Glen Cayley, technical director of Shell's European exploration and production activities, said Monday the amount is a "significant spill" in the context of annual amounts of oil spilled in the North Sea.
He says the flowline to the Gannet Alpha platform is now leaking around five barrels a day. The leak began last week.
Cayley says he expects waves to disperse the oil sheen and he does not expect it to reach the shore. The platform is still operating.
Shell says the spill covers a surface area of 19 miles by 2.7 miles (31 kilometers by 4.3 kilometers) at it largest point.