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Under-Explored Lower Paleozoic Strata in Illinois

Beverly Seyler, John P. Grube, Joan E. Crockett, Philip M. Johanek, Bryan G. Huff, Rex A. Knepp., Steven R. Gustison, Randy D. Lipking, Illinois State Geological Survey, Champaign, Illinois 61820

Lower Paleozoic units represent 50 percent of the volume of rock in the Illinois Basin yet only two percent of all exploratory wells penetrate these strata. Devonian, Silurian and Ordovician reservoirs account for 7.5 percent of the petroleum produced in Illinois. These reservoirs provide a guide to the potential for Lower Paleozoic production and the opportunity for new exploration targets in the Basin. Prolific Devonian reservoirs, both siliciclastic and several types of carbonate, have been established though lightly explored. Silurian reservoirs, commonly associated with various reef settings, exhibit compartmental characteristics that have both field development and new play potential. Non-reef Silurian production has also been established. Ordovician reservoirs are limited to the Trenton carbonate, closed structure fields leaving the prolific hydrothermal play as pure potential. Diagenetic alteration has played a key role in the formation of reservoirs in existing Lower Paleozoic reservoirs. These units have not been extensively explored, in part because of the greater drilling depths and the practice of penetrating deeper horizons mostly in fields where shallower production has been established. This strategy has brought sporadic results because many of the traps in Lower Paleozoic units are stratigraphic and structural closure may shift with depth. Updated contoured structure maps on key horizons illustrate shifting of some features with depth.

An Illinois Basin Consortium project funded in part by the U.S. DOE is focusing on existing petroleum reservoirs and the potential for new discoveries in Lower Paleozoic units. Major tasks for this project include creating a digital catalogue of existing reservoirs and developing models of the stratigraphic and structural framework of Lower Paleozoic units using available subsurface data. These products will be available in digital format on an ArcIMS website. Preliminary results suggest that applying new exploration strategies to the Illinois Basin can result in significant new discoveries in Lower Paleozoic units.



AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York