Fears low oil prices will persist for years sent shares of U.S. crude producers tumbling on Friday after OPEC failed to agree on a unified output cap, effectively letting its 13 members pump at will under a policy aimed in part at squeezing out U.S. rivals. OPEC ministers, who control a third of the world’s oil supply, ended their meeting in Vienna on a discordant note, unable to decide as a group how much it should pump in aggregate. It appeared for a while OPEC would raise its current 30 million barrels per day (bpd) cap on production, but negotiations broke down after Iran said it would not accept any limits until it emerges from Western-imposed sanctions. Iraq’s oil minister echoed those sentiments, asking after the meeting why OPEC members should accept a production cap if non-OPEC oil producers do not have one. The comments exacerbated oversupply worries and sent U.S. oil prices CLc1 below $40 a barrel, dragging down U.S. energy stocks. Global oil prices have already dropped more than 50 percent in the past year, and futures trading indicate Wall Street does not expect them to rise above $50 per barrel until July 2017 at the earliest.