PREDICTIVE STRATIGRAPHIC FRAMEWORK FOR ORGANIC-RICH LAKE BASINS: GREEN RIVER FORMATION, LAKE UINTA, COLORADO AND UTAH

Early to middle Eocene Green River Formation deposits (ca53 to 46 Ma) in Lake Uinta formed in two subbasins, the Piceance and Uinta basins and represent mixed siliciclastic-carbonate-evaporite and organic-rich mud rocks formed in a deep (10’s metres), stratified lake environment during the time of the Eocene climate optimum. The Piceance basin contains the World’s richest oil shale deposits known today. Organic-richness in lake deposits is controlled by variation in net production and dilution that varies vertically and laterally within depositional cycles and between lake evolution stages.

The distribution of facies associations and organic-rich deposits is characterized by both, short-term and long-term variations in lake conditions. Short-term changes (100 and 400 Ky) in the lake basin are controlled dominantly by climate fluctuations from arid to humid. These climate fluctuations are marked by complex small-scale (decimeters to meters) and large-scale (10’s m) shallowing-upward and deepening-upward depositional cycles within an overall aggrading depositional trend. Small-scale depositional cycles characterize the local depositional conditions whereas large-scale depositional cycles (≈ 400 Ky duration) reflect larger-scale changes within the basin area. Depositional cycles mark abrupt changes in lithofacies and oil shale richness that are controlled by periods of low, rising, and high lake and bounded by sequence boundaries, transgressive surfaces, and maximum flooding surfaces, respectively. Peaks in organic richness occur during lake level rises and maximum floods, and vary during high lake times.

Long-term changes in Lake Uinta are reflected by lake stages, that are about 1-1.5 million years in duration, and that characterize variation in sedimentological patterns, depositional trends, and organic-richness throughout the basin evolution. Lake stages are controlled by a combination of climate and tectonics, and comprise the following stages: fresh to mesosaline (S1), transitional (S2), highly fluctuating (S3), rising (S4), high (S5), and closing (S6). Organic richness is highest during S1, at the beginning of S2 and S3, late in S4, and during S5. Lake system predictions thus require understanding both short- and long-term changes and their respond to changing climate and tectonics.