American attention to oil shale has grown and waned for more than 150 years. The OPEC oil embargo of 1973 and 1974, during which gasoline prices soared, renewed interest in commercial production.

The Interior Department began investigating how to commercialize the Green River deposits in 1967, according to the U.S. Geological Survey.

"The dramatic increases in petroleum prices resulting from the OPEC oil embargo ... triggered another resurgence of oil-shale activities during the 1970s and into the early 1980s," adds the USGS 2006 report, "Geology and Resources of Some World Oil-Shale Deposits." Federal leases in Utah, Colorado and Wyoming were offered through competitive bids under the Federal Prototype Oil Shale Leasing Program. Two tracts in Utah and two in Colorado were leased to oil companies.

No shale oil was produced on the federal tracts in Colorado, but Unocal Oil Co. developed oil-shale facilities on private land in Colorado.

Also, oil-shale experiments were carried out for decades on the Naval Oil Shale Reserve near Rifle, Colo. The Deseret News visited the reserve's Paraho plant, located at Anvil Points, in the early 1970s, finding it a bustling project with heaps of shale to be heated and a huge retort structure.

According to the Colorado School of Mines in Golden, the Paraho project later closed down but the pilot plant experimentally produced fuel from Australian oil shale.

The USGS report says three energy companies spent about $80 million on the Utah tracts. Several thousand barrels of oil were produced before the experiments ended.

Exxon began developing a huge oil-shale project near Parachute, Colo., but dropping oil prices rendered it uneconomical. Exxon ended the project on May 2, 1982 — a date infamous in Colorado as "Black Sunday." The Post Independent of Glenwood Springs, Colo., recently estimated that 2,000 people lost their jobs in a single day. In a searing dislocation, the local economy plummeted and expensive homes lost value.

The Unocal plant near Rifle, Colo., was the last major oil-shale project on the Green River Formation, according to the USGS. Construction began in 1980, $650 million was spent in the effort, and around 4.4 million barrels of shale oil was produced. But in 1991, it too closed. Today, with gasoline generally above $3 a gallon, interest in oil shale has been revived.

Michael Vanden Berg, a geologist with the Utah Geological Survey, said liquid petroleum is much more valuable than coal. It is worth more per British Thermal Unit, or unit of energy produced.

Hanging up oil-shale development, he believes, is not so much the price of crude oil as "coming up with a viable technology that is environmentally benign."