Government scientists should try to find out how much crude can be recovered from a promising reservoir beneath North Dakota's already prolific oil patch, Sen. Byron Dorgan says. The U.S. Geological Survey says a study now would be premature.

The Three Forks-Sanish formation is made up of sand and porous rock directly below the rich Bakken shale in western North Dakota.

"The question is: What's there and what's recoverable using today's technology?" said Dorgan, D-N.D., who said he will make a formal study request to the USGS next week.

Rich Pollastro, a USGS geologist, said the federal agency does not have enough data for a comprehensive study of the Three Forks-Sanish formation.

"We would need at least five years of new history before we would even consider it," Pollastro said.

Geologists and oil companies are not sure if the Three Forks-Sanish is a separate oil-producing formation or if it acts as a trap, catching oil that leaks from the Bakken shale above. Some say it could be a combination of both.

New horizontal wells aimed at the Three Forks-Sanish in recent months have been successful. But those wells came online after the USGS released a study in April that estimated up to 4.3 billion barrels of oil can be recovered in the Bakken, where oil-producing rock is sandwiched between layers of shale about two miles under the ground.

The potential of the underlying Three Forks-Sanish formation was factored into the agency's estimate for the Bakken, though it was based on production from traditional vertical wells that were spudded decades ago, Pollastro said. The Three Forks-Sanish added about 500 million barrels to the estimate, he said.

The last formal study on the formation in North Dakota was done 40 years ago, Pollastro said.

Continental Resources Inc., based in Enid, Okla., said it tapped its first Three Forks-Sanish well in May in Dunn County. The well averaged about 700 barrels of oil daily in its first week of production, spurring the company's stock to rise 23 percent. Continental, which is the largest leaseholder in the Bakken with more than a half-million acres, has since targeted 27 new wells at the Three Forks-Sanish. Fourteen of the wells are producing.

Continental spokesman Brian Engel said some wells in the new formation are outperforming Bakken wells.

"The results are even better, in some cases," he said. "We're quite pleased."

Engel said a federal study of the formation would "educate the public on what we're working with and it would bring a lot more knowledge to what we think is a new formation."

The Three Forks-Sanish formation was first targeted in the 1950s, in an oil play known as the Antelope Field in McKenzie, Williams and Divide counties in northwestern North Dakota, said Julie LeFever, a geologist with the state Geological Survey in Grand Forks.

LeFever, who has studied the Bakken formation for more than two decades, said the state is doing its own assessment of the Three Forks-Sanish.

How much oil may be trapped in the formation below the Bakken is unknown. But LeFever said a comprehensive government study of the Three Forks-Sanish likely would spur more investment in North Dakota's oil industry.

"Everybody knows there's oil in the Bakken and the Three Forks-Sanish - to tell a geologist or an oil company that is not a news flash," LeFever said. "I think this is something they can take to investors."

The Geological Survey has called the Bakken formation the largest continuous oil accumulation it has ever assessed. Dorgan also requested the agency do that study, which took 18 months.

The state completed its own independent study of the Bakken formation at about the same time the federal study was released. The results were similar, though the state did not account for oil from the Three Forks-Sanish.

The Bakken 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. Companies target the shale horizontally, and use pressurized fluid and sand to break pores in the rock and prop them open to recover the oil. The technology has come far in the past couple of years but is still being perfected, industry officials say.

LeFever said the Three Forks-Sanish is typically about 250 feet thick, and underlies the entire Bakken formation. She believes oil found in the Three Forks-Sanish has "migrated" from the Bakken over millions of years.

"I'm beginning to think a lot of our Bakken reservoir is in the Three Forks-Sanish," she said.