A PETROGRAPHIC INVESTIGATION OF THE STRATIGRAPHY BENEATH THE BALL STATE UNIVERSITY GROUND-SOURCE GEOTHERMAL SYSTEM

Ball State University (BSU) is in the process of constructing the world’s largest ground-source geothermal system that utilizes the shallow lithology and groundwater as a thermal reservoir. Although the stratigraphy is generally known, a detailed stratigraphic architecture will aid the understanding of the true nature of the subsurface capacity to store and transport thermal energy. We consider data from five research wells drilled in 2012–2013 during construction of Phase 2 of the BSU geothermal conversion. Data include drilling logs and cuttings as well as gamma logs collected in collaboration with the Indiana Geological Survey. Two local cores were examined at the Indiana Geological Survey to provide correlation and aid in interpretation. Complementing these data are drilling logs for the 600+ boreholes drilled to accommodate the heat exchange loops. These data were examined to construct a detailed stratigraphic log and a map of subsurface geology for the Phase 2 field.

Our results expectedly show layered dolostone, limestone, and shale overlain by glacial till. The glacial till ranges from 10–30 m in thickness, and mantles a karstified bedrock surface that descends southeast toward the pre-glacial Anderson River. Formations in the study area slightly thicken towards the southwest. Local dip is irregular but generally trends towards the southwest at approximately 1º. Silurian strata and their thicknesses include the Pleasant Mills (11m), Waldron Dolostone (5m), Salamonie Limestone (12m), and Cataract Formation (12m). Most limestone/dolostone units are massively bedded. Upper Ordovician shale and dolostone of the Maquoketa Group include the Brainard Shale (7m), the Whitewater Formation (24m), and the Dillsboro Formation (33m). Both the Brainard and Dillsboro are interbedding on the mm to cm scale. All five wells end in the Ordovician Kope Formation.