ABSTRACT: Sequence Analysis of the Pennsylvanian Morrow Formation, E. Colorado and W. Kansas

BLAKENEY-DEJARNETT, BEVERLY and LEE F. KRYSTINIK, Union Pacific Resources Co., Fort Worth, TX

Exploration strategy and field development of the Pennsylvanian Morrow Formation in eastern Colorado and western Kansas has been enhanced substantially by the application of sequence stratigraphic concepts. Linear sandstone reservoirs encased in marine mudstone were originally interpreted as fluvial-dominated deltaic lobes. Hydrocarbons were thought to be trapped in the structurally updip seaward terminus of a sandy deltaic lobe. However, after continued drilling, engineering analyses indicated little reservoir pressure depletion and much higher volumes of produced and in-place hydrocarbons than previously predicted. A re-evaluation of available cores and petrophysical logs, within the context of sequence stratigraphy, documented evidence in support of a series of valley-fill deposits associated with glacial/eustatic cycles.

Armed with the valley-fill model, explorationists expanded the play concept from field extension opportunities to a regional trend, which significantly increased estimates of oil in place. The exploration concept changed from small sandy lobes to long (up to 40 mi or 65 km), continuous sandstone bodies.The trap configuration also changed dramatically. Instead of exploring for fluvial-dominated deltas oriented perpendicular to structural strike, the model suggested that the largest fields should be found where valley-fill sandstones trend parallel to structural strike. In existing fields, secondary recovery operations were modified, and new development drilling locations were identified once individual valleys were differentiated. Although geochemical analyses indicated evidence for lo g-distance migration, no conduits for hydrocarbons seemed available until sequence stratigraphic analysis enabled us to identify the long, continuous sandstone trends that connected source rock to reservoir.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)