Significant commercial production could be 10 to 20 years away, Boak said. But if the economic, technical and environmental issues can be resolved, he said, oil shale could help bridge the gap until renewable or alternative energy becomes more common.

Oil prices hovering above $90 a barrel could make the attempt all the more enticing.

About half the oil shale underlying the region is in the Piceance Basin of northwest Colorado. That's where it's the "deepest, thickest, richest," Boyd said: 2,000 feet down to the base of the oil shale formation.

Besides Shell, Chevron USA and Midland, Texas-based EGL Resources Inc. received 10-year federal research and development leases in the basin last year.

Early this year, the Interior Department approved a 10-year lease for Alabama-based Oil Shale Exploration Co. for the only oil-shale experiment on federal land in Utah.

"The sheer magnitude of the resource is just world-class," said Robert Lestz, oil shale technology manager for Chevron USA.

The research leases could lead to larger ones for commercial production. By year's end, the BLM is expected to release a draft environmental review of commercial oil shale development. The analysis is meant to provide a framework; more detailed reviews would be done as specific projects are proposed.

ExxonMobil Corp., which is increasing its natural gas production in Colorado, failed to land one of the research leases.

It was Exxon's shutdown of its $5 billion project near Parachute on May 2, 1982, that marked the end of the last oil shale boom and sent the western Colorado economy reeling for years.

Some politicians are urging caution. A measure by Rep. Mark Udall, D-Colo., in the House version of the 2008 Interior Department's appropriations bill would prohibit using federal funds to prepare final regulations for a commercial oil shale leasing program or conduct commercial lease sales.

Udall has said he wants to make sure oil shale is developed responsibly to avoid another economic bust.

The 2005 federal energy bill required the BLM to prepare an environmental impact statement and commercial regulations.

Environmentalists worry that an oil shale boom could strain area water supplies and increase air pollution if more coal-fired power plants are built to power operations.

Lestz of Chevron said he believes the technology wasn't adequate to mine oil shale in the 1970s and 1980s and doesn't know if it is yet.

Chevron is working with the University of Utah and the Los Alamos National Laboratory in New Mexico on its in situ process.