According to Bloomberg, ConocoPhillips will spin off its refining and marketing arm into a new business, two weeks after Marathon Oil Corp. split itself up to increase returns for investors. ConocoPhillips, based in Houston, will divide into two stand-alone, publicly traded operations. The division is expected to be completed in the first half of 2012. Marathon Petroleum Corp., Marathon’s former refining business, debuted on the New York Stock Exchange this month. Marathon rose as much as 11 percent on the day the split was announced. ConocoPhillips is the second-largest U.S. oil refiner with capacity of 2 million barrels per day, according to its website. It owns 12 U.S. refineries, and has a two-refinery joint venture with Alberta-based oil producer Cenovus Corp. The plants account for about 10 percent of U.S. refining capacity, according to data compiled by Bloomberg. Following the split, ConocoPhillips’ refining arm will be the largest U.S. independent refiner, with more capacity than Valero Energy Corp., according to data compiled by Bloomberg. ConocoPhillips also owns five refineries outside the U.S.