OPEC Secretary General Abdalla Salem El-Badri, center, is interviewed in Vienna ahead of the oil… ENLARGE OPEC Secretary General Abdalla Salem El-Badri, center, is interviewed in Vienna ahead of the oil cartel’s meeting on Thursday. Agence France-Presse/Getty Images Energy company stocks and the currencies of major oil-producing nations stumbled Friday as OPEC’s decision to maintain crude output levels despite a glut rippled across the globe. The Organization of the Petroleum Exporting Countries’ decision knocked down U.S. benchmark oil prices on Friday by 10% to $66.15 a barrel, the lowest level since September 2009. Uneasy investors dumped energy stocks. Among the hardest hit were U.S. domestic oil producers including Continental Resources Co., the biggest producer in North Dakota’s Bakken Shale. Its shares plunged on Friday nearly 20%, to $40.98. Exxon Mobil Corp. fell 4.2%, BP PLC dropped 5.5% and Royal Dutch Shell PLC lost 7%, all in abbreviated New York […]