The world’s richest deposit of oil shale lies in western Colorado’s Piceance Basin in the Eocene Green River formation. And that resource is even larger than previously thought. The U.S. Geological Survey’s previous assessment, done 20 years ago, placed resources at 1 trillion barrels. A spanking-new assessment shows 1.5 trillion barrels of in-place resource.

The heart of the Green River formation boasts in-place resources of more than 2.8 million barrels per acre, sprawling across 92,000 acres in Rio Blanco County, Colorado. Townships 1s to 2s, ranges 97w to 98w, contain an astonishing 285 billion barrels in place.

Geologically, the oil-shale interval in the Piceance Basin is divided into 17 rich and lean zones, and the USGS assessed each interval separately. These are roughly time-stratigraphic units that are distinctive and laterally continuous sequences, and can be traced across the basin.

The 2009 assessment employed a bounty of new data, donated by oil companies, which were incorporated into prior work. This go-round, the USGS used about twice as many oil-yield data points, measured with the standard Fischer assay method. All data used for the assessment are to be made available to the public in digital form. 

The growth in reserves is mainly due to assessment of new areas and new intervals. However, the USGS noted that much of this previously unassessed resource is low-grade. The agency made no attempt to estimate recoverable reserves, as there is no economic method of recovery at present.

Nonetheless, interest in the potential of oil shale is high because it can be processed to yield liquid transportation fuel. Methods under investigation are surface mining and retorting and in-situ retorting.

"For the first time in 20 years, we have an updated assessment of in-place oil shale in the Piceance Basin of Colorado," says Secretary of the Interior Ken Salazar. "The USGS scientific report shows significant quantities of oil locked up in the shale rocks of the Piceance Basin. I believe it demonstrates the need for our continued research and development efforts."