ABSTRACT: Potential for Pre-Devonian Plays in Illinois

WHITAKER, STEPHEN T., and DONALD F. OLTZ, Illinois State Geological Survey, Champaign, IL

Hydrocarbon exploration and development in the Illinois basin began in the 1880s and has led to the production of over 4.5 billion barrels of oil, mostly from Mississippian clastic and carbonate reservoirs. Little attention has been given to sub-Devonian hydrocarbon exploration despite the fact that the few deep penetrations in the basin have revealed reservoir-quality rock in strata as old as the Cambrian. Although seismic data indicate that the sedimentary section in the basin reaches depths in excess of 23,000 ft, the deepest well is only 15,000 ft.

Silurian strata, containing the majority of current sub-Devonian plays in Illinois, have produced about 45 million barrels of oil. Three major Silurian plays have produced economic hydrocarbons: reefs, pinchouts of dolomitic reservoirs, and development of dolomitized reservoirs in basal Silurian paleovalleys. Potential new fields are likely in all three plays.

Upper Ordovician Trenton (Galena) production in Illinois has been established in 22 fields for 13.5 million barrels of oil. This production is limited to fractured limestone and dolomitic limestone reservoirs on structures. Possible locally developed dolomites on Trenton paleostructures offer a potential play.

There has not yet been any production in Illinois from rocks older than the Trenton due to the lack of exploration in lower Paleozoic strata. Several formations in these strata possess reservoir-quality rocks, and some shows have been reported.

Studies to identify potential source rocks within the deeper Paleozoic strata in Illinois so far have been limited and inconclusive. Because of the lack of deep penetrations and lack of core, sampling is not yet sufficient to define source potential in these strata.

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