An unconventional oil and gas revolution is under way in the United States, but its full ramifications are only beginning to be understood. The basic facts are clear enough. Half a decade ago, it was assumed that the U.S. would become a large importer of liquefied natural gas; now the domestic natural gas market is oversupplied, thanks to the ability to produce shale gas through hydraulic fracturing and horizontal drilling technologies.

Shale gas alone is now 10% of the overall...