Outcrop Characterization of Fluvial Sandbodies in Lower Williams Fork Formation,

Coal Canyon Area, Colorado

By

Rex D. Cole

Mesa State College, Grand Junction, CO

Edmund R. Gustason

Consultant, Littleton

Steve Cumella

Williams Production RMT Company, Denver,

CO

Channel sandbodies of the Late Campanian Williams Fork Formation are prolific gas reservoirs in the southern Piceance Basin. A detailed outcrop characterization study of the Williams Fork in Coal Canyon, approximately two miles north of Palisade, Colorado, shows considerable sedimentologic variation in the lowermost 510 feet of the Late Campanian Williams Fork Formation. Within this interval, the percentage of fluvial sandstone ranges from 17.5 to 41.7 %, based on seven detailed measured sections (total logged interval=1,894 feet). Sandbodies are distinctly lenticular and isolated in mudrock. Detailed sedimentologic analysis and mapping (GPS) of 42 individual sandbodies have been completed to date. In this group, apparent channel widths range from 106 to 3,854 feet (average=1,050 feet), and thicknesses range from 5 to 45 feet (average=13.7 feet). Corresponding width-to-thickness ratios range from 13.2 to 251.9, with an average of 81.7. Most sandbodies are multi-storied (2-4 channel-fill sequences) and commonly show lateral-accretion surfaces suggestive of point-bar deposition. Apparent depositional trends are to the southeast (azimuth mean=123º). The sandstone is very fine to lower medium grained, moderately sorted, lithic, carbonate cemented, locally bioturbated, and well stratified (horizontal lamination, ripple lamination, trough and tabular-tangential cross-stratification). Channel scours are commonly accentuated by thin lags of sideritic mudchips. Associated mudrocks consist of drab-colored siltstones, claystones, and mudstones that are frequently bioturbated and carbonaceous. Coal seams, ranging in thickness from a few inches to more than 10 feet, occur in the lower 250 feet of the study interval. Siderite concretions are also common.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming