Refining profits that buttressed earnings for Exxon Mobil Corp. and Royal Dutch Shell Plc as crude prices plunged are now slumping, further pressuring all of the world's biggest oil companies as they move into 2016. Global refining margins, the estimated profit from turning oil into gasoline and diesel, fell 34 percent in the fourth quarter, the steepest decline in eight years, to $13.20 a barrel, data on BP Plc's website show.