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Subsurface stratigraphy of the Upper Cretaceous middle Williams Fork Formation, Piceance Basin, northwest Colorado: implications for reservoir sandstones

Renee Foster, Paul Weimer, Jonathan Cantwell, Renaud Bouroullec, and Edward (Gus) Gustason

The Upper Cretaceous middle Williams Fork Formation is a high net:gross sandstone interval within the continental strata of the Williams Fork Formation of the Piceance Basin in northwestern Colorado. This unit produces a significant portion of the natural gas production in the basin. Regional correlation of 3000 well logs across the basin identified three informal units based on the channel-fill deposit architecture and net sandstone distribution: middle Williams Fork Formation 1, 2 and 3. Correlations through the Colorado River Valley show the most detail across the Piceance Basin due to well density. The middle Williams Fork Formation 1 is a transitional zone of meandering to braided channel-fill deposits. These deposits range in thickness between 5 to 25 meters, and widths approximately less than to slightly greater than 800 meters (0.5 miles). The transitional boundary separating the lower Williams Fork Formation and the middle Williams Fork Formation is subtle and represents a change in net sandstone content. Throughout the Colorado River Valley, the primary incised valley complex of the middle Williams Fork Formation 2 trends northeast. This interval is identified as the highest net sandstone zone of the middle Williams Fork Formation. It is composed of amalgamated multi-story, multi-lateral channel-fill belt deposits that are between 18 to 100 meters thick and three to four miles wide across the basin. This interval is significant to the development of the basin's natural gas production as the middle Williams Fork Formation 2 likely contains a large portion of reserves. Overlying the middle Williams Fork Formation 2 large incised valley–fill feature is the middle Williams Fork Formation 3 channel-fill sandstone belt deposits. The channel-fill #1 sandstone deposit is a small episode of incision into the shale that overlies the middle Williams Fork Formation 2 amalgamated channel-fill sandstones. channel-fill #2 sandstone deposit is an areally extensive channel-fill belt that may show evidence of greater tidal influence. Collectively, these channel-fill deposits represent the last regressive pulse of the Western Interior Seaway before entering into a regional transgression.


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