By Jessica Resnick-Ault
NEW YORK (Reuters) - Benchmark Brent crude prices edged higher on Monday, supported by a North Sea pipeline outage and a workers' strike in the Nigerian energy industry, but U.S. prices slid slightly, highlighting concerns about growing U.S. output.
Brent crude futures (LCOc1), the international benchmark, settled up 18 cents at $63.41.
U.S. crude futures (CLc1) fell 14 cents to $57.16.
Brent had traded as high as $63.91 earlier in the day but fell back after Ineos, operator of the North Sea Forties pipeline, said the crack that shut it down had not spread.
"The Forties pipeline outage is continuing to be supportive of the market," said John Kilduff, partner at Again Capital. "We're just watching this as to see how the market reacts to not having these barrels available."
The 450,000-barrel-per-day link that provides some of the physical crude underpinning Brent has been shut since Dec. 11, forcing Ineos to declare force majeure on all oil and gas shipments from it last week.
"There is still no reliable information about how long the repair work will last and when the pipeline will go back into operation," Commerzbank said in a note. "This should preclude any fall in the Brent price for the foreseeable future".
Early in the session Brent was under further pressure as a major Nigerian oil union began a nationwide strike, but the action concluded the same day it began, after a domestic oil and gas company recalled laid off staff. Strikes could resume in January, according to the president of the Petroleum and Natural Gas Senior Staff Association of Nigeria.
U.S. production (C-OUT-T-EIA) has soared 16 percent since mid-2016 to 9.8 million bpd, approaching the output of top producers Saudi Arabia's 10 million bpd and Russia's 11 million bpd.
This has undermined market-balancing efforts by the Organization of the Petroleum Exporting Countries and a group of non-OPEC producers, including Russia, to withhold production.
Largely because of rising U.S. shale output, the International Energy Agency said global oil markets would show a supply surplus of about 200,000 bpd in the first half of 2018.
The U.S. Energy Information Administration showed a similar surplus for that period and indicated a supply overhang of 167,000 bpd for all of 2018.
(Additional reporting by Henning Gloystein in Singapore and Libby George in London; Editing by David Evans, Lisa Von Ahn and Frances Kerry)