The end of the U.S. shale boom is about to claim another victim: oil refiners. Refiners are showing signs of slowing down after an unprecedented rise in U.S. crude production sent them on a five-year bull run. Citigroup Inc. downgraded five refining stocks Wednesday, including a subsidiary of billionaire Carl Icahn’s CVR Energy Inc. Profit margins from turning oil into gasoline and diesel fell last week to the lowest level since 2010, according to data compiled by Bloomberg. Why? As output rose, a ban on most exports left U.S. refiners awash in crude, allowing them to buy it for less than competitors in Europe and Asia. Now that producers have put the brakes on drilling, that advantage is eroding. The change may hurt the bottom line at Exxon Mobil Corp. and Royal Dutch Shell Plc, which report earnings later this month. Refining profits […]