Crude prices fell more than 1 percent on Thursday as the restart of two key oilfields in Libya pumped more crude into an already bloated market.
U.S. gasoline futures also led the complex lower, falling to the lowest in at least eight years for this time of year after inventories rose by the most in nearly three months and demand remained weak.
Benchmark Brent crude was down 19 cents at $51.63 a barrel by 2:35 p.m ET (1735 GMT), nearly 9 percent below this month's peak.
U.S. light crude ended Thursday's trade 65 cents, or 1.3 percent, lower at $48.98, having fallen to a fresh four-week low.
Libya's 300,000 barrels per day (bpd) Sharara oilfield and 90,000 bpd El Feel oilfield have restarted after the end of protests that had blocked pipelines there, a Libyan oil source and local official said.