An area of shale and other rock in North Dakota and Montana is estimated to hold the largest potential oil resources in the 48 contiguous states, according to an assessment released this week by the U.S. Geological Survey.

The area, known as the Bakken Formation, might contain 3 billion to 4.3 billion barrels of oil that could be extracted using current technology, the survey said.

The United States had an estimated 21 billion barrels of proven oil reserves in 2006, according to the Energy Department. The new assessment by the Geological Survey could raise these reserves once drilling starts.

The survey reinforces what oil companies that have flocked to the region already knew: A boom is afoot. But geologists and industry officials alike cautioned that the number was simply an estimate, easily skewed higher or lower by technological advances or economic changes. If the price of oil drops, companies will not be willing to spend as much to extract it, and the Bakken Formation, which also extends into Canada, requires an expensive technique called horizontal drilling.

The new estimate covers undiscovered, technically recoverable oil resources, which it defines as those "producible using currently available technology and industry practices." That definition is one notch below the "proven" oil reserve category.

Sen. Byron L. Dorgan, D-N.D., who requested the survey, said the resources should attract investment to his state and make the United States "marginally" less dependent on foreign oil.

"The technology always improves and advances," Dorgan said, "so there's probably much more recoverable in the future."

The survey reveals a 25-fold increase from the last U.S. Geological Survey assessment of the Bakken Formation in 1995, which estimated that 151 million barrels could be extracted.

With oil prices rising above $110 a barrel on the New York Mercantile Exchange this week, unconventional resources such as shale rock are becoming more profitable to develop.