SEDIMENTARY HETEROGENEITY OF THE MANCOS SHALE

An understanding of the diversity of mudstone is crucial to the assessment of the potential of low-permeability reservoirs as unconventional resources. Resource availability within these reservoirs depends not only on hydrocarbon potential, but also on the reservoir's geomechanical response to hydraulic fracturing. Determining relationships between sedimentary heterogeneity and geomechanical properties ensures efficient recovery of natural gas and oil from low-permeability plays, such as the Mancos Shale. The Upper Cretaceous Mancos Shale, deposited in the foredeep of the Rocky Mountain foreland basin during the Sevier Orogeny is found across multiple basins in western North America, bordered by uplifts of Laramide structures. Across these basins, the Mancos varies from 1,000 to 5,000 feet thick, an order of magnitude greater than most low-permeability formations, highlighting the necessity to locate an ideal target interval for hydrocarbon production. Three cored intervals from the Colorado Plateau were analyzed in order to define distinct lithofacies and constrain the range of variation in the Mancos. Cored intervals from the Uinta Basin, UT, the Piceance Basin, CO, and the San Juan Basin, NM were studied, across a range of stratigraphy from the Lower Blue Gate to the Juana Lopez Member of the Mancos Shale. Core analysis resulted in the classification of eleven lithofacies and various permutations therein. The range in variation is not only a factor of primary sedimentation, but is also due to post-sedimentation deformation and diagenesis. Distinctive variations include changes in carbonate and clay content, sand to silt ratio, bioturbation index and ichnofacies, fossil content, presence of incipient fractures, laminae and differing lamination styles, and presence of soft sediment deformation and other sedimentary structures. The variety of sedimentary structures such as starved ripples, climbing ripples, and scours indicates deposition by gravity driven processes as opposed to deposition by suspension settling. The complexity of the variations in the core provides a broad range of samples for geomechanical testing, from which relationships between geomechanical and sedimentary properties can be studied.