WATFORD CITY, ND - JULY 23: Workers with Raven Drilling line up pipe while drilling for oil in the Bakken shale formation on July 23, 2013 outside Watford City, North Dakota. North Dakota has been experiencing an oil boom in recent years, due in part to new drilling techniques including hydraulic fracturing and horizontal drilling. In April 2013, The United States Geological Survey released a new study estimating the Bakken formation and surrounding oil fields could yield up to 7.4 billion barrels of oil, doubling their estimate of 2008, which was stated at 3.65 billion barrels of oil. (Photo by Andrew Burton/Getty Images)
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