Abstract: Reservoir Geology within a Sequence Stratigraphic Framework - Incised Valley-Fill Sandstones of the Pennsylvanian Morrow Formation, Southern Stateline Trend, Colorado and Kansas
BOWEN, DAVID W., and PAUL WEIMER
Incised valley-fill deposits of the Pennsylvanian Morrow Formation of eastern Colorado and western Kansas have proven to be prolific oil reservoirs. Two major trends have produced 66.5 million barrels of oil to date and ultimately will produce over 100 million barrels of oil. This study focuses on the southern area of the Stateline trend along the Colorado-Kansas border. Five depositional sequences are productive from incised valley-fill reservoirs in three fields. Characterization of these fields was facilitated by developing a sequence stratigraphic framework for correlation. Within the sequence framework, individual valley-fill properties were defined, attributed to specific sequences, and their geometries mapped. Three-dimensional seismic data was highly beneficial in determining the dimensions and the geometries of these valley-fill systems. Structure proved to be an important component of these traps.
Complications to development geology arose when several valley-fill systems of different ages were superposed. As many as five valley-fills stacked vertically which caused reservoirs to intersect and potential seals to be breached. Problems also arose due to reservoir complexity resulting from high-frequency relative sea level fluctuations. For example, one valley-fill comprises a higher-order sequence. Mudstones deposited during flooding of the valley (TST) formed an internal seal that effectively compartmentalized two reservoirs within a single valley-fill. DST data obtained during field development confirms this interpretation.
The Morrow Formation is an important analog for similar incised valley-fill systems worldwide. The Morrow shows the potential significance of this type of reservoir with proven reserves of over 100 million barrels of oil.
AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado