CHEYENNE - The U.S. Geological Survey has lowered its estimates of the amount of recoverable coal in the nation's most prolific coal field, but not enough to jeopardize the characterization by some that the United States is the Saudi Arabia of coal.

The USGS report released this week estimates there are 77 billion short tons of recoverable coal in the Gillette field, down 29 percent from the 109 billion short tons estimated in 2002 in generally the same area.

Fred Freme, coal industry statistician with the federal Energy Information Administration, said Friday the new estimate may sound like a significant change, but considering the billions of tons of mineable coal in the country it's not.

"It's not like that change means we're going to run out of coal in the next 10 years," Freme said.

The USGS, which is assessing coal reserves in the nation's largest coal fields, started with the most productive one - the coal field around Gillette in northeast Wyoming. The Gillette coal field is part of the coal-rich Powder River Basin in northeast Wyoming and southeast Montana and has coal seams up to 200 feet thick.

"A lot of times we talk about the United States as the Saudi Arabia of coal, that type of thing," said Jim Luppens, who heads the USGS coal assessment program. "… A lot of people would question how accurate is that because there really hasn't ever been a real in-depth analysis like what we're trying to do with this Gillette coal field."

Coal is the most reliable and affordable energy source in the United States. However, coal-fired power plants produce carbon dioxide, which some say is a major cause of climate change.

Researchers are trying to find cleaner ways to use coal. If they are successful, coal may remain a huge energy source for the United States well into the future, and so it is important to get more accurate estimates of the nation's coal reserves.

"I think that's a pretty important number to have in terms of long-term energy planning, that sort of thing," Luppens said.

Wyoming is the largest coal-producing state, digging out 453.6 million short tons of coal in 2007. The Gillette coal field alone produced 431 million short tons, or about 38 percent of the nation's entire annual coal production.

"This is why we started with the Gillette coal field in this new assessment," Luppens said. "It is the single most important coal field in the United States right now in terms of total production."

Luppens said the USGS had more and better data to analyze the Gillette coal field than it did in 2002.

The information showed an ancient river channel, the dimensions of which geologists had not fully understood, that limited the formation of coal.

"If you got a channel out there and no coal, that's obviously going to cut the previous estimates of coal," Luppens said. "So that's one of the main reasons we came up with the lower estimates this time."