Sequence stratigraphy, depositional history, and hydrocarbon potential of the Mancos Shale, Uinta Basin, Utah

The Mancos Shale, an organic-lean marine mudstone dominated by detrital quartz and clay, was deposited into the Upper Cretaceous Western Interior Seaway. It is a proven source rock with potential as a target for hydrocarbon production from horizontal drilling and hydraulic fracturing, but prospective reservoir target intervals from its 4,000 ft (1220 m) thickness must be identified and characterized. The distribution of lithofacies throughout the Mancos has not previously been studied in detail, so this formation remains undifferentiated and the relationship with paleodepositional up- and downdip strata undefined. Previous core-based analysis has provided a depositional and sequence stratigraphic framework for predicting the distribution of lithofacies in the Mancos. However, a unified, basin-wide facies and sequence stratigraphic correlation which tests existing outcrop and core-based models is lacking. 157 wireline logs were chronostratigraphically correlated across the basin to build a regional subsurface map and cross sections that highlight stacking patterns, regional facies relationships, stratal architecture, and sequence stratigraphy within a depositional framework. A sequence stratigraphic model is established for the Mancos Shale, which incorporates seminal outcrop-based models from strata of central Utah, Mancos core, and stacking patterns identified from wireline log data. The Mancos thickens to the north and west, matching regional trends of tectonically driven subsidence, the most significant control on accommodation. The Mancos Shale consists of 29 4th order T-R cycles, which can be stacked to form four 3rd order cycles and a single 2nd order cycle. Whereas the stacking patterns of 2nd and 3rd order cycles are consistent across the basin, suggesting allocyclic control, 4th order cycles are more variable, influenced by both allocyclic and autocyclic controls. Deposition of the Mancos evolved from a low gradient ramp, to a deeper water basin, which was then infilled by basin-floor fans. Two organic-rich facies associations, each corresponding to transgressive and early highstand sequence sets, heterolithic facies of the Juana Lopez and lowermost Blue Gate (FA1) and sediment starved shelf deposits of the Lower Blue Gate (FA2) offer the most prospective intervals for unconventional hydrocarbon production. Preliminary petrophysical analysis and basin modeling corroborate this analysis.