Nine weeks of maintenance at UK's Buzzard oilfield in 2014

(Expands on likely impact, adds market reaction)

LONDON, Feb 7 (Reuters) - Britain's biggest oilfield, Buzzard, will undergo a total nine weeks of maintenance in 2014, rather than the two weeks traders had expected, according to the latest information seen by Reuters.

Two weeks of maintenance are planned at the North Sea field in May, four weeks in August and three weeks in September, according to details outlined in a Suncor investor presentation released this week.

Suncor, which has a 29.9 percent stake in Buzzard, announced second and third quarter maintenance at the field in its fourth quarter earnings release. The second quarter maintenance came as a surprise to the market.

Duration, timing and production impact details to Suncor were given in its investor presentation.

The work in May and August is estimated to cut crude volumes from Suncor's portion of the Buzzard field by around 55,000 barrels per day apiece.

The September work is expected to have an impact of about 15,000 bpd. Actual barrels lost will be higher if the field is in full shutdown during these periods.

"It's approximate in terms of date - the (nine weeks) duration is based on the information we have been provided and the estimated crude impact would be net to Suncor," Sneh Seetal, a spokeswoman for Suncor, said on Friday.

Buzzard is the biggest contributor to the Forties crude stream, one of the four crudes that underpins the Brent benchmark. The field has a maximum operating capacity of about 210,000 barrels per day (bpd).

Some analysts cautioned that the maintenance was still unconfirmed, but Brent crude oil futures rose as traders digested the news. "That's a lot of August-September downtime," one said.

Buzzard has already experienced four unplanned outages this year. Analysts at JBC Energy said the extended planned turnaround may be a bid to stabilise output.

"This implies that Brent prices will see spikes going forward, as Forties is generally the price-setter for the global benchmark," the analysts said in a note on Friday.

Buzzard is operated by Nexen (KSE: 005720.KS - news) , a unit of Chinese state company CNOOC (HKSE: 0883.HK - news) . Nexen did not immediately return requests for comment on the latest maintenance details but in January told Reuters that it wanted to improve Buzzard's reliability in 2014. (Reporting by Claire Milhench; additional reporting by Nia Williams in Calgary; editing by Keiron Henderson)