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A Facies and Sequence Stratigraphic Model for the Mancos Shale, Uinta Basin: Identifying Unconventional Horizontal Targets

Lauren Birgenheier, Brendan Horton, Laini Larsen, Andrew D. McCauley, John McLennan, and Robert Ressetar

The Mancos Shale is unusually thick (~4000 ft., Uinta Basin), regionally extensive and ubiquitous across the Rocky Mountain region, relatively low TOC (average 1 – 2 %, max 6.7%) and carbonate poor (average 18% carbonate, 41% clay, and 41% detrital silica). Effective development of the Mancos play will require an accurate, detailed sedimentologic and stratigraphic characterization with the aim of locating and predicting prospective horizontal target reservoir facies and production sweet spots. To date, such analyses have been lacking. Using detailed core description from wells in the Uinta Basin, along with thin section, x-ray diffraction, x-ray fluorescence, and QEMSCAN analysis, a facies model has been developed. This model diagnoses prodelta, mudbelt, and sediment starved shelf environments within the broadly offshore Mancos system. Geochemical, geomechanical, and petrophysical evaluation suggest the distal sediment starved shelf deposits contain relatively fine-grained facies with higher calcite and TOC content. These facies offer the most prospective combination of hydrocarbon production, storage capacity and reservoir potential to fracture in a brittle fashion. Core analysis and basin-wide detailed log correlation suggest that desirable target reservoir facies (distal sediment starved shelf deposits) are found in the late transgressive sequence set and early highstand sequence set. In contrast, a petrophysical water saturation study of Mancos wells in the Uinta Basin suggests that proximal mudbelt facies have characteristically higher water saturations. This implies that these relatively coarser grained (higher detrital quartz sand and silt content), lower TOC and lower carbonate content would resist successful hydrocarbon production. These proximal mudbelt facies correlate to late highstand and lowstand sequence set deposition. Basin-wide log chronostratigraphic correlation defines key subunits within the Mancos and constrains the most prospective horizontal targets to three discrete transgressive intervals in the lower Mancos. Here, a regionally significant depositional and sequence stratigraphic framework is identified that can be tied to lithologic properties. This is critical for resource play evaluation and completion of the Mancos Shale in the Uinta Basin as well as for other similar offshore deposits of the Cretaceous Western Interior Seaway.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013