Depositional Systematics and Exploitation of Morrow Valley-Fill Complexes in Cheyenne County, Colorado
Lee F. Krystinik, David W. Bowen, Hilary G. Swanson
The Lower Pennsylvanian Morrow Formation in Cheyenne County, Colorado, contains sandstone reservoirs interpreted to be valley-fill deposits. These reservoirs are exceptionally permeable (up to 20 darcys), thick (20-70 ft, 6-21 m), and highly productive (up to 650 bbl/day, 103 m3/day). Previous depositional interpretations of the Morrow in southeast Colorado have spanned the spectrum of fluvial and deltaic environments, with little predictive success.
Detailed sedimentologic analysis of 65 cores reveals that the Morrow is composed of a series of multistory, northwest to southeast-trending valley-fill complexes incised into marine limestone and shale. Valleys are associated with at least six distinct regionally extensive unconformity horizons indicated by weathering profiles, roots, coal, and calcrete deposits. The valleys are incised up to 80 ft (25 m) and are filled by coarse, trough cross-stratified sandstone grading upward and laterally into rippled fine sandstone or rooted flood-plain mudstone. The sequences are capped by transgressive burrowed sandstone which grades into marine mudstone. These lithofacies have unique well-log characteristics, allowing calibration of logs for correlation.
Each sandstone body within a valley complex is the amalgamated deposit of laterally migrating channels. Channel stacking is documented by multiple scour lags, repeated fining-upward grain-size profiles, vertical changes in sedimentary structures, and changes in transport vectors. These stacked channel deposits are well-interconnected reservoirs and are continuous along valley axes.
Use of the valley-fill concept in conjunction with core-derived log calibrations and transport vectors can improve predictability and substantially reduce drilling risk.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.