Subsurface Stratigraphy of the Paleogene Wasatch Formation in the Piceance Basin, Northwestern Colorado
Dawn Tschanz, Paul Weimer, Stephen Cumella, and Edmund (Gus) Gustason
The Wasatch Formation (Paleocene to lower Eocene) in the Piceance Basin of northwest Colorado consists of thick (greater than 5500 feet) continental strata that was deposited in an intermontane basin that developed at the end of the Cretaceous and is associated with the Laramide orogeny. The basin was flanked by the eroding high of the Douglas Creek Arch (west), by the Uncompahgre Plateau (southwest), by the Sawatch Uplift (southeast), and by the White River Uplift (east). The Wasatch Formation produces natural gas in ten fields. To date, the regional understanding of the formation is primarily from the outcrops encircling the basin, and is limited in the subsurface. This study divides the Wasatch Formation into twelve distinct stratigraphic intervals in the subsurface based on the regional correlation of approximately 2200 wireline logs. Based on these correlations, a series of cross-sections, isopach maps, and net sandstone maps were constructed for each interval. In addition, well cutting data, outcrop measured sections, and outcrop descriptions from literature were tied to the subsurface correlations to give a better understanding of the lithology. The lithology allowed the identification of lithofacies and lithofacies assemblages, and thus facies interpretation and identification of regional structural events were possible. The twelve intervals of the Wasatch Formation consist of fluvial and related strata at the base, changing upward to interbedded fluvial and lacustrine strata. Intervals 1 through 5 are mostly Distributary Fluvial Systems that flowed primarily to the north, northeast, and the east as they were derived from the adjacent. In contrast, the fluvial strata of Interval 6 were deposited by Axial Systems that were derived from the Uncompahgre Plateau to the southwest. Intervals 7-11 include the fresh water strata in the northern part of the basin and the fluvial strata to the south. There is a vertical change in lake types between Intervals 8 (overfilled) to Interval 11 (balanced-filled). The top of the studied strata is a major flooding surface. Currently, the Wasatch Formation produces from two different sandstones in Interval 6: a lower sandstone to the north, and an upper sandstone to the south. An important outcome of this study is the framework to aid in new exploration targets.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012