The ability of rock to generate oil and gas is directly related to the type and quantity of kerogen and to its thermal maturity; therefore, thermal maturity is a commonly used tool for oil and gas exploration. The purpose of this study ws to provide a thermal-maturity map for the lower part of the Upper Cretaceous Mesaverde Group in the eastern part of the Uinta Basin. Prior to this study, thermal-maturity data were not available for the Uinta Basin. This study uses coal rank to show the thermal maturity of the associated rocks. The map was prepared in cooperation with the U.S. Department of Energy under its western gas sands project.