ABSTRACT: Porosity Development in the Knox Megagroup of the Illinois Basin

SIKORSKI, PETER, Purdue University, West Lafayette, IN

The Cambrian-Ordovician Knox Megagroup of the Illinois basin is considered equivalent to the Arbuckle of the Great Plains, the Prairie du Chien of Michigan, and the Ellenburger of Texas; all the latter locally have prolific hydrocarbon production. This production, in part, apparently is related to post-Sauk subaerial weathering. The Knox of the Illinois basin likewise was beveled and subaerially weathered. It has exhibited minor hydrocarbon shows but is not yet productive.

The Knox is predominantly a carbonate sequence, specifically dolomite, with two minor included clastic formations. Intercrystalline porosity, fracture porosity, and vugs characterize the dolomites. This porosity is related to the development (formation of karst dissolution and brecciation features along with multiple episodes of dolomitization, silicification, and cementation) of its paleoaquifer system. Fractures and the intergranular porosity of the minor sandstone formations and stringers within the dolomite play a significant role in fluid migration and accumulation. Original porosity may be occluded by calcite, dolomite, anhydrite, barite, and/or quartz cement.

Study of logs and samples indicate that substantial porosity is found at depth within the Knox, rather than confined exclusively to immediately below the sub-Tippecanoe unconformity. One of the few deep cores from the Knox, 2827 m subsea and 478 m below the unconformity in the Illinois part of the basin, exhibits open vugs as large as 1 cm.

Only several dozen holes in the deep south-central part of the Illinois basin penetrate the sub-Tippecanoe unconformity; some holes penetrate only a few meters. This suggests the advisability of drilling more and complete sections of Knox to adequately test its exploration potential.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)