<P> Black shales similar to the eastern shales are widely distributed west of the Mississippi River . These include some, such as the Woodford Shale of Oklahoma, which are near - stratigraphic equivalents to the Devonian / Mississippian shales of the eastern US . </P> <P> The largest oil - shale resource in the world is contained in the Eocene Green River Formation in Colorado, Utah, and Wyoming in three basins: the Piceance Basin, Green River Basin, and Uinta Basin . The Green River oil shales have been the focus of most efforts of the past hundred years to establish an American oil shale industry . </P> <P> The Green River oil "shale" is actually a marl, and some beds yield up to 70 gallons of oil per short ton of shale . Estimated in - place resources are capable of generating 4.2 trillion barrels of oil . Although the Piceance basin of western Colorado has the smallest lateral extent of Green River shale, it contains the greatest amount of high - grade shale . Using a cut - off of 25 gallons per ton, the Piceance has about 352 billion barrels of in - place resource . </P> <P> The Elko Formation of northeastern Nevada is an oil shale, late Eocene to early Oligocene in age . Like the Green River Formation, it contains beds deposited in a lake . Faulting and subsequent erosion have removed most of the original extent of the deposit, leaving isolated bodies of the formation within an area about 100 miles long and 30 miles wide in Elko County, Nevada . </P>

When did the west begin to thirst for oil