The U.S. Geological Survey has called the Bakken the largest continuous oil accumulation it had ever assessed.
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Oil drillers targeting the rich Bakken shale formation in western North Dakota and eastern Montana have produced 1 billion barrels of crude, data from the two states show.

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 to recover oil trapped in a thin layer of dense rock nearly two miles beneath the surface.

North Dakota has generated 852 million barrels of Bakken crude, and Montana has produced about 151 million barrels through the first quarter of 2014, data show.