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. In the Illinois Basin region, such a comprehensive effort of subsurface temperature data compilation and analysis hasn’t been attempted since the 1970s. This paper presents the methodologies that were implemented to develop this new geothermal database for the Illinois Basin region as part of the NGDS program. The database contains temperature observations from more than 26,000 wells in Indiana, Illinois, and Kentucky, providing a substantial increase in information on the thermal state of the subsurface. We also present some early results of data analysis, including new geothermal gradient maps and temperature anomalies, and how these results are being used in conjunction with other studies in the Illinois Basin. After compiling bottom hole temperature (BHT) data from geophysical logs, we conducted a number of quality assurance steps and filtering analyses. Primary tests involved graphical screening for outliers and mean statistics calculations to remove data that were beyond two standard deviations of the expected value. For Indiana and Illinois, many records with BHT values of exactly 100 degrees Fahrenheit were removed from the database because they resulted from the field operator using depth charts to assign a BHT value, rather than an actual temperature observation at depth. The BHT values at depths greater than 3,000 feet were corrected to account for error related to the circulation of drilling fluids. The Harrison equation was applied to all wells between 3,000 and 12,900 feet to approximate 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. Results from this project include updated maps of geothermal gradients throughout the Illinois Basin that show a higher resolution of subsurface thermal conditions than earlier products. Depth maps of temperature anomalies appear useful for identifying important subsurface structural features and areas that may have deep convection of groundwater. These data are being used to help evaluate results from new paleohydrologic model reconstructions that are aimed at identifying the impact of Pleistocene glaciations on the distribution of freshwater and brines across the Illinois Basin. Results of this project are also helping to improve the calculation of geologic storage resources for carbon sequestration in deep saline formations throughout the basin, because the density of carbon dioxide is partially dependent on the temperature of the subsurface storage formation.
|Original language||English (US)|
|Title of host publication||Abstracts - AAPG, Eastern Section Meeting|
|Publisher||American Association of Petroleum Geologists. Eastern Section, varies], United States|
|State||Published - 2012|