Subsurface Stratigraphy of the Upper Cretaceous Lower Mancos Shale and Related Units, Piceance Basin, Northwestern Colorado.

Nathan Rogers, Paul Weimer, Stephen Cumella, Renaud Bouroullec, Edmund (Gus) Gustason, and Dag Nummedal

The Upper Cretaceous lower Mancos Shale and related units in the Piceance Basin 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 environment of the shallow Western Interior seaway. This study is based on a detailed analysis of wireline logs from ~1,500 wells across the basin. The study interval ranges in thickness from less than 2,000 feet to the southeast to more than 4,500 feet to the northwest. Three intervals were identified with distinct stratal patterns: lowermost Mancos, the Niobrara, and the Prairie Canyon (Mancos 'B'). The lowermost Mancos (lower Cenomanian to upper Turonian) is dominantly a fossiliferous mudrock facies with abundant bentonite beds, condensed sections with significant potential source rock intervals, thin fine-grained sandstone and siltstone lithofacies, and carbonate-concretion intervals. The total thickness varies from 200 feet to 400 feet from the top of the Dakota Sandstone to the base of the equivalent 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 Niobrara interval (Coniacian through Santonian) is dominantly interbedded calcareous shale and shaly limestone facies. The carbonate content increases to the eastern (more distal) part of the basin. Shale content increases toward the more proximal deposits to the west. The thickness of the Niobrara interval varies from 700 feet to 1,700 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 Prairie Canyon interval (lower to middle Campanian) is dominantly interlaminated fine-grained sandstone to silty shale. The thickness of the interval varies from 700 feet to more than 1500 feet, with the thickest part of the interval central part of the basin. Correlations of subtle silt beds within this unit have identified clinoforms indicating a northerly prograding wedge. The Mancos 'B' interval contains a significant gas accumulation on the Douglas Creek arch, and much potential for Mancos 'B' gas exists deeper in the basin.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012