Reservoirs in the lower Pennsylvanian Morrow Formation of eastern Colorado, southwestern Kansas, and northwestern Oklahoma have produced greater than 8 tcf of gas and 200 million bbl of oil. This prolific depositional system produces from reservoirs representing a range of depositional environments from updip, fluvial-dominated, incised-valley fills to deep-water basin-floor systems. The valley fills of the Morrow Formation are of particular significance because they can be mapped in great detail from subsurface control over very long distances. Facies distributions in the valleys change systematically downdip. As a result, reservoir characteristics and trapping mechanisms vary with these changes in internal valley stratigraphy.
This paper focuses on the reservoir geology of Nicholas and Liverpool Cemetery fields. These fields produce gas from the extreme downdip region of an incised-valley-fill system in the lower Morrow Formation. The valley systems in this downdip region are deeper and wider and demonstrate greater marine influence than the updip regions of the valley systems farther north and into the hinterland.
Compartmentalization in these downdip reservoirs differs significantly from updip valley-fill reservoirs. The reservoirs in this downdip region are more highly compartmentalized because the dominant reservoir facies in these fields is a series of bayfill delta deposits. These deposits are isolated by shale deposits in the valley. This depositional setting contrasts markedly with predominantly fluvial reservoirs in updip regions of the valleys. Understanding the scale, geometry, and internal complexity of this depositional system is important because the associated sandstones are important gas reservoirs in southwest Kansas. Cores, wire-line logs, pressure data, and production data collected from the Morrow Formation at Nicholas and Liverpool Cemetery fields provide valuable information from which to describe and interpret this downdip valley fill. This paper describes the downdip incised-valley-fill reservoirs in this producing complex and documents the trapping relationships of these reservoirs and their production characteristics.