State data show that 1 billion barrels of oil have been produced from the rich Bakken shale formation in western North Dakota and eastern Montana.
Drillers first targeted the Bakken in Montana in 2000 and moved into North Dakota about five years later using advanced horizontal drilling and hydraulic fracturing techniques.
Data show that North Dakota has tallied 852 million barrels of Bakken crude, and Montana has produced about 151 million barrels.
The Bakken encompasses some 25,000 square miles in North Dakota, Montana, Saskatchewan and Manitoba. About two-thirds of the acreage is in western North Dakota.
Oklahoma-based Continental Resources is the biggest leaseholder in the Bakken. The company says two-thirds of the production from the Bakken has come in the past three years.