The UK's Buzzard oil field contributed only 16% of Forties blend output in the week of February 18-24, down from 33% the previous week, BP data showed Friday.
This is the lowest contribution of Buzzard to Forties seen so far in 2013, and BP did not provide data for previous years.
The fall coincided with a fall in production at Buzzard, leading into scheduled field maintenance in order to carry out pigging of field pipelines. The data does not go up to the post-maintenance period, starting on March 5, when Nexen announced that Buzzard had already ramped up to 200,000 b/d.
BP did not provide actual production figures.
Buzzard was scheduled to contribute 40.6% of Forties output in February, BP data showed, with the share set to fall to 36.3% in March. Buzzard's share is projected to reach 39.5% and 40.6% in April and May, respectively.
BP data showed that Forties production in February reached 464,500 b/d, falling to a projected 444,100 b/d in March. Forties production in April and May is projected to reach 481,700 b/d and 468,400 b/d, respectively.
Buzzard's nameplate capacity is 220,000 b/d but but in the 12 months to November it averaged only 156,420 b/d, according to figures from the UK's Department for Energy and Climate Change.
Buzzard is the single largest oil field feeding into the Forties blend, itself the basis for the Dated Brent oil benchmark.
