Regional sequence stratigraphic setting and reservoir geology of Morrow incised-valley sandstones (lower Pennsylvanian), eastern Colorado and western Kansas

Abstract
Oil and gas exploration for the lower Pennsylvanian Morrow Formation of eastern Colorado, western Kansas, and northwestern Oklahoma provides a subsurface data set that transects the entire range of lowstand depositional systems from incised-valley-fill systems to deep-water basin-floor systems in one composite depositional sequence. One compound incised-valley fill that is a part of this system contains three facies tracts with unique reservoir characteristics: (1) the updip facies tract is dominated by amalgamated fluvial channel sandstones, (2) the transition facies tract consists of fluvial channel sandstones interbedded with finer grained estuarine sandstones, and (3) the downdip facies tract consists of ribbonlike fluvial channel sandstones isolated in estuarine shale.
A 175-mi-long (283-km-long) longitudinal cross section through one trunk of the incised-valley-fill drainage shows that internal valley-fill strata change significantly as a function of the interplay of varying depositional systems down gradient in the valley. Key contrasts in reservoir performance are documented as a function of changes in reservoir characteristics, trap controls, and trap configurations from updip to downdip in this valley-fill drainage.
The strata of the Morrow Formation were deposited in a cratonic basin during a period in the Earth's history when the climate was cooler than today. High-frequency changes of sea level across an extremely low-gradient depositional surface controlled erosion and deposition. These facies tracts reflect the response of valley-fill sedimentary processes to high-frequency relative sea level changes resulting from glacio-eustasy. The resultant valley-fill systems have many characteristics in common with published valley-fill models, but have significant differences as well.