US crude oil futures closed Friday's session down 48 cents at $59.15 per barrel after weekly data from oilfield services firm Baker Hughes showed U.S. drillers continued to draw down the number of rigs exploring for oil.
The oil rig count fell by 24 to a total of 679, down from 1,527 at the same time last year. The report marked the 21st consecutive week that energy firms took more rigs offline than they added.
Brent was down 37 cents at $66.42 a barrel by 2:38 p.m. EDT (1438 GMT). It rose to a 2015-peak of $66.93 on Thursday and increased 21 percent in April. West Texas Intermediate traded as high as $59.90 a barrel before settling.
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Oil prices eased off 2015 highs on Friday as the dollar strengthened and after Iraq said its crude oil exports hit a record in April.
Brent and U.S. crude rallied between 20 and 25 percent in April, helped by a weaker dollar and bets that a global supply glut would ease, following the June-to-January sell-off that halved prices from above $100 a barrel.
News that Iraq's oil exports rose in April to a record 3.08 million barrels per day from 2.98 million bpd in March served as a reminder of ample supply in the market.
Iraq's data highlights increasing output from OPEC members, whose supply in April rose to its highest in more than two years at 31.04 million bpd, according to a Reuters survey.
Trading was thin, with some major markets closed for the May Day holiday.
"What is driving prices these days is less physical markets which remain very weak but more expectations of future tightening," said Amrita Sen, chief oil analyst at Energy Aspects.
Despite a sharp drop in new U.S. shale drilling in recent months, there have been few signs that a global supply glut is easing, and there are some signs that U.S. oil production may not fall significantly in the near future.
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Futures and options trades suggest U.S shale producers are locking in production costs for next year, which could pave the way for a rebound in production.
"If markets don't tighten as quickly as people are expecting, the sell-off can be large," Sen said.
A Reuters survey showed OPEC oil supply in April rose to its highest in more than two years to 31.04 million barrels a day.
Saudi Arabia, the biggest exporter, kept output near a record high in April, sources in the survey said.
Saudi Arabia approved a restructuring of Saudi Aramco that includes separating it from the oil ministry, a Saudi-owned TV station reported on Friday, in a decision analysts said aims to make the state oil giant more transparent and keep it away from political sway.
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There were no indications that the restructuring, which Al Arabiya reported on Friday citing sources, will lead to fundamental changes in how the world's top crude exporter decides its energy policy.
Aramco officials could not be immediately reached for comment but Arabiya's reports closely reflect official thinking.