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Fluvial Sand-Body Dimensions in the Lower Williams Fork Formation (Upper Cretaceous), Southwestern Piceance Basin, Colorado

Cole, Rex D.1, Steve Cumella2
1 Mesa State College, Grand Junction, CO
2 Williams Production RMT Company, Denver, CO

The lower Williams Fork Formation in the southwestern Piceance Basin is a classic low net-to-gross sequence that was deposited in a lower coastal-plain setting. Fluvial systems were sinuous to anastomosing, flood plains poorly drained, and peat-forming mires, marshes, swamps, and lakes common. The lower Williams Fork includes the Cameo-Wheeler coal zone and is stratigraphically equivalent to part of the productive interval in gas fields in the Parachute-Rifle-Collbran area. Detailed sedimentological examination and mapping of fluvial sand bodies in the lower Williams Fork (500 to 700 ft thick) was conducted in Coal Canyon, near Palisade, Colorado. Within the study area (~800 acres), 136 fluvial sand bodies were mapped using differential GPS receivers. Stratigraphic control for the sand bodies was defined by 15 measured sections (total=4,441 ft). The field data indicate a range in average thickness from 0.5 to 29.0 feet (mean average=9.3 feet), and a range in apparent sand-body width from 40.1 to 2,791.1 feet (average=528.4 feet). Paleocurrent data (N=1,646) collected from 99 of the 136 sand bodies indicates a unimodal distribution with a vector mean of 75ยบ. Sand bodies in the study area can be grouped into five genetic types. Type A sand bodies were deposited by anastomosed channels. Type B sand bodies were deposited by sinuous channel systems that did not develop significant meander belts. Type C sand bodies, on the other hand, were deposited by more robust sinuous channel systems that did produce significant meander belts. Type D and E sand bodies are interpreted as crevasse-channel and crevasse-splay, facies, respectively.