Predictive Sequence Stratigraphic Framework for Organic-Rich Lake Basins: Green River Formation, Piceance and Uinta Basins, Colorado and Utah - Oil Shale and Unconventional Oil Play
J. FREDERICK SARG and KATI TÄNAVSUU-MILKEVICIENE
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, U.S.A.
Early to middle Eocene Green River Formation deposits (ca 53 to 46 Ma) in Lake Uinta formed in two subbasins, the Piceance Creek basin and the Uinta basin and represent mixed siliciclastic-carbonate and organic-rich lake deposits formed during the time of the Eocene climate optimum. The world’s richest oil shale deposits occur in the Piceance Creek basin, Colorado and an unconventional shale oil resource play is active in the Uinta basin, Utah, in areas where the Green River organic-rich mudstones are mature. More conventional clastic and microbialite carbonate reservoirs also contribute to this play.
Deposition is comprised of organic-rich and organic-poor mudstones i.e. oil shale, siliciclastics, and carbonates, formed in a deep (10’s meters), stratified lake environment. Complex small-scale (decimeters to meters) and large-scale (10’s meters) depositional units form prograding and aggrading shallowing-upward depositional cycles within the overall deepening-upward depositional trend and mark abrupt changes in lithofacies and oil shale richness. Three idealized small-scale depositional units or cycles have been defined. The first type of depositional unit is deposited in a littoral to sublittoral environment that starts with progradational siliciclastic-rich deltaic and shoreline deposits, passes upward into progradational to aggradational carbonate shoal and microbial carbonates, and is covered by mud- to silt-sized littoral to sublittoral deposits and/or oil shale. In the profundal zone, two types of depositional units occur: (1) a depositional unit starts with lean laminated oil shale with silt and sand layers or beds and pass upwards into siliciclastic turbidites, and (2) a depositional unit that starts with bedded evaporates and/or lean laminated oil shale. Both depositional units continue upwards into interbedded rich laminated oil shale and gravitational oil shale. Organic richness of these deposits varies vertically and laterally within the depositional cycle and the richest deposits form dominantly in the beginning of the rich transgressive unit or at the end of the depositional unit during periods of high lake level.
Deposition is controlled by variations in runoff and vegetation that influence the inflow of siliciclastics and nutrients, and therefore also the distribution of facies associations and formation of organic-rich deposits. Two scales of depositional units are identified here, medium-scale units that are 10’s of meters thick, typically 10-60 meters thick, and small-scale, meter thick units. Depositional units are bounded by sequence boundaries and correlative conformities, and are divided into units that represent low, rising, and high lake levels. During times of low runoff, lake level was low, vegetation decreased, fewer nutrients were brought into the lake, and lean oil shale formed. Thin marginal deposits formed during low lake level, at times, evaporite deposition occurred in the deeper part of the basin. During the change to a wetter period, runoff increased and nutrient input increased. The rising lake level is marked by sharp-based sandstones or marginal carbonates. In the profundal area, siliciclastic turbidite and rich laminated oil shale and gravitational oil shale deposits formed. Subsequent wet periods increased vegetation and runoff, bringing high lake levels, and increased nutrients, resulting in rich oil shale. Profundal units are composed of gravity flow and laminated oil shale deposits. Twelve medium-scale sequences correlated to published age dates represent 400 Ky eccentricity cycles. Published early Eocene hyperthermals correlate with five of these sequence boundaries. Small-scale sequences within the medium-scale sequences appear to represent 100 Ky eccentricity cycles.