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BISMARCK, N.D. (AP) — North Dakota oil drillers and U.S. Sen. John Hoeven want federal geologists to reevaluate the amount of recoverable crude oil in the state, saying a new assessment likely would show stronger production potential and attract investment.

The U.S. Geological Survey once said two massive shale formations found in North Dakota held the largest continuous oil accumulation it ever assessed. But the title was given to a formation in Texas’ Permian Basin last year, after a USGS assessment found nearly three times the amount of recoverable oil than in the Bakken and Three Forks formations.

Hoeven now wants the USGS to take into account other formations in western North Dakota’s oil patch that could be exploited using technology developed for the Bakken and the Three Forks directly below it.

“The industry wants a broader study,” the North Dakota Republican said. “The industry feels it will show a larger resource if the other formations are included.”

Hoeven said top officials at the USGS have given assurances that the study would be done, though it could take “a year or two” to begin.

USGS spokesman Dave Ozman said he was aware of the request but no formal decision has been made to do a new assessment.

North Dakota geologists say nearly 20 different geologic formations have been tapped in the state since oil was first brought to the surface 66 years ago. The Bakken and Three Forks, which only have been tapped in the last decade using hydraulic fracturing, now account for 95 percent all of North Dakota’s more than million-barrel-per-day oil production, ranking it second behind Texas.

Know-how gained from the Bakken and Three Forks is being tested on other formations in the state and it has been successfully applied elsewhere, creating a rivalry for drilling rigs and investment.

Ron Ness, president of the North Dakota Petroleum Council, said investment dollars have dropped in the state as companies and investors have more options.

“We’ve certainly seen a lot of competition from other oil shales,” said Ness, whose group represents more than 500 companies working in the state’s oil patch.

Ness said a new assessment that would consider underdeveloped formations would give a more accurate picture of the state’s oil capability.

“Having the best information available would do a lot for development and reaffirm and show the potential,” he said.

The Bakken formation encompasses some 25,000 square miles within the Williston Basin. About two-thirds of the acreage is in western North Dakota, where the oil is trapped in a thin layer of dense rock nearly 2 miles beneath the surface. The Three Forks formation underlies most of the Bakken.

In 2013, the USGS released data that showed 7.4 billion barrels of oil could be recovered from the Bakken and Three Forks spanning parts of North Dakota and parts of Montana, nearly double the amount the agency previously estimated for the region in 2008.

The USGS’s study last year found Wolfcamp shale, located in the Midland Basin portion of Texas’ Permian Basin, contains 20 billion barrels of recoverable oil.

Billionaire oilman Harold Hamm, chairman and chief executive officer of Oklahoma City-based Continental Resources Inc., has long said that government estimates of recoverable oil in the Bakken and Three Forks formations are too conservative.

Hamm, whose company is one of the oldest and biggest operators in North Dakota, has for years maintained the Bakken and Three Forks alone hold more than 20 billion barrels of recoverable oil, which would rival new the federal estimate for the Texas formation.