In a relatively short period of time—less than five years—technological breakthroughs have unlocked massive quanti- ties of natural gas in shale deposits that were previously thought to be unprofitable. The International Energy Agency’s latest estimate of global natural gas resources is more than double the estimate it put forward in 2008. And much of that gas is in the United States.

For decades, drillers were unable to extract profitable quantities of natural gas from low-permeability shale, coal beds, and tights sands deposits. In recent years, however, drillers have perfected two techniques that have long histories in the oil and gas industry: horizontal drilling and hydraulic fracturing. The combination of these techniques ushered in what is known as the “shale gas revolution.” The results of that revolution are only now coming into focus.