Regional sequence stratigraphic setting and reservoir geology of morrow incised valley sandstones (Lower Pennsylvanian), eastern Colorado and western Kansas

The regional sequence stratigraphic setting and reservoir geology of Lower Pennsylvanian Morrow incised valley systems are examined from a subsurface data set of wireline well logs, core, and limited seismic data. A detailed cross-section down the axis of a single valley system for 175 miles (283 km) illustrates: (1) the sequence stratigraphic framework of Morrow Formation valley-fill strata, (2) the stratigraphic complexities within a single compound valley-fill system, and (3) the organization of lithofacies within this compound valley as a function of relative position along the trend of the valley.; Three informal facies tracts comprise the Morrow valley-fill deposits. These are designated updip, transitional, and downdip. Amalgamated fluvial channel sandstone units characterize the updip facies tract, mixed fluvial and estuarine sandstones characterize the transitional facies tract, and fluvial channel sandstones and estuarine shales characterize the downdip facies tract.; The Morrow was deposited during an icehouse period of Earth history. High amplitude changes of relative sea level acted on the low gradient, slowly subsiding shelf in eastern Colorado and western Kansas to produce high-frequency fourth-order sequences in the Morrow. These sequences, however, are limited areally to the incised regions of third-order compound valley-fill strata. High-frequency sequences are best recognized in the transitional and downdip facies tracts where basinward shifts of lithofacies occur across fourth-order sequence boundaries.; Morrow Formation sandstones trap oil and gas through a combination of structural and stratigraphic relationships. Complications to reservoir geology occur due to complex interbedding of rock types within the transitional and downdip facies tracts of the valley-fill reservoirs. This interbedding results largely from the superposition of high-frequency sequences.