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

Gabriela I. Keeton¹, Matthew J. Pranter¹, Edmund (Gus) R. Gustason, III², and Rex D. Cole³
¹University of Colorado, Department of Geological Sciences, Boulder, Colorado, USA
²Enerplus Resources, 1700 Lincoln Street, Denver, Colorado, USA
³Colorado Mesa University, Department of Physical and Environmental Sciences, Grand Junction, CO, USA
[email protected]

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; 1485 ft in total length from Coal Canyon and Plateau Creek Canyon) and subsurface data (N=5 cores, 441 ft in total length) were used to assess how the facies, and architectural elements in outcrop and core relate to gamma-ray-log response (total and spectral values) and 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 the stratigraphic variability associated with changes in fluvial depositional style. Through borehole-image analysis, sine curves were identified on borehole images and classified into bedding and non-bedding features and associated dip and azimuth values were recorded. Spectral gamma-ray readings were sampled at 1-ft 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 anomalies are assessed through petrographic analysis. The study documents the gamma-ray and borehole-image responses through a catalog that associates 1) architectural elements to spectral gamma-ray signatures, and 2) sedimentary attributes to borehole images.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects