STRATIGRAPHIC VARIABILITY, SECONDARY POROSITY DEVELOPMENT, AND CORRELATION OF THE CAMBRIAN POTOSI DOLOMITE ACROSS THE ILLINOIS BASIN

The Cambrian (Upper Furongian) Potosi Dolomite in the Illinois Basin consists of dense dolomite intercalated with highly porous and permeable intervals. Here we focus on facies trend, porosity development, and correlation of the Potosi across the Illinois Basin using wireline geophysical logs, cores, and well cuttings. The Potosi is a relatively pure dolomite unit that conformably underlies and overlies the relatively impure Franconia and Eminence Formations. It ranges from 100 feet (30 m) in northern Illinois to over 600 feet (183 m) in southern Illinois. The Potosi is a dolomitized fine to coarsely crystalline mudstone to grainstone facies containing microbialites and relics of bioclasts, ooids, and peloids deposited in a shallow marine ramp setting.

Throughout the Illinois Basin, the Potosi is commonly dense, but it is interlayered with vugular, fractured and/or cavernous intervals generally lined with drusy quartz. Horizontal and vertical to near-vertical mineralized to open fractures, collapse breccia, and solution-enhanced fractures are common. Individual highly porous intervals are up to 10 ft. (3 m) thick and commonly cause lost circulation of drilling mud when penetrated. We interpret the characteristics of the porous zones in the Potosi as paleokarst features unrelated to unconformity, and that they were likely controlled by basinal and hydrothermal fluid flow through initial pore system. The development of this buried paleokarst and related secondary porosity appear to have been related to multiple stages of diagenesis of precursor limestone and early to late-stage dolomite formation. An early porosity network formed by early seawater dolomitization at certain intervals may have driven the initial flow of basinal and hydrothermal fluids. Reservoir quality in the porous intervals was later enhanced by carbonate dissolution and brecciation. The porous intervals in the Potosi are widespread in the Illinois Basin, thus, the formation has an excellent potential to serve as a combined reservoir and seal for storing anthropogenic CO₂ and waste material.