PALEOKARST EVENTS IN THE CAMBRO-ORDOVICIAN KNOX SUCCESSION, ILLINOIS BASIN

The Upper Cambrian-Lower Ordovician rocks in the Illinois Basin consist of alternating carbonate and siliciclastic units in the north; the siliciclastic intervals thin southward and, in the southern and deeper part of the basin, the succession consists chiefly of carbonate rocks classified as the Cambro-Ordovician Knox Group. The Knox (up to 2,300 m) conformably overlies the Mt. Simon Sandstone and is capped by the major sub-Tippecanoe unconformity. The dominant lithology of the carbonate units is fine to coarsely crystalline dense dolomite containing vugular, fractured cavernous intervals. Here, we focus on facies and stratigraphic variability of the Knox in Illinois and southwest Indiana using the available subsurface data and interpret the characteristics of the porous intervals as paleokarst features formed by basinal fluids unrelated to unconformity.

The units in the Knox Group, especially the Cambrian Franconia (Derby-Doerun) Formation, Potosi Dolomite, and Eminence Formation, and the Ordovician Oneota Dolomite commonly consist of dense dolomite in which intercrystalline porosity is very low or absent but contain fractured, brecciated, vugular, and cavernous intervals. The pore spaces are characterized by partial to complete infilling with chalcedonic silica, calcite, or dolomite. Multiple cavernous intervals are present that generally display anomalously high porosity, with caliper log signature departing from the baseline because of enlarged well diameter. Well data indicate that several hundred barrels of drilling fluid was lost during drilling in some intervals suggesting excellent reservoir permeability. We interpret the characteristics of the porous zones as hypogene paleokarst features unrelated to an unconformity and that they were likely controlled by basinal and/or hydrothermal fluid flow through earlier formed pore systems. The fractured/cavernous intervals are confined by thick impervious dolomite intervals, which could serve as effective seal. Thus, the Knox carbonates have excellent potential to serve as a combined reservoir and seal for storing anthropogenic CO2 and waste material.