Geologists have determined Utah has far less oil shale than previously estimated, but extracting usable energy from those still-vast deposits remains more of a possibility than a probability.

For the first time, the Utah Geological Survey has investigated the Uinta Basin independently of oil-shale regions in Colorado and Wyoming and found that a 44-year-old federal estimate of more than 80 billion barrels of Utah shale oil potential is closer to 77 billion barrels of crude-oil equivalent, the agency announced Wednesday.

But that calculation doesn't take into account how much energy it would take to convert the shale -- a rock called organic marlstone -- into something useful, economical and environmentally acceptable.

Estimates that there could be 1.2 trillion barrels of recoverable shale oil are based on a 1964 U.S. Geological Survey of Utah, Colorado and Wyoming, said UGS investigator Michael Vanden Berg. About half the shale is in Colorado, with Utah and Wyoming splitting the rest.

The UGS analyzed data from 293 wells throughout eastern Utah's Uinta Basin, then mapped the thicknesses of the oil shale. The study found that the richest zones are in central Uintah County. The resulting estimate of how much crude-oil equivalent might be there took into account areas where shale development is prohibited, such as national parks and wilderness-study areas, and excluded zones where oil and gas production is active.

The geologists used the petroleum industry's estimate that a ton of shale would yield 25 gallons of shale oil to hone their conclusion, Vanden Berg said.

Even so, it's all theoretical. "It's an in-place resource," he said. "That's very different from a recoverable resource."

The U.S. Bureau of Land Management likewise acknowledges Western shale-resource estimates aren't based on reality. But the agency had to include some number in its recently completed environmental study, Washington, D.C.-based BLM spokeswoman Heather Feeney said in February.

"Right now, there is no technologically or economically viable way to recover oil shale," Feeney said. "In this particular geological formation, it's difficult to determine exactly what portion is recoverable."

To calculate the amount of Utah shale oil that could be recovered economically, there would have to be a technology that works here because every deposit is different, Vanden Berg said. "A technology that works in Brazil is not necessarily going to work here."

Only one company, Oil Shale Exploration Co., has a lease to research, develop and demonstrate the economic feasibility of shale oil on federal land in Utah. OSEC's federal lease so far has allowed it to export waste rock left over from the 1980s shale bust to Alberta, Canada, for testing a process developed in Australia for an oil-shale experiment that recently was shut down, costing Calgary-based Suncor its $100 million investment.

Amy Hansen, an OSEC spokeswoman, said the 300 tons of shale shipped to Canada yielded 9,000 gallons of kerogen, which could be further refined into kerosene or diesel fuel. But no North American refineries accept kerogen.

In June, OSEC announced a partnership with Petrobras of Brazil and Mitsui & Co. Ltd. of Japan to spend $12 million to study the feasibility of Petrobras' Petrosix technology on Utah oil shale on 40,000 acres of private land near Vernal.

Oil-shale critic Randy Udall, director of the nonprofit Community Office for Resource Efficiency in Aspen, Colo., says that per pound, oil shale contains one-tenth the energy of crude oil, one-sixth that of coal and a quarter the energy that comes from burning dung cakes.

phenetz@sltrib.com