Mineral, Chemical, and Textural Relationships in Rhythmic-bedded, Hydrocarbon-productive Chalk of the Niobrara Formation, Denver Basin, Colorado

Indigenous hydrocarbons are produced from organic-rich chalk beds of the Upper Cretaceous Niobrara Formation in the Denver basin, eastern Colorado. The types of hydrocarbons produced from these chalks are determined by the level of thermal maturity associated with present-day burial or paleoburial conditions. Detailed analyses of deeply-buried chalk from core of the Smoky Hill Chalk Member of the Niobrara Formation in the Champlin Petroleum #2 Boxelder Farms well combined with core data from other Niobrara wells have helped identify many depositional and diagenetic relationships. Porosity of the chalk is proportional to maximum burial depth and inversely proportional to the amount of non-carbonate material (acid-insoluble residue content) in the chalk. Total organic carbon content in the chalk is proportional to the amount of acid insoluble residue and relative abundance of pyrite in the acid-insoluble fraction. Quartz is inversely proportional to the amount of insoluble material, and the amount of clay tends to increase as insolubles increase, suggesting that detritus in these chalks is greatly influenced by reworked, altered, volcanic products rather than siliceous clastics. Mixed-layer illite/smectite clay of bentonite beds in the Niobrara Formation appears to be a good geothermometer, and its composition provides an indication of thermal maturity of the indigenous hydrocarbons. Scanning electron microscopy of the chalk fabric shows progressive cementation with increasing burial depth. Oxygen isotopes of the carbonate become progressively more negative with increasing burial. Oxygen Isotopes, therefore, record the effect of progressive cementation in these chalks and support the idea that pressure solution and reprecipitation of the carbonate is the primary process for porosity reduction in chalk reservoirs of the Niobrara Formation in the Denver basin and adjacent areas.