Buzzard is the largest of the fields that contribute to the Forties crude blend, the most important of the North Sea crudes underpinning the Brent crude benchmark.

"Buzzard is doing well," one of the sources, an equity holder in Forties output, said.

The field restarted on Nov. 3 after an extended, two-month shutdown for maintenance and output was briefly halted earlier this month due to a power outage.