Luncheon – May 2, 2003

Title: Reservoir Architecture of Lower Coastal Plain Sandbodies in the Basal Williams Fork Formation (Upper Cretaceous), Coal Creek Canyon, Southwestern Piceance Creek Basin, Colorado
Speaker: Dr. Rex Cole, Department of Physical and Environmental Sciences, Mesa State College, Grand Junction, Colorado
Date: May 2, 2003
Publication: The Outcrop, May 2003, p. 4-5

Between Grand Junction and De Beque, Colorado, the Williams Fork (Hunter Canyon) Formation forms spectacular outcrops in the canyon walls of the Colorado River and its local tributaries (Coal Canyon, Main Canyon, and Plateau Creek Canyon). In this area, the Williams Fork is approximately 2,300 feet thick and consists of mudrock, carbonaceous mudrock, coal, sandstone, and fossiliferous sandy limestone (rare). This presentation focuses on the lower 650 feet of the Williams Fork in Coal Canyon, which is stratigraphically equivalent to part of the productive interval at Grand Valley field (gas), approximately 25 miles to the northeast. Within the Coal Canyon study area, which is approximately 800 acres, 15 measured sections with outcrop gamma-ray measurements were completed (4,272 total feet) and 136 channel-form and lenticular sandbodies were mapped by GPS (1,314 total waypoints). Sedimentological observations (e.g., thickness, number of stories, stratification, etc.) and paleocurrent data (N = 1,647) were collected at most waypoints. Four local coal seams were used for stratigraphic control between the measured sections.

The majority (N = 96) of the 136 sandbodies in the study interval have channel-form geometries. Sandbody thickness ranges between 4.5 and 45.0 feet, whereas the apparent width (defined as the linear distance between the two terminations of a sandbody outcrop) ranges from 50 to 2,800 feet. most channel-form sandbodies thicker than five feet are poly-genetic; i.e., they consist of vertically and laterally amalgamated channel-fill elements. Individual channel fills range in thickness from less than one foot up to 25 feet and have lateral dimensions ranging from less than 10 feet up to 150 feet. Lateral-accretion surfaces are common in many of these channel-fill sequences, as are vertical-accretion mudrock intervals (mud plugs). Most channel-fill sequences fine upward, have mudchip lags on major scour surfaces, and contain small- to medium-scale cross-stratification. These sedimentologic observations suggest that the majority of the channel complexes were sinuous. The paleocurrent data indicate sediment transport to the east-northeast (vector mean = 750 ). Teredolites and Arenicolites burrows are common, suggesting that some of the channels were invaded by saltwater wedges.

Forty of the 136 sandbodies in the study interval have lenticular geometries with little internal heterogeneity. They range in thickness from 0.5 to 15 feet and in apparent width from 40 to 350 feet. Most lenticular sandbodies are ripple laminated, but may be totally bioturbated, especially at their lateral terminations into mudrock. These sedimentologic characteristics suggest that the lenticular sandbodies were deposited as off-channel splays and possibly small splay deltas.

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