--> --> Core facies analysis of the Niobrara-equivalent of the Mancos Shale (Mancobrara), Piceance Basin, CO

AAPG Rocky Mountain Section Meeting

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Core facies analysis of the Niobrara-equivalent of the Mancos Shale (Mancobrara), Piceance Basin, CO


The Niobrara-equivalent interval of the Mancos Shale (Mancobrara) in the Piceance Basin of western Colorado is an emerging gas resource play. The Mancobrara consists of hundreds of feet of argillaceous mudstone, silty and sandy mudstone, calcareous mudstone and bentonite. There are a few easily recognized and correlatable carbonate-rich intervals and thick bentonite beds but it is difficult to subdivide the Mancobrara into mappable genetically related units. A few lithostratigraphic, chemostratigraphic and sequence stratigraphic frameworks have been published but, because of the lack of publicly available core material, none of these stratigraphic frameworks have utilized core and core analyses to define genetically related intervals or the significant surfaces that bound them. This paper discusses the results of a detailed core facies analysis of the Mancobrara in the Piceance Basin. Approximately 1600 feet of core from 6 wells was described. Lithofacies were defined based on physical parameters, such as whether a mudstone is laminated or nonlaminated, grayscale, abundance of skeletal material (foraminifera and bivalves) or fecal pellets, and macrobioturbation index. Lithofacies were integrated with elemental geochemistry (XRF), XRD, and TOC/RockEval data to define several discrete core facies. Vertical facies successions (e.g., upward coarsening successions of laminated silty mudstone to thinly interbedded mudstone and sandstone) and their important bounding surfaces were compared with wireline log responses to develop a depositional model and form the building blocks for developing a sequence stratigraphic framework. This also helped define cyclicity in the Mancobrara that can be compared to parasequence stacking patterns in the Emery Sandstone to the west and alternating chalk-marl cyclicity in the Niobrara Formation to the east.