April 13, 2012. Royal Dutch Shell Plc, Europe's largest oil company, said an oil sheen between two of its platforms in the Gulf of Mexico is dissipating, and the company is trying to determine where it came from. The sheen, estimated at about six barrels that covered 10 square miles, has broken up. Shell said that an inspection found "no sign of leaks" and "no well control issues" from its operations in the area. The company's Mars and Ursa platforms off the coast of Louisiana are still operating. The platforms handle the equivalent of about 60,000 barrels of oil a day for Shell, about 1.8 percent of the company's output.