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Pump jacks are seen in the Midway Sunset oilfield, California, April 29, 2013. The nearby vast Monterey shale formation is estimated by the U.S. Energy Information Administration to hold 15 billion barrels of technically recoverable oil, or four times that of the Bakken formation centered on North Dakota. Most of that oil is not economically retrievable except by hydraulic fracturing, or fracking, a production-boosting technique in which large amounts of water, sand and chemicals are injected into shale formations to force hydrocarbon fuels to the surface. Picture taken April 29, 2013.  REUTERS/Lucy Nicholson (UNITED STATES - Tags: ENERGY BUSINESS) - RTXZ5IM Lucy Nicholson / Reuters