The Stratigraphy and Depositional Systems of the Green River Oil Shale in the Piceance and Greater Green River Basins, Colorado and Wyoming
The Green River Formation is an Early Eocene lacustrine depositional system that contains the richest and thickest oil shale deposits in the world, and occupied three basins in the western United States, including the Greater Green River basin in Wyoming, the Uinta basin in Utah, and the Piceance basin in Colorado. The depositional history and facies of the three basins are different. Each basin appears to contain different evaporative phases at different times (e.g. Piceance - nahcolite and halite, Greater Green River - trona, and Unita - nahcolite), and the richness of the oil shale units appears to vary between the basins. Stratigraphic correlation between the basins has been initiated using new radioisotopic ages combined with sequence, bio-, and magneto-stratigraphy.
Correlation from outcrop to subsurface in the Piceance basin has been accomplished using gamma ray logs, where anomalously high gamma ray values (Th, U, K) are present at the tops of siltstone units, directly below erosive sequence bounding surfaces. The Rich/Lean (R/L) zonation established by the USGS in the 1970’s, using the Fischer Assay Oil Yield (gallons/ton) data measured from cores are in conformity with the gamma ray marker beds, as well as tuff beds, and the sodium-rich beds of nahcolite and halite. At the lake margins, each sequence is typically composed of a basal siliciclastic sandstone unit deposited during periods when low lake level was falling rapidly or was low. During these low lake level times, salinity increased and nahcolite, evaporites were deposited in the lake center. These units are overlain by deposits of mixed limestone and oil shale deposited during times of rising lake level. High lake levels are represented by thick oil shale units (R units) that pass upwards into lean units of siltstone (L units).
New 40Ar/39Ar dating in the Piceance and Greater Green River basins has allowed a tentative correlation between these lacustrine systems. Each basin shows a history from freshwater overfill (Piceance seqs. 3-7) to balanced-fill (Piceance seq. 8) to underfill (Piceance seqs. 9-13). Northerly sourced volcaniclastic sediments progressively returned each basin, from north to south, to a balanced-fill, and a final overfill state. Each basin evolved from a fresh water system to a hypersaline system, and ended in a probable mesosaline state when the very organic-rich Mahogany oil shale unit was deposited.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009