* Britain's biggest oilfield shut by power outage - traders
* Expectations for Cushing crude stocks rise weigh on U.S. crude
* Brent August crude contract expiring on Thursday (Rewrites throughout, updates prices, changes byline, moves dateline from previous LONDON)
By Robert Gibbons
NEW YORK, July 16 (Reuters) - Brent crude prices rose on Thursday after a power outage shut production at Britain's largest oilfield and as the August contract approached expiration at the end of the session.
U.S. crude futures gave up early gains and then turned lower on data from industry intelligence firm Genscape showing more crude inventory gains at the Cushing, Oklahoma, hub, brokers and traders said.
Brent August crude LCOQ5 was up 69 cents at $57.74 a barrel at 10:52 a.m. EDT (1452 GMT), having reached $58.21.
September Brent crude LCOU5 was up only 29 cents at $57.41, putting August Brent at a premium to September LCOc1-LCOc2 .
U.S. August crude was down 20 cents at $51.21, after rising to $52.71 intraday.
"Another build at Cushing would be bearish for U.S. crude, and the Buzzard outage and contract expiration are supporting Brent," said Phil Flynn, analyst at Price Futures Group in Chicago.
Britain's Buzzard oilfield, the most important source of crude oil underpinning the global benchmark Brent, was closed after power supplies failed, traders said. ID:nL5N0ZW1XZ
It normally pumps 170,000 to 180,000 barrels per day (bpd) but went down early on Thursday, traders said.
A spokeswoman for Buzzard operator Nexen, a unit of China's CNOOC 0883.HK , declined to comment.