U.S. oil prices tumbled about 3 percent to a nearly 6-1/2-year low on Thursday as data showing a big rise in key U.S. stockpiles intensified worries over a growing global glut.
U.S. crude settled down $1.07, or 2.5 percent, at $42.23 a barrel. It fell as low as $41.92 earlier, which was the lowest since March of 2009. Benchmark North Sea Brent crude oil was down 60 cents at $49.60 a barrel, ahead of Friday's expiry of its front-month contract.
A rise in the dollar, after higher U.S. retail sales in July and strengthening employment data, added to the weight on oil.
Oil has fallen by nearly a third since late June, a decline that continued this week after a spate of refinery outages sapped demand for crude.
The largest of those refineries—BP's 413,500-barrel per day (bpd) facility in Whiting, Indiana, also the biggest in the U.S. Midwest—has been forced to shut two-thirds of its capacity for repairs to a leak that could last a month or more.
Losses deepened on Thursday morning after market intelligence firm Genscape reported that stockpiles at the Cushing, Oklahoma delivery point for U.S. crude futures has risen more than 1.3 million during the week to Aug. 11, adding to fears that the Whiting outage would cause stocks of surplus crude to swell. If confirmed, it would be the biggest Cushing build since March.