The Bakken shale formation in western North Dakota and eastern Montana has produced 1 billion barrels of crude since production began, data from the two states showed.
North Dakota has generated 852 million barrel, and Montana around 151 million barrels through the first quarter of 2014.
Drillers first targeted Bakken shale in Montana in 2000.
Producers moved into North Dakota five years later when advancements in horizontal drilling and fracking techniques allowed them to viably recover oil two miles beneath the surface.
Continental Resources, founded by billionaire Harold Hamm, said two-thirds of Bakken production happened in the past three years.
The company said, “This milestone validates the immense potential of the Bakken field, and development is just beginning.”