Subsurface Stratigraphy of the Upper Cretaceous lower Williams Fork Formation, Piceance Basin, Northwest Colorado

Joseph Nicolete, Paul Weimer, Stephen Cumella, Renaud Bouroulec, Edmund (Gus) Gustason, and Dag Nummendal

The Upper Cretaceous lower Williams Fork contains important reservoir and source rocks for natural gas accumulation in the Piceance Basin in northwestern Colorado. The lower Williams Fork is divided into five informal intervals. Interval 1 contains the coal-rich lower coastal-plain strata coeval to the shoreline deposits of the Rollins Member of the Iles Formation. The Rollins shoreline prograded more than 40 miles (64.4 km) from the northwest to the southeast. Within this interval, coals also become younger to the southeast. For most of the Piceance Basin, intervals 2-5 consist of interbedded sandstones, siltstones, and shales deposited in river channel, overbank, and crevasse-splay settings and are largely devoid of coal. To the southeast near the Mamm Creek field, interval 2 overlies a regional flooding surface at its base, and includes shoreline sandstones and coeval coals. Interval 3 is composed primarily coals and interbedded coastal-plain shales. Interval 4 also overlies a regional flooding surface, and includes shoreline sandstones and coeval coals similar to interval 2. Shoreline trends changed throughout the deposition of the lower Williams Fork Formation. For intervals 3 and 5, landward pinchouts of the coal were used to estimate the orientation of the coeval shoreline. The youngest Rollins shoreline had an azimuth of about 10 degrees and the shoreline azimuth of interval 5 was about 130 degrees. The rotation occurred as the Cretaceous interior seaway receded to the east as it was transgressing farther north, likely associated with the early movement of Laramide uplifts. The coals of the lower Williams Fork are a significant source of the natural gas in the basin-centered gas accumulation of the Piceance Basin. The updip pinchouts of the coals in intervals 2-4 play an important part in the areal distribution and thickness of the gas column. The lower Williams Fork was deposited within the highstand-systems tract of a third-order sequence. This HST is bounded by a maximum flooding surface at the top of the underlying Cozzette (75.0 Ma) and a sequence boundary at the base of the overlying middle Williams Fork (73.5 Ma). Higher frequency sequences (fourth order) are likely recorded by the transgressions and regressions of the shoreline sandstones of intervals 2 and 4. Other high frequency sequences may be present within the aggradational stages of the shoreline evolution.

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