ABSTRACT: The Mississippian Cypress Formation on Interstate 57, Union County, Illinois-A Demonstration of Lateral and Vertical Heterogeneity
COLE, ROBERT D., and DENNIS J. HAGGERTY, Illinois State Geological Survey, Champaign, IL
The Cypress Formation, the most prolific Mississippian producer in the Illinois basin, has been targeted for both regional and detailed outcrop and subsurface study. An outcrop of Cypress exposed in four parallel roadcuts on Interstate 57, each approximately 1,000 ft long and ranging from 7 to 27 ft in thickness, was examined in detail. Stratigraphic sections were measured every 50 ft along 700-ft segments of each roadcut to form a grid of 60 measured sections. Each section was logged with a hand-held gamma ray spectrometer/scintillometer, and air permeability readings were taken with a minipermeameter every 6 in. vertically. Two core plugs, 4 in. in length by 2 in. in diameter, were drilled from each section to determine both permeability and porosity in the laboratory.
These roadcuts exhibit significant lateral and vertical heterogeneity. Offshore bars, low-amplitude, lenticular scour and fill intervals, and restricted, plane-laminated sheet and wedge-planar sandstones are the most common depositional facies present in these roadcuts. These near-shore, shallow marine deposits grade laterally into shales at each end of each roadcut. No single depositional facies extends the entire length of the study area, and many pinch out within the distance between two measured sections. Several shaly to flaggy-bedded sandstone intervals would probably inhibit vertical communication of fluids in the subsurface, and lateral communication is apparently affected by the restricted distribution of depositional facies.
Approximately 1,800 permeability readings were taken and 106 core samples were cut to correlate quantitative engineering data with the detailed geological descriptions. In the lab, the core samples were analyzed for air permeability and porosity. These values were then compared to those from the minipermeameter. This procedure was repeated for all the plugs, and the permeabilities from each procedure were correlated. Results show a wide range of permeability in the outcrop ranging from a high of 1 darcy to a low of 1.8 md. Porosity values were consistently between 15% and 19%. A detailed three-dimensional computer model was constructed which demonstrates the scale of flow units and other aspects of heterogeneity. We believe that this study offers an explanation for production variatio s exhibited by some Cypress sandstone reservoirs.
AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)