US light crude oil CLc1 dropped 0.14 cents at $48.95 barrel. Brent crude was down 71 cents, or 1.3 percent, to $53.78 a barrel after reaching its highest level since April at $54.87.
According to the U.S. Energy Information Administration, U.S. weekly crude stocks increased 4.6 million barrels the week-ending September 1, topping analysts' forecast for a 4.0-million-barrel build in a Reuters poll.
While refineries, pipelines and offshore platforms resume operations after Harvey, another Atlantic hurricane, known as Irma, is approaching the U.S. coast and is set to hit Florida tomorrow. Brent found some support from news that Saudi Arabia will trim oil supply allocations to its customers worldwide in October by 350,000 barrels per day (bpd).
Oil futures were mixed on Thursday, with Brent rising to a 5-1/2 month high while US crude slipped on a bigger-than expected crude stock build, as the restart of USA refiners after Hurricane Harvey was countered by the threat of Hurricane Irma. As of late Thursday, data from S&P Global Platts showed that 12.8% of US refinery capacity was down due to the storm.
"Most refineries are restarting and we expect a near-full recovery by month-end", USA investment bank Jefferies stated. Yet another hurricane, Katia, is in the Gulf of Mexico.
"Hurricane Irma is already impacting the natural-gas market with demand out of the southeast region dropping roughly 1.5 [billion cubic feet a day] over the past 24 hours", said Robbie Fraser, commodity analyst at Schneider Electric.
US crude production output fell nearly 8 percent because of Harvey, from 9.5 million barrels per day (bpd) to 8.8 million bpd, according to the Energy Information Administration (EIA).
Port and refinery closures along the Gulf coast and harsh sea conditions in the Caribbean have hit shipping. It was predicted to hit Florida on Sunday or Monday.
The U.S. National Hurricane Center said Irma had weakened from a Category 5 storm to Category 4 and moving northwest through the central Bahamas toward Florida.