The Cambro-Ordovician Prairie du Chien and Knox Groups in the Subsurface of Central Illinois: Facies, Reservoir Potential, and Correlation

Askari, Zohreh; Lasemi, Yaghoob; Leetaru, Hannes E.

The Cambro-Ordovician Prairie du Chien and Knox Groups (Sauk II-III) in central Illinois consist of alternating dolomite and siliciclastic units. The Lower Cambrian Knox Group (1000-1400 feet thick) overlies the Mt. Simon Sandstone and includes, from base to top, the Eau Claire Formation, Galesville and Ironton Sandstones, Franconia Formation, Potosi Dolomite, and the Eminence Formation. The Lower Ordovician Prairie du Chien Group (450-700 feet thick) underlies, with the major sub-Tippecanoe unconformity, the St. Peter Sandstone and comprises the Gunter Sandstone at the base followed by Oneota Dolomite, New Richmond Sandstone, and the Shakopee Dolomite. To determine lateral and vertical lithofacies variations and evaluate the reservoir and seal potential of these deposits, a number of deep wells in the central third of Illinois were examined using subsurface data.

Detailed facies analysis indicates that the carbonate units consist of dolomitized mudstone to grainstone facies with relics of bioclasts, ooids, intraclasts and peloids, recording deposition on a shallow marine ramp setting. The sandstone intervals are porous and permeable and are texturally and compositionally mature. They may contain fine to coarsely crystalline dolomite intercalations, indicating deposition in a shallow to marginal marine settings. In addition, several permeable vugular or fractured/cavernous dolomite intervals that grade to dense fine to coarsely crystalline dolomite are present within the dolomite units. Several hundred barrels of fluid were lost in some of these porous intervals during drilling indicating high permeability. The permeable sandstone and dolomite intervals are laterally extensive and could serve as important reservoirs for natural gas or CO2 storage. The dominant lithology of the Knox and the overlying Prairie du Chien Group is fine to coarsely crystalline dense dolomite. The intercrystalline pore space of the dolomite was lost as a consequence of later stage diagenetic dolomite overgrowth or cementation. The dense dolomite intervals, therefore, could serve as an effective seal for the encompassing porous and permeable sandstone and dolomite intervals.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013