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

A 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; 1400 ft in total length from Coal Canyon and Plateau Creek Canyon) and subsurface data (cores from four wells; 1300 ft in total length) were used to assess how the facies, facies associations, and architectural elements in outcrop and core relate to gamma-ray-log response (total and spectral values) and resistivity-based borehole images, respectively. Stratigraphic attributes for the lower, middle, and upper Williams Fork Formation and their associated spectral-gamma-ray and borehole-image responses document stratigraphic variability associated with changes in fluvial depositional style. 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 soft-sediment deformation); the associated dip and azimuth values were also recorded. Spectral gamma-ray readings were sampled at one-foot increments for four measured sections that span the Williams Fork Formation. Spectral gamma-ray signatures were tied to architectural elements (crevasse splays, single-story and multistory sandstone bodies) and paleosols and anomalies were assessed through petrographic analysis of framework grains, matrix and cements. The study documents the gamma-ray and borehole-image responses in a catalog that associates 1) architectural elements to spectral gamma-ray signatures, and 2) sedimentary attributes to borehole images.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California