 In the late 1990s, BP Plc boss John Browne used an oil-price slump to usher in the age of the supermajor, buying U.S. rivals Amoco and Arco to create a transatlantic giant. He kicked off a round of mergers that saw his company and four others dominate the industry -- Exxon Mobil Corp. and Chevron Corp. in the U.S., Royal Dutch Shell Plc and Total SA in Europe. 