Morrow Formation Facies Geometries and Reservoir Quality in Compound Valley Fills, Central State Line Area, Colorado and Kansas

Lee F. Krystinik

Pennsylvanian Morrow valley-fill reservoirs are highly productive from a 2-km wide, 32-km long group of fields along the central Colorado and Kansas border. Many wells can produce 200-800 bbl/day, but 40-acre offsets to these wells with similar pay thicknesses and well log characteristics can be nonproductive. Variable production is caused by complex facies relationships created when a coarse, permeable valley sequence scoured into and reoccupied a preexisting valley containing fine, low-permeability sandstone.

A detailed study of 26 cores integrated with well logs and production data shows that most production comes from the coarse-grained, younger fluvial deposit which can scour deeply the older, more estuarine valley fill. The older erosion surface scours as much as 25 m into underlying shallow marine carbonates and is filled by a low-permeability (< 100 md), laterally variable sequence of fluvial to estuarine, trough to rippled cross-stratified sandstone and burrowed black shale. The younger erosion surface scours as much as 15 m into the underlying fluvial to estuarine sandstone and is filled by trough cross-stratified, coarse-grained to pebbly, highly permeable sandstone (up to 15 darcys).

The fine-grained valley fill can be differentiated from the coarse valley fill by integrating core and log data. Once identified, the two deposits can be mapped and used predictively as the field continues to develop to the north and south.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.