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AAPG Rocky Mountain Section Meeting

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Stratigraphic Framework for the Mancos-Niobrara of the Piceance Basin

Abstract

stratigraphic framework is proposed for the emerging prolific Mancos- Niobrara horizontal play in the Piceance Basin. Developments in regional stratigraphic correlations are allowing operators to better understand the heterogeneity of this play. The Mancos Shale is an Upper Cretaceous succession that easily weathers in outcrops along the flanks of the Piceance Basin with well penetrations in over 1800 wells within the basin. A series of cross sections and isochore maps provide a stratigraphic framework for this thick predominately shale interval. The Mancos interval extends from the top Dakota (middle Cenomanian) to the Rollins Sandstone (Upper Campanian). The thickness of the Mancos unit within this study varies from under 1500 feet in the southeast to more than 5000 feet in the north of the basin. The Mancos is divided into four informal units in the subsurface: Lower, Middle, Upper, and Uppermost. Each of these units are further divided into more detailed subunits based upon internal stratigraphy which approximate chronostratigraphic entities. The “Lower Mancos” consists of 3 subunits from the top of the Dakota Group to the base of the Niobrara Formation and includes the Dakota Silt, Mowry, Frontier, Juana Lopez, and Storm King Mountain members. The “Middle Mancos” consists of 12 subunits, and is equivalent to the Niobrara Formation, with informal units informally called the Fort Hays Chalk, Rangely, sand prone Wolf Mountain, Tow Creek, and Buck Peak benches. The “Upper Mancos” consists of seven subunits and extends from the Mancos “B”/Prairie Canyon zone in the area of the Douglas Creek Arch to the Castlegate condensed section. This condensed section is a prominent high gamma-ray marker which correlates to the very organic-rich Sharon Springs in the Denver Basin. The “Uppermost Mancos” consists of eight shale intervals intertonguing with the eight upper Cretaceous sandstones of the Morapos, Castlegate, Loyd, lower Sego, upper Sego, Corcoran, Cozzette, and Rollins. The Rollins Sandstone marks the end of the Mancos seaway in the Piceance Basin. The utilization of this detailed stratigraphic zonation is beneficial to better predict and classify hydrocarbon source and reservoir potential. Production from the Mancos-Niobrara is either oil from fractured Niobrara in the Rangely Field, gas from the “Upper Mancos” Mancos B mainly on the Douglas Creek Arch, or gas from the more recent drilling of various horizontal targets in the Niobrara.