Since 2008, deep-pocketed foreign investors have subsidized the U.S. energy boom, as oil and gas companies spent far more money on leasing and drilling than they made selling crude and natural gas.

But the rivers of foreign cash are running dry for U.S. drillers. In 2013, international companies spent $3.4 billion for stakes in U.S. shale-rock formations, less than half of what they invested in 2012 and a tenth of their spending in 2011, according to data from IHS Herold, a research and consulting firm.

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