Subsurface Variations of Sandstone-Body Thickness in the Williams Fork Formation (Cretaceous) from Douglas Creek Arch to Coal Canyon, Colorado
Scott Schindelar, Beau Taylor, Marko Gorenc, Rex Cole, and Matthew Pranter
This project involves a log-based (N = 28) investigation of the stratigraphic variation of sandstone bodies in the Upper Cretaceous Williams Fork Formation (Kwf) in a subsurface transect (73 miles) from the Douglas Creek Arch to Coal Canyon, which is near Palisade, CO. Along the transect, the Kwf is separated into two intervals. The lower Kwf has an average thickness of 722 ft, and was deposited in marginal-marine and coastal-plain settings, whereas the upper Kwf, which averages 858 ft thick, was the product of alluvial-plain sedimentation. The boundary between the upper and lower Kwf is not well-defined, but is expressed by a change in net-to-gross ratio (total sand thickness divided by the total interval thickness). The NTG ratio for the lower Kwf ranges from 27.7 to 63.2, and averages 41.7 (std. deviation = 8.8), whereas the range for the upper Kwf is 30.4 to 78.3, and averages 56.2 (std. deviation = 11.3). Six gamma-ray log signatures are present in the log suite: (1) bell-shaped polygons, which are interpreted as channel-fill and point-bar deposits; (2) irregular-cylinder polygons, which probably reflect deposition by low-sinuosity fluvial systems, or vertically stacked point bars, channel fills, or estuarine facies; (3) cylinder polygons, which are interpreted as low-sinuosity or anastomosed fluvial deposits; (4) funnel-shaped polygons, which represent progradational deposition (e.g., splay delta and delta front); (5) low-intensity spikes, which probably represent crevasse-splay deposits; and (6) high-intensity spikes that represent coal seams. The total number of polygons defined from the 28 wells is 1,970, with polygon thickness values ranging from 1.3 to 178.1 ft (mean = 10.7 ft; std. deviation = 14.0 ft). The cylinder and irregular cylinder polygons had the greatest average thicknesses (50.9 and 26.0 ft, respectively), followed by the funnel- (10.0 ft), bell- (8.1 ft), and spike-shaped polygons (6.2 ft for low-intensity, and 3.4 ft for high-intensity). Work is currently in progress to correlate the well-log data to nearby outcrops, so more definitive genetic interpretations can be made.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012