BILLINGS, Mont. — Vast coal seams dozens of feet thick that lie beneath the rolling hills of the Northern Plains once appeared almost limitless, fueling boasts that domestic reserves were sufficient to power the U.S. for centuries.

But an exhaustive government analysis says at current prices and mining rates the country's largest coal reserves, located along the Montana-Wyoming border, will be tapped out in just a few decades.

The finding by the U.S. Geological Survey upends conventional wisdom on the life span for the nation's top coal-producing region, the Powder River Basin.

It also reflects the changing economic realities for companies seeking to profit off extracting the fuel as mining costs rise, coal prices fall and political pressure grows over coal's contribution to climate change.

“You're looking at a forty-year life span, maximum, for Powder River coal,” said USGS geologist Jon Haacke, one of the authors of the analysis.

Claims the U.S. had reserves sufficient to last as long as 250 years came from “greatly inflated” estimates of how much coal could be mined, Haacke added. They were based on data put out by the U.S. Energy Department, which for decades has made little distinction between coal reserves that reasonably could be mined and those that could not.

The perception of coal's abundance began to shift in 2008, when the USGS team released initial data that called into question the longevity of U.S. supplies.

Yet assertions America was the “Saudi Arabia of coal” persisted, including in 2010 by President Barack Obama and continuing in recent months by industry supporters. The Department of Energy states on its website that based on current mining rates, “estimated recoverable coal reserves would last about 261 years.”

Belying that outlook is both the USGS assessment and the industry's recent changes in fortune. Mine production has dropped after many electric utilities switched from coal to cheaper natural gas. Two of the three biggest domestic coal companies, Arch Coal and Alpha Natural Resources, declared bankruptcy in the past 18 months.

Leslie Glustrom, an environmental activist from Boulder, Colo., said she believes the end for the Powder River Basin is coming even more rapidly than the USGS study suggests.

And she said it has little to do with a “war on coal” that Republicans frequently accuse the Obama administration of waging.

“This is not a political problem. It's a geologic problem,” Glustrom said.

With coal prices down to $9.55 a ton, the reserve estimate has plummeted to just 16 billion tons, Haacke said.