Marathon Oil (MRO) led oil stocks lower Wednesday after it reported a smaller-than-expected Q4 profit on rising costs for exploration in the U.S., where it's racing rivals to get a position in oil-rich shale formations in Texas and North Dakota. Marathon said before the market opened that earnings per share plunged 30% to 55 cents, undershooting Wall Street's target by 12 cents. Exploration costs rose 70% during the quarter to $238 million.