Effects of Diagenesis on Reservoir Quality Within Two Cypress Reservoirs in the Illinois Basin

BEATY, D. SCOTT, KENNETH R. MCGEE, and BEVERLY SEYLER, Illinois State Geological Survey, Champaign, IL

One billion bbl of oil have been produced from the Chesterian Cypress Formation in the Illinois basin. These heterogeneous reservoirs may consist of deltaic, marine-reworked deltaic, and/or reworked marine sandstone within mixed siliciclastic-carbonate environments. Thin section, x-ray diffraction, and scanning electron microscopy coupled with energy dispersive x-ray analysis indicate that the effects of diagenesis play a significant role in reservoir quality at Mattoon and Parkersburg fields in Illinois. Five separate Cypress sandstones may be present at Mattoon field (Coles County), a structural/stratigraphic trap. Parkersburg field (Edwards Company), a predominantly stratigraphic trap, produces from three distinct Cypress strata.

In these fields, reservoir quality is reduced when quartz overgrowths and later stage, blocky mosaic ferroan-calcite cement occlude pore throats. Authigenic clay minerals occur as pore-lining particles that inhibit fluid-flow. Clay minerals present are illite, mixed-layered illite/smectite, chlorite, and kaolinite. Reservoir quality is enhanced through dissolution of early ferroan-calcite cement, dissolution of detrital feldspar, and microfracturing.

Completion, stimulation, and production programs within the heterogeneous Cypress sandstone reservoirs would be improved by recognition of mineral relationships and diagenetic overprints. Developments programs may need to include the use of clay stabilizers in mud clean-out acid treatments.

AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)