Print this pageThe Geneva Dolomite, a basal member of the Middle Devonian Grand Tower Formation in the Illinois portion of the Illinois Basin, is an exploration target that has recently generated much interest. Wells from fields discovered in the 1990s have initial production rates exceeding 300 barrels of oil per day from depths of 3000 to 4000 feet. Cumulative production from some of these wells is greater than 100,000 barrels in approximately two years. A study in Marion, Fayette and Effingham counties, Illinois, shows that pronounced closure, fracturing, and development of secondary porosity, including dolomitization and solution enhanced moldic porosity, are needed for reservoir development and entrapment of petroleum from the Geneva Dolomite.
Draping of younger, Middle Devonian strata over Silurian reefs has resulted in approximately 100 feet of structural closure on the Geneva carbonates at Sandoval and Raccoon Lake fields. Miletus Field shows 60 feet of structural closure, possibly reef related. St. James Field is an example of a Geneva reservoir located over a structural fold with no apparent reef influence. Regional mapping shows that thinning of the Middle Devonian carbonate interval overlying the Geneva Dolomite reservoirs is common.
Examination of Geneva rock in Indiana quarries and core from central Illinois petroleum reservoirs shows similar characteristics. The dark brown, sucrosic, Geneva Dolomite with well developed moldic porosity is strikingly similar in both the quarries and the core. Dolomitization and porosity development is widespread and appears to be facies related. The greatest porosity development is correlated with facies that were originally composed of bioclastic packstones and wackestones.
To account for the widespread and uniform dolomitic character of the Geneva, we suggest a regional mixing-zone model of dolomitization. The model would require that the Geneva carbonates were subaerially exposed, subjecting the sediments to an influx of meteoric waters. The influx of these waters through the Geneva carbonates would establish a mechanism for dolomitization, the associated enhanced porosity and permeability and the solution and brecciation observed in the Geneva. Widespread subaerial exposure of the Geneva carbonates prior to deposition of the overlying sediment would establish the top of the Geneva as a sequence or parasequence boundary.
AAPG Search and Discovery Article #90926©1999 AAPG Eastern Section Meeting, Indianapolis, Indiana