A long-awaited federal report on oil that could be recovered in parts of North Dakota, Montana and two Canadian provinces is to be released this week, potentially boosting investment in already promising oil territory.

The Bakken shale formation encompasses some 25,000 square miles in North Dakota, Montana, Saskatchewan and Manitoba. About two-thirds of the acreage is in western North Dakota, where the oil is trapped in a thin layer of dense rock nearly two miles beneath the surface.

Ron Ness, president of the North Dakota Petroleum Council, said the number of wells in the Bakken increased from about 300 in 2006 to 457 at the end of last year. Bismarck-based MDU Resources Group, Inc., announced its first venture into the Bakken this week.

The study being released by the U.S. Geological Survey was done at the request of Sen. Byron Dorgan, D-N.D., over the past 18 months.

"Technology continues to advance," Dorgan said Monday. "This is not going to be a red light or green light about oil development in the Bakken - clearly there already is a big green light there. But I think the question is pretty clear -How much of that oil is recoverable using today's technology?"

The Geological Survey in 1995 estimated the amount of recoverable oil in the Bakken at 151 million barrels, said Brenda Pierce, a geologist and program coordinator for the agency's energy resources program.

Pierce said she would not disclose the study's findings until Thursday. Asked whether the estimate would be an increase from the 1995 figure, she said, "There is industry in there and having success. There's your answer."

Julie LeFever, a geologist with the state Geological Survey in Grand Forks, has been studying the Bakken for more than two decades. She calls it an "unconventional resource."

The oil is trapped in microscopic pores of rock, and to capture it, most companies "fracture stimulate" horizontal wells by forcing pressurized fluid and sand to break pores in the rock and prop them open to recover oil.

"It's not something you would see in most oil formations," LeFever said. With technology, she said, "the success rates are going up, but we're not all the way there yet."

She said estimates of the total amount of oil in the Bakken Formation have varied wildly over the years, from 10 billion barrels to 500 billion barrels. The higher estimate was done by Leigh Price, a USGS geologist who died in 2000 before his study was published.

Dorgan urged the agency to review Price's work part of a national inventory of the nation's oil resources.

Pierce, of the USGS, said her agency used raw data from Price's study, but also relied on agency experts and information from oil companies drilling in the Bakken.

The study does not estimate how much oil may be in the formation - only what the agency believes can be recovered using current technology.

State officials plan to release a separate study later this month, which may include an estimate on the total amount of oil the formation holds in North Dakota.

Ness, of the North Dakota Petroleum Council, said the federal study could be helpful by stimulating interest and long-term investment in the Bakken's potential.

"As far as producers in the field, (the study) may or may not have an impact," Ness said. "They're going to drill wells to find out what's out there, anyway."

MDU Resources announced Monday that its first two wells in the Bakken began producing oil last week.

One of the wells is averaging 838 barrels of oil daily, and the second averaged some 1,800 barrels over a two-day period, the company said. Both wells are in Mountrail County.

It's the first venture into oil development in North Dakota for the Bismarck-based energy, mining and construction company, said Tim Rasmussen, a company spokesman.

He said the Bakken was a good place to begin.

"It's a rapidly emerging domestic play in the U.S., and the companies actively producing up there are seeing good results," Rasmussen said. "As a domestic producer of energy, we saw opportunity there."

Rasmussen said MDU had planned to drill 25 wells this year, in Mountrail and Burke counties. "But based on our early success, we're looking at opportunities for further accelerate our drilling beyond that," he said.

The company said it has three drill rigs operating in the Bakken.

The Bakken represents about 16 percent of oil produced in North Dakota, up from about 5 percent in 2006, Ness said.

There were 3,870 operating wells operating in North Dakota' oil patch last month, compared with 3,648 a year ago, state officials say.