DEVELOPMENT OF A NEW GEOTHERMAL DATABASE FOR THE ILLINOIS BASIN

The U.S. Department of Energy is currently funding a broad consortium of institutions to develop a new National Geothermal Data System (NGDS) for assessing the geothermal resources of the United States. During the 1970s, course maps of heat flow were produced. Numerous wells have since been drilled in the Illinois Basin region, of which bottom-hole temperatures (BHTs) were recorded during the drilling process. Unfortunately, the bottom-hole temperatures have never been viewed on high resolution maps. We compiled bottom hole temperature data from geophysical logs and created a detailed database that contains observations of temperature from more than 26,000 wells in Indiana, Illinois, and Kentucky, providing a substantial increase in information on the thermal state of the subsurface. Quality control of the data was based on reduction of duplication, correction of erroneous depths or temperature and elimination of faulty data acquisitions. For Indiana and Illinois, many records with bottom-hole temperature values of exactly 100ºF were removed from the database because they resulted from the field operator using depth charts to assign a BHT value, rather than an actual observation of temperature at depth. The BHT values at depths between 3,000 and 12,900 feet were corrected using the Harrison equation to account for error related to the circulation of drilling fluids and better represent an equilibrium, or ‘in-situ,’ temperature. Geothermal gradients were calculated for each well by using a mean ambient surface temperature at the well location from values obtained from the PRISM Climate Group. Mean geothermal gradients were also calculated for specific geologic intervals. Updated maps of geothermal gradients and temperature anomalies throughout the Illinois Basin show a higher resolution of subsurface thermal conditions than earlier products. Preliminary results for this study suggest that higher gradient heat flows occurred at shallow depths whereas low gradient heat flows occurred at deeper depths.