Reservoir Geology Of Incised Valley-Fill Sandstones Of The Pennsylvanian Morrow Formation, Southern Stateline Trend, Colorado And Kansas

Abstract

ABSTRACT Incised valley-fill deposits of the Pennsylvanian Morrow Formation of eastern Colorado and western Kansas have proven to be prolific oil reservoirs. Since the discovery of oil from a Morrow sandstone reservoir at Sorrento Field in 1979, 66.5 million barrels of oil have been produced along two major incised valley trends. Ultimate cumulative production from these reservoirs is estimated to be 107 million barrels of oil. Understanding the trapping mechanisms and internal complexities of these reservoirs provides insight to significant opportunities for further exploration and development along these trends and important analog data for similar types of incised valley-fill reservoir plays worldwide. The Morrow Formation is Early Pennsylvanian in age. Several fourth-order sequences comprise the upper Morrow siliciclastic interval. These high order sequences developed as a result of sea level fluctuations associated with Early Pennsylvanian glacio-eustasy. During the Morrowan, the study area was part of the broad, gently dipping northwest shelf of the Anadarko basin. Rising and falling sea level alternately submerged and exposed this shelf, causing maximum shoreline displacements of 90 to 125 miles (145 to 200 km) across the low-gradient muddy shelf. Extensive drainage systems trending northwest-to-southeast developed during each of these sea level cycles as rivers incised into the muddy shelf strata during lowstand periods. The reservoirs within these valleys are primarily amalgamated fluvial channel sandstones. Marine influenced sandstones are locally important reservoirs that formed from deposition in estuarine systems developed as valleys became flooded from rising sea level. Interfluve areas were sites of soil formation during valley incisement and valley-fill accumulation. This study focuses on the southern area of the Stateline Morrow trend along the Colorado-Kansas border. Four sequences produce oil and/or gas from incised valley-fill reservoirs in three fields along this trend - Second Wind, Jace, and Moore-Johnson. Establishing the sequence stratigraphic framework for these incised valley-fill sandstones allows for the mapping of trap geometries for petroleum accumulation. The sequence stratigraphic framework is developed by correlating wireline logs and integrating core data. Once the sequence framework is established, individual valley-fill reservoirs can be defined, attributed to specific sequences, and their geometries mapped. Three-dimensional seismic data is of great benefit in determining the dimensions and the geometries of the oldest two valley-fill systems. Complications to development geology arise when several valley systems of different ages are superposed and cross-cut. Moore-Johnson, Jace, and Second Wind fields illustrate problems with field exploitation due both to superposition of valleys and to internal reservoir complexity resulting from high-frequency relative sea level fluctuations.