Depositional and Diagenetic Controls on Production in Morrow Valley Fills, Central State Line Area, Colorado/Kansas

Lee F. Krystinik

Production from wells in Pennsylvanian Morrow valley-fill deposits along the central state-line trend of Colorado and Kansas is controlled by sedimentary facies and diagenesis. The reservoir sandstone units fill paleovalleys incised up to 25 m (80 ft) into the underlying marine shale and limestone. In the state line area, multiple valley incisions occurred, superimposing valley fills with distinct reservoir characteristics. Most of the production comes from a coarse-grained permeable fluvial deposit that filled a paleovalley incised into an underlying fine-grained and low-permeability estuarine valley fill. The depositional and diagenetic distinctions between these two valley fills explain most of the complexity observed along this 2-km (1-mi) wide, 32-km (20-mi) long tre d.

The coarse-grained younger deposit has large, open pore throats generated by the dissolution of pore-filling carbonate cement. Migration of fine kaolinite booklets through the coarse deposit occurs without blocking pore throats because the pore-throat radii are larger than the clay particles. Although capable of good initial production, the fine-grained, older, more estuarine valley fill is subject to blockage of pore throats by particulate kaolinite, which causes precipitous declines in production.

The fine-grained valley fill can be differentiated from the coarse (highly productive) 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 #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.