Date of Award
Master of Science (MS)
The Upper Cretaceous lower Mancos Shale and related units in the Piceance Basin of Colorado are composed of interbedded shale, siltstone, fine-grained sandstones, and shaly limestones deposited in the distal portions of clastic wedges shed eastward into the marine shelf of the Western Interior seaway. This study is based on an analysis of wireline logs from ~1,800 wells across the basin. The study interval ranges in thickness from less than 2,000 feet to the southeast to more than 5,250 feet to the northwest. Three intervals were identified with distinct stratal patterns and lithologies: Lower Interval, the Middle Interval (Niobrara), and the Upper Interval (Prairie Canyon -- Mancos "B").
The Lower Interval is dominantly a fossiliferous mudrock facies with bentonite beds, condensed sections with significant source rock intervals, fine-grained sandstone and siltstone lithofacies, and carbonate-concretion intervals. The total thickness varies from 200 to 400 feet from the top of the Dakota Sandstone to the base of the Fort Hayes limestone of the Niobrara. Sediment was sourced from the west and wireline logs show distinct, subtle, upward coarsening cycles with numerous interspersed ash beds.
The Middle Interval (Niobrara) is dominantly interbedded calcareous shale and shaly limestone facies. The carbonate content increases to the eastern part of the basin. Shale content increases toward the more proximal deposits to the west. The thickness of the Niobrara Interval varies from 700 to 1700 feet thick due to thinning on the transcontinental arch to the southeast. There are regionally distinct source and reservoir facies within this interval. Wireline-log correlations were used to identify distinct zones of high resistivity and to identify hydrocarbon-bearing marls and carbonates that are in the hydrocarbon-maturation window.
The Upper Interval (Prairie Canyon - Mancos "B") is dominantly interlaminated fine-grained sandstone to silty shale. The thickness of the interval varies from 700 to more than 1500 feet, with the thickest section in the central area of the basin. Correlations of subtle silt beds within this unit have identified clinoforms indicating a northerly prograding wedge.
Rogers, Nathan Thomas, "Subsurface Stratigraphy of the Upper Cretaceous Lower Mancos Formation and Related Units, Piceance Basin, Northwestern Colorado" (2012). Geological Sciences Graduate Theses & Dissertations. 55.