SEOUL, Jan 26 : South Korea's S-Oil said on Monday it had logged a 91 per cent surge in fourth-quarter operating profit and expects first-quarter refining margins to remain robust due to steady demand, supply disruptions and the planned closure of a U.S. refinery. The refiner, which is 63 per cent owned by Saudi Aramco, reported a quarterly operating profit of 424 billion won ($293 million). Its refining division saw operating profit climb 55 per cent to 225.3 billion won. The refining margin