A Regional Lithostratigraphic Model of the Eau Claire Formation (Cambrian)

Richard W. Lahann¹, Cristian Medina¹, John A. Rupp¹, Thomas R. Lovell², Brenda B. Bowen², David A. Barnes³, John B. Hickman⁴, Ralph Bandy⁴, and Joel Sminchak^5
1Indiana University, Indiana Geological Survey, Bloomington, IN, [email protected]
²Purdue University, Department of Earth and Atmospheric Sciences, W. Lafayette, IN, [email protected]
³Western Michigan University, Michigan Geological Survey, Kalamazoo, MI, [email protected]
⁴University of Kentucky, Kentucky Geological Survey, Lexington, KY, [email protected]
⁵Battelle Memorial Institute, Columbus, OH, [email protected]

Several studies have been published that evaluate the potential of the Mount Simon Sandstone (Cambrian) to serve as a CO2 storage reservoir. However, relatively few studies have examined the sealing properties of the overlying Eau Claire Formation (Cambrian) or the regional variation of those sealing properties.

For this study, suites of wireline logs from 77 wells from Michigan, Ohio, Kentucky, Illinois, and Indiana were used to define electrofacies for the Eau Claire interval. The electrofacies were defined using a clustering program, the software “Geological Analysis of Maximum Likelihood System” (GAMLS). One well per county was chosen in an attempt to avoid spatial bias in the clustering process. Many counties were not represented either owing to an absence of drilling or inadequate log control. The wells that were used contained a gamma-ray log and at least two porosity-related logs (sonic, density or neutron). The log data were conditioned within GAMLS prior to clustering. The cluster run was seeded with the gamma-ray logs and then divided into twelve electrofacies, which were assigned to seven lithofacies: (1) argillaceous dolostone/dolomitic sand, (2) dolostone, (3) clean silt, (4) muddy silt, (5) silty shale, (6) dolomitic shale, and (7) clean shale. The choices of lithofacies were based on mean log responses for the cluster mode. The abundance of silt-sized feldspar made differentiation of high-gamma siltstones from shales problematic. The validity of the lithofacies assignments were confirmed by core description, petrology, and inorganic geochemistry for selected sites in Illinois and Indiana. Confirmation using locations in Ohio, Michigan, and Kentucky is in progress.

Examination of the GAMLS models for lithofacies across the five-state region indicated the development of a significant silty package in the lower half of the Eau Claire in NW Indiana. This package thins to the southeast. The other distinctive regional pattern observed was an increase in sandy dolomite from west to east across Kentucky into the laterally equivalent Conasauga Formation or Group in Kentucky and Ohio. Additionally, an increase in shale content was interpreted towards the center of the Michigan basin. How these multiple lithofacies vary both vertically within the interval that is designated as the confining unit and how they vary laterally across the region will control the effectiveness of the seal and control storage practices.

AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012