Chapter 21: Log-Derived Indicator of Thermal Maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming

Abstract The Cretaceous Niobrara Formation in the Denver basin is a highly organic, hydrocarbon-source rock. Accurate values of vitrinite reflectance (Ro) present in the Niobrara penetrated by 23 control wells were measured directly or derived by plotting the log of RO from two or more vertically-distributed measurements versus depth. These vitrinite-reflectance values when plotted against borehole resistivities for the “K” zone of the Niobrara in the wells, substantiate a linear log-log correlation between the two parameters that allows the mapping of organic maturation levels over a large part of the Denver basin through the use of a large number of available electric logs