Northeastern Montana lies within the Williston Basin, a bowl-shaped geologic depression centered in North Dakota that slowly subsided over hundreds of millions of years and accumulated up to 16,000 feet of sedimentary rock in its deepest part. The basin became a major oil and natural gas province because it contains organic-rich deposits to generate hydrocarbons, reservoir rocks where they can accumulate, and geologic traps such as faults, folds, and lateral rock changes that hold them in place. One deeply buried unit in the basin is the Bakken Formation, a dolomite bed between two organic-rich shales deposited in deep and coastal waters about 350 million years ago during the Late Devonian age. Discovered in 2000, the Elm Coulee Oil Field covers about 750 square miles, with oil at depths of 8,500 to 10,500 feet in rock only about 45 feet thick. Its oil is extracted through horizontal drilling and hydraulic fracturing, production began in 2001, doubled Montana's oil output within a few years, and by 2013 nearly 150 million barrels had been extracted.