Oil prices opened the final trading session of 2015 in a downbeat mood after record U.S. crude inventories reinforced concerns over a global supply glut that has pulled down prices by a third over the past year. Crude inventories in the United States, the world’s largest petroleum producer, rose 2.6 million barrels last week, the U.S. Energy Information Administration said. Crude prices held losses after falling 3 percent in the previous session, with U.S.