Royal Dutch Shell says it still does not know what caused the worst North Sea oil spill in a decade but the leak is now only pumping one barrel — or 42 gallons — of oil into the cold water per day.

Company spokeswoman Sally Hepton said Wednesday the company is still trying to isolate and shut down the leak.

Shell had said the main spill was caused by a leak at an oil flowline at the Gannet Alpha platform 112 miles (180 kilometers) east of the Scottish city of Aberdeen. It said that was under control but it is struggling to control a second leak from a relief valve near the flowline.

Shell says the oil sheen on the North Sea now covers 6,400 acres (26 square kilometers).