A Sequence Stratigraphic Framework for the Mancos Shale, Uinta Basin, UT: Identifying Prospective Intervals within an Emerging Natural Gas Resource

McCauley, Andrew; Birgenheier, Lauren; Ressetar, Robert

Collaborative research between the Utah Geological Survey and the University of Utah seeks to provide a multidisciplinary assessment of the hydrocarbon potential of the Mancos Shale in the Uinta Basin of Utah, including the identification of stratigraphic target intervals. The Cretaceous Mancos Shale is organic lean, gray, silty shale deposited into the Western Interior Seaway and is dominated by detrital quartz and clay material with a few carbonate-rich intervals. Economic development of the relatively lean 4,000' thick Mancos Shale will require a detailed understanding of lateral and vertical heterogeneity of lithofacies, geomechanical properties, and hydrocarbon potential.

We used 125 well logs from the basin to establish a chronostratigraphic framework for the Mancos, including the correlation of parasequences and overlying flooding sequences, as well as regional surfaces of erosion. Cross sections extend core and outcrop based stratigraphy by other workers into the subsurface, which is placed in a sequence stratigraphic framework based on stratal architecture and stacking patterns in core and well logs. Basinwide stacking patterns distinguish regionally significant surfaces from local lateral heterogeneities, identify offshore sequence stratigraphy, and allocyclic versus autocyclic controls on deposition. Geochemical, geomechanical, and petrophysical analyses of core were used to identify the most prospective reservoir facies, which are characterized by relatively high concentrations of TOC and calcite deposited in a sediment-starved shelf environment associated with the transgressive and early highstand systems tracts. This study establishes a regionally significant depositional and sequence stratigraphic framework that can be tied to lithologic, petrophysical, and geomechanical properties, critical steps for resource play evaluation and completion of the Mancos Shale in the Uinta Basin as well as other similar and widespread offshore deposits of the Western Interior Seaway.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013