Exploration Model for Shallow Silurian (Kankakee) Carbonate Reservoirs in Western Illinois: ABSTRACT

Abstract

Reservoirs in shallow (600-650 ft deep) basal Silurian Kankakee carbonates at Buckhorn consolidated, Siloam, and Kellerville oil fields in western Illinois have produced nearly 2 million bbl of oil, but were developed essentially by random drilling. A new exploration model that combines lithologic studies and isopach mapping has been developed at the Illinois State Geological Survey. Isopach mapping of Silurian and Devonian rocks between an organic facies in the Mississippian-Devonian New Albany Shale and the top of the Ordovician Maquoketa Shale reveals thickened sequences that coincide with most of the oil fields. These thickened intervals apparently reflect subtle paleovalleys eroded into the Maquoketa shale during the Ordovician-Silurian hiatus. During the initial Silurian marine transgression, these paleovalleys at the base of the Kankakee were filled with carbonates to form the thickened sequences. Differential erosion at the top of the Kankakee does not satisfactorily explain the locally thickened sequences in the Kankakee. Lithologic studies suggest that subsurface fluid flows concentrated along these paleovalleys contributed to subsequent diagenesis of valleyfill carbonates. Diagenetic alteration of these carbonates resulted in development of basal Kankakee reservoirs within the paleovalleys. This concept of Kankakee reservoirs occurring within paleovalleys at the Ordovician-Silurian unconformity is a new exploration modelmore » that can aid in the search for similar traps in western Illinois.« less