WASHINGTON — The United States has an oil reserve at least three times that of Saudi Arabia locked in oil-shale deposits beneath federal land in Colorado, Utah and Wyoming, according to a study released yesterday.

But the researchers at the RAND think tank caution the federal government to go carefully, balancing the environmental and economic impacts with development pressure to prevent an oil-shale bust later.

"We've got more oil in this very compact area than the entire Middle East," said James Bartis, RAND senior policy researcher and the report's lead author. He added, "If we go faster, there's a good chance we're going to end up at a dead end."

For years, the industry and the government considered oil shale — a rock that produces petroleum when heated — too expensive to be a feasible source of oil.

However, oil prices, which spiked above $70 a barrel this week, combined with advances in technology could soon make it possible to tap the estimated 500 billion to 1.1 trillion recoverable barrels, the report found.

The study, sponsored in part by the U.S. Department of Energy, comes about a month after the president signed a new energy policy dramatically reversing the nation's approach to oil shale and opening the door within a few years to companies that want to tap deposits on public lands.

The report also says oil-shale mining, above-ground processing and disposing of spent shale cause significant adverse environmental impacts. Shell Oil is working on a process that would heat the oil shale in place, which could have less effect on the environment.