MEEKER, Colo. — What better evidence of the daunting challenge that oil shale presents: Shell Frontier Oil & Gas, seen as the leader in the quest to free millions of barrels of oil in massive rock formations in a three-state area, doesn't expect to start commercial production any time soon.

The company has been researching ways to tap the vast resource for more than a quarter century and has been running tests since 1996 on private land amid the sagebrush-covered hills and pinon pine and juniper forests of northwestern Colorado.

And yet in July, Shell withdrew a state mining permit to start work on a federal research and development lease granted by the Bureau of Land Management.

"There were a myriad of factors," Shell spokesman Tracy Boyd said.

One was ongoing research and testing. The results could change what Shell will ask for in its permits for work on three 160-acre parcels of federal land approved by the BLM for demonstration projects.

What isn't changing, Boyd said during a recent tour of Shell's research site, is the company's belief that the oil shale formations under western Colorado, eastern Utah and southwest Wyoming could help meet the nation's growing demand for energy.

"We (the industry) have this huge resource sitting here in the United States of unconventional oil in oil shale that is awaiting for someone to crack the technical nut," Boyd said.

Shell may apply for permits again in a year or so, he added. The company hopes to make a decision about commercial production within the next decade.

Federal and industry estimates peg the amount of oil trapped in the rocks from about 1 trillion to 1.8 trillion barrels, or three times the proven reserves of Saudi Arabia. Of that, roughly 800 million barrels are considered recoverable.

The catch is extracting the oil from the rock, something that's been tried on and off for nearly a century. The shale, or kerogen, is a precursor that wasn't buried deeply enough or naturally processed long enough to complete the transformation to oil.

Turning the shale to oil requires heating it: above ground after mining or, as Shell has done, in the ground, a process called in situ — "in place."

"There's talk about being the Saudi Arabia of oil," said Jeremy Boak, project manager at the Colorado Energy Research Institute based at the Colorado School of Mines in Golden. "It's probably never going to be the Saudi Arabia of oil shale production rates."