BISMARCK, N.D. - Billionaire oilman Harold Hamm believes North Dakota's oil reserves are double the federal government's estimates.

Hamm was the keynote speaker Monday at an energy conference in Bismarck. He said the U.S. Geological Survey's estimate of 4.2 billion barrels of oil in the Bakken shale formation could be "100 percent off."

Hamm is the chairman and chief executive officer of Continental Resources Inc., an independent oil and gas company based in Enid, Okla. His company was one of the first to tap the Bakken formation in North Dakota's oil patch 20 years ago.

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.

Hamm also said he believes domestic reserves are growing, and not just in North Dakota.

A service of YellowBrix, Inc.