Workers use perforating tools to create fractures in rock. An EPA study finds that "fracking" to reach and extract deep pockets of hydrocarbons has not caused widespread drinking water pollution. Brennan Linsley/AP hide caption

itoggle caption Brennan Linsley/AP

Pumpjacks at the Inglewood oil fields in California in March. Some of the most controversial methods of oil extraction, like fracking, oil sands production and Arctic drilling, are also expensive. That's made them less profitable as the price of oil continues to fall. Richard Vogel/AP hide caption

itoggle caption Richard Vogel/AP