The organic-rich marls within the Niobrara Formation act as source rocks while the chalks act as reservoirs. The hydrocarbon potential of a source rock is highly dependent on its organic content and maturity. Different empirical relationships were developed to estimate TOC from logs. This study will look into and compare the different methods of quantifying TOC from logs and how they apply to the Niobrara formation. Most of these methods were initially developed for Devonian Shales (Passey et al., 1990; Schmoker 1980). With the Niobrara being Cretaceous in age, there is a need to test and analyze those methods and their applicability within the Niobrara Formation.
The Schmoker, Delta Log R, and Uranium log derived TOC methods were tested on the Niobrara Formation in the studied well. Results show that the distinct petrophysical log responses of the Niobrara Formation play a significant role in controlling the effectiveness and applicability of these methods in quantifying TOC in the Niobrara Formation. Prinicipal component analysis (PCA) performed on Niobrara well logs resulted in five different log groups each influenced by a different measured formation property in the Niobrara. Petrophysical analysis of the well logs and the PCA groupings shows that the Niobrara exhibits distinctive high resistivities in the chalks caused by lithologic and clay bound water variations. This petrophysical behavior of the Niobrara has compromised the effectiveness of the Delta Log R method in the Niobrara Formation. The bulk density log behavior in the Niobrara Formation was found to not be influenced by changes in organic matter which has also compromised the effectiveness of the Schmoker log derived TOC method in the Niobrara Formation. PCA analysis showed a close relationship between the GR and Spectral uranium logs influenced by TOC. Both the GR and Uranium logs were found to be the best proxies of organic richness in the Niobrara Formation. Uranium or GR derived TOC proved to be the best method to estimate TOC in the Niobrara Formation. A petrophysical workflow was developed to allow for geochemical rock types to be defined from high resolution core TOC data and then related to open hole log combinations using Decision Tree Analysis. Decision Tree Analysis then allows for the same rock types to be predicted from well logs in wells that lack core data.