Abstract: Tidal Flats and Tempestites: An Integrated Approach to an Atypical Facies of the Cypress Sandstone

T. Joshua Stark, Richard Smosna, Kathy Bruner, Robert Davis, Jon Musselman

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 storm-swept tidal flat.

Developed above marine shales, a lower shoreface/platform facies is interbedded with carbonate-rich tempestite storm deposits. High clay concentrations within the sands create nonreservoir conditions, characterized by low permeabilities and high irreducible water content. As proximal tidal environments built 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 to core lithologies, and were successfully used to identify various facies within the Cypress Sandstone section. Early development wells flowed oil, due to overpressured reservoir conditions.

AAPG Search and Discovery Article #90984©1994 AAPG Annual Convention, East Lansing, Michigan, September 18-20, 1994