Outcrop-to-Subsurface Correlation of Isolated Fluvial Sandstones, Grand Hogback, Piceance Basin, Colorado
Bryan McDowell and Piret Plink-Björklund
Colorado School of Mines
In western Colorado, the Williams Fork Formation has proven to be a significant gas producer from isolated and amalgamated fluvial deposits. These isolated sandbodies are difficult to interpret in the subsurface due to their variable well log responses, overall complicated stratigraphic architecture, and high lateral and vertical variability. Outcrop studies along the Grand Hogback of Colorado have yielded four types of sandbodies in the Upper Williams Fork Formation based on their reservoir potential: (1) single-story, (2) multi-story, (3) laterally-amalgamating, and (4) crevasse/other. Outcrop GR curves were taken alongside stratigraphic sections (approx. 6 inch spacing) and used to create cumulative GR, K, U, and Th histograms and pseudo-GR curves to compare to subsurface well data. This data set shows distinct differences between sandbodies, notably: (1) highest radioactivity associated with crevasse splays/channels, (2) lowest radioactivity associated with multi-story channels, and (3) a range of data associated with single-story sandbodies. These differences appear interrelated with lithofacies content and the stratigraphic architecture of individual sandbodies. The focus of this study is to test the validity and accuracy of this method for subsurface expression and interpretation related to hydrocarbon production. We attempt to use a combination of outcrop GR measurements, stratigraphic sections, sandbody geometry, and nearby well data to differentiate sandbody types and lithofacies in the subsurface using wireline logs and simple statistical methods to aid future exploration in the Upper Williams Fork Formation.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013