FILE - In this Feb. 13, 2015, file photo, stacks and burn-off from the Exxon Mobil refinery are seen at dusk in St. Bernard Parish, La. A barrel of U.S. crude fell below $40 per barrel, Friday, Aug. 21, 2015, for the first time since the end of the global economic crisis. (AP Photo/Gerald Herbert, File)

Barrel of US crude drops below $40

Friday’s fall, to $39.86, was just the latest indicator of a vast shift in the energy landscape over the past year.