--> Abstract: The Depositional System of the Green River Oil Shale in Western Colorado, by Yuval Bartov, Dag Nummedal, and Rick Sarg; #90078 (2008)

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The Depositional System of the Green River Oil Shale in Western Colorado

Yuval Bartov, Dag Nummedal, and Rick Sarg
Colorado Energy Research Institute, Colorado School of Mines, Golden, CO

The Green River Fm, an Early Eocene lacustrine system that contains the richest and thickest oil shale deposits in the world. This study focuses on the Piceance basin in Western Colorado which is a small Laramide foreland basin. The depositional history of the lake was reconstructed by correlating depositional sequences at the basin margin to the basin center subsurface using well logs. The correlation is based on gamma ray signal which is controlled by the amount of K- feldspars U and Th that are delivered to the basin from the hinterlands. The sequences were picked in the subsurface by comparing the log signature of nearby wells and identifying the geometries of the depositional surfaces. The log characteristic of the typical sequence begins with a drop in the gamma signal after a prominent gamma ray high. This indicates the increased productivity and sediment supply that follows a lake-level fall and that brings the detrital rich sediments into the lake. The correlated surfaces are thus time significant. The facies distribution in the basin shows a direct connection between these reconstructed lake-levels and the geochemistry of the lake waters. As lake-level fell a sequence boundary formed and the lake water was restricted to the basin center. This increased salinity enabled the precipitation of halite, nahcolite and Dawsonite in the basin center. As lake-level rose, influx of fresh water diluted the upper part of the water column. Restricted bottom water provided the perfect conditions for organic mater preservation leading to the rich oil shale deposits. As lake-level reached the basin margin, shallow water carbonates were deposited. These transgressive carbonates are associated with oil shale rich beds toward the basin center.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas