ABSTRACT: Reservoir Quality Porosity in the Sauk Sequence, Illinois Basin

LONGMAN, MARK W., Independent, Lakewood, CO

Cambrian-Ordovician cores from the Sauk Sequence in the Illinois basin reveal several types of porosity capable of providing excellent reservoir quality. Both primary and secondary porosity are present within the Upper Cambrian sandstones of the Mount Simon, but the best reservoir rocks in this formation are well-sorted bimodal quartz sandstones deposited on the flanks of exposed basement knobs such as the St. Jacob Dome. Up to 18% primary interparticle porosity is present locally at a depth of 5000 ft. Secondary porosity formed by dissolution of cements was observed deeper in the basin.

Reservoir-quality porosity is rare in sandstones of the Upper Cambrian Eau Claire. This formation's best porosity occurs in the upper carbonate unit where up to 17% porosity is present. Pores are interparticle and vuggy in dolomitized ooid grainstones.

A variety of pore types occurs within the Knox Group, but only some offer good reservoir quality. Microporous cherts are present, particularly in the upper part of the Knox, but are unlikely to have significant permeability. Dolomites with intercrystalline porosity offer better reservoir potential directly related to size of the crystals. The best reservoir quality is seen in porous sandstones, particularly those interbedded with dolomites of the Shakopee in Indiana. Dolomites in which vuggy porosity is enhanced by fractures may also be important, but an enigma with regard to porosity in the Knox is the lack of porosity enhancement associated with dissolution at exposure surfaces at the top of the Oneota, Potosi, and Shakopee formations.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)