic environments are recognized throughout the basin. Upon the southern flank of the Moorman syncline in the Stringtown field, a distal facies of the Cypress depositional system is recognized as a stormswept tidal flat. Developed above marine shales, a lower shoreface/platform facies is interbedded with carbonate-rich tempesite storm deposits. High clay concentrations within the sands create non-reservoir conditions, characterized by low permeabilities and high irreducible water content. As proximal tidal environments build seaward, a scour channel downcut into the lower shoreface/platform facies. Continuing tidal flat progradation deposited lower and middle tidal-flat pay sands on the scour surface. Overlying upper tidal-flat strata provide the seal for this reservoir. Regional transgression destroyed the balance of the prograded tidal flat, preserving the sequence only within the deep scour features. Lithologically, the pay sands are classified as quartzarenites and sublitharenites, the latter containing appreciable concentrations of shale rock fragments and feldspars. The development of secondary moldic porosity is the primary determinant of reservoir quality, derived from the dissolution of rock fragment fabrics. Geophysical well logs and Formation MicroScanner data analysis were correlated tomore » core lithogies, and were successfully used to identify various facies within the Cypress Sandstone section. Early development wells flowed oil, due to overpressured reservoir conditions.« less
Published Version
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