A FACIES AND SEQUENCE STRATIGRAPHIC MODEL FOR THE MANCOS SHALE, UINTA BASIN

The Mancos Shale is unusually thick (~4000 ft., Uinta Basin), regionally extensive and ubiquitous across the Rocky Mountain region, relatively low in total organic carbon (average 1 – 2 %, max 6.7%), and carbonate poor (average 18% carbonate, 41% clay, and 41% detrital silica). An extensive sedimentologic, sequence stratigraphic, geochemical and geomechanical analysis of Mancos Shale, Uinta Basin, from core and wireline logs indicate that the most distal, organic rich intervals of the formation are found in two stratigraphic phases or facies associations. Facies association 1 (FA1) is found in the Juana Lopez Member. FA1 is a heterolith - interbedded and interlaminated sandstone, siltstone and organic rich mudstone. Sandstone beds contain current ripples and hummocky cross stratification, indicative of deposition above storm wave base in an environment that was subject to regular sediment reworking by wave activity and hyperpycnal flows. FA1 is interpreted as prodelta deposits deposited along a shallow, low gradient ramp. The updip fluvial dominated shorelines and coal mires of the Ferron and Frontier Sandstone contributed brackish water conditions and terrestrial organic matter. The interval is strongly progradational as a result of the low accommodation tectonic setting. Facies Association 2 (FA2) is found in the Niobrara equivalent interval deposited during Ocean Anoxic Event III and consists of laminated mudstone deposits that contain a higher calcite content (coccolith fecal pellets). FA2 was deposited on the sediment starved shelf, below storm wave base and beyond the main influence of hyperpycnal flows. The system was deposited in a higher accommodation tectonic setting than FA1 in which updip wave dominated shorelines, such as the Emery Sandstone, trapped coarser grained material along the shoreline. Core analysis and basin-wide detailed log correlation suggest that desirable shale hydrocarbon target reservoir facies (FA1 and FA2) are found in the late transgressive sequence set and early highstand sequence set. Tectonic events that governed temporal changes in the accommodation regime between FA1 and FA2 may be broadly applicable to Cretaceous-age Rocky Mountain shale systems in other basins and so can provide a useful first order stratigraphic and geographic facies prediction tool.