Silurian Pinnacle Reef Distribution in Illinois: Model for Hydrocarbon Exploration

Stephen T. Whitaker

Approximately 92 million bbl of oil have been produced in Illinois from buried Silurian pinnacle reefs and from younger strata draped over these reefs. Better understanding of Silurian reef distribution and the use of appropriate exploration methods should lead to the discovery of new reef-associated hydrocarbon reserves.

Evidence presented in this study suggests that Silurian pinnacle reef development was not limited to hinge-line trend around a subsiding basin center. Instead, isolated reefs grew through most of Illinois along a broad ramp dipping gently southeastward under a relatively shallow sea that opened to the south during the Silurian. Uplift of the Wabash platform in Indiana enabled concurrent pinnacle reef development along its flanks and formed the Fort Wayne and Terre Haute banks. These reef banks merged with and extended the scattered trends in Illinois.

Erosion of Silurian strata prior to the Middle Devonian, particularly along the emerging Sangamon arch, removed or reduced the pinnacle reef structures across much of the central Illinois. These reef remnants are not easily detected by exploration methods commonly used in the basin, yet they can be oil-productive. Applications of geophysical and detailed lithologic surveys can greatly enhance the ability to locate these reefs.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.