The Basin Fill History of the Green River Formation in the Piceance Creek Basin, Colorado

Yuval Bartov and Dag Nummedal
Colorado School of Mines, Golden, CO

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. 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 analysis of the sedimentary sections indicates six lacustrine stages for the Green River Formation in the Piceance Creek basin: a) Filling – initial lacustrine environments with gradual deepening. b) Fluctuating – shorelines were fluctuating at the basin marines. c) Low stand lake levels were low and shoreline regressed far into the basin. d) Saline – the salinity of the lake was significantly high with evaporate deposition e) deep lake – lake levels increased and lowered the salinity. f) very deep lake with extensive oil shale deposits at the basin margin.