Exploring the basin fill history of the Green River Formation in the Piceance Creek and the Uinta Basins

Yuval Bartov, Colorado Energy Research Institute, Colorado School on Mines, Golden, CO 80401, phone: 303-2733841, [email protected] and Dag Nummedal, Colorado Energy Research Institute, Colorado School of Mines, Golden, CO 80401.

The Green River lakes occupied the Uinta, Piceance and Great Green River Basins during the Early to Middle Eocene time. These foreland basins were initiated during the late Cretaceous and early Cenozoic Laramide Orogeny. The Green River Formation in the Piceance Creek Basin contains over 2000 ft of lacustrine deposits composed of alternating, kerogen-rich mudstones, carbonates and sandstones and forms the world's larges oil shale reserve of about 1 trillion BOE. In this study we used outcrops, cores and over 100 well-logs to identify the major sequences across the Piceance Creek Basin and into the Uinta Basin, in order to quantify the various controlling factors of the basins' fill. The sections that we analyzed are composed of numerous high frequency sequences manifested by distinctive flooding surfaces. These prominent surfaces have a relatively conformable flat-lying stacking thus indicate small incision between sequence boundaries that are probably tied to high frequency low amplitude lake level changes. The sections also indicate an important sedimentary source from the Douglas Arch that divides the two basins, and a sedimentary fill that keeps pace with the spatially variable basin subsidence. The chronostratigraphic correlation also points to an in-phase relationship between the two lakes not only during the Mahogany Ledge time but throughout the entire Parachute Creek Member. The correlated signals in the gamma-ray logs between the two basins indicate either a physical connection between the two lakes with an over-filled condition or a similar response to climate changes in closed under-filled conditions. At the time this abstract is written we cannot differentiate between the two possibilities.