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Characterization of Fluvial Sandstones Based on Outcrop Gamma-ray Data and Borehole Images, Williams Fork Formation, Piceance Basin, Colorado

Gabriela Keeton, Matthew Pranter, Edmund (Gus) Gustason, and Rex Cole

This study investigates spectral-gamma-ray and borehole-image-responses of fluvial architectural elements and associated sedimentary attributes. Detailed sedimentary analysis of the Williams Fork Formation in the Piceance Basin, Colorado, based on outcrop and subsurface data shows the stratigraphic variability of the associated fluvial systems. The lower Williams Fork Formation consists primarily of mudrock with isolated to amalgamated channel sandstones that were deposited by a meandering-fluvial system within a coastal-plain setting. The middle and upper Williams Fork formations are interpreted to have been deposited by a low-sinuosity braided-fluvial system in an alluvial-plain setting. Outcrop measured sections (N=4; 1485 ft [452.6m] total length) from Coal Canyon and Plateau Creek Canyon were used to assess how the facies and architectural elements in outcrop relate to gamma-ray-log response (total and spectral values). Cores from 2 wells in the Piceance Basin (539 ft [164.2 m] total length) were used to assess how the facies and architectural elements relate to resistivity-based borehole images. Through borehole-image analysis, sine curves were identified on borehole-images and classified into 'bedding features' (i.e. lamina, beds, bed sets) and 'non-bedding features' (i.e. scours) and associated dip and azimuth values were recorded. Spectral gamma-ray readings were sampled at 1-ft (0.3 m) increments for measured sections that span the Williams Fork Formation. Spectral-gamma-ray signatures were correlated to architectural elements (crevasse splays, single-story, multistory sandstone bodies) in terms of spectral-log motifs and thorium, potassium and uranium cross-plots. Average total count decreases for each architectural element (crevasse splays followed by single story and multistory sandstone bodies) due a drop in potassium and thorium values that is associated with an increase in grain size. Anomalies were assessed through petrographic analysis of framework grains and cements.


AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012