Oil prices slipped further Tuesday, as concerns about weakening demand growth in China added to expectations that a global oversupply could last for years.
US benchmark West Texas Intermediate for September delivery was down 19 cents at $41.68 a barrel compared with Monday's close.
WTI has lost more than 30 percent of its value in the past two months, and on Friday dropped to its lowest level since March 2009 at $41.35.
On Tuesday in London afternoon trading, Brent North Sea crude for October was down 30 cents at $48.44 a barrel.
Prices are downbeat as strong output from US shale oil producers and the Organization of the Petroleum Exporting Countries cartel has outpaced tepid growth in demand.
BMI Research, a subsidiary of financial information provider Fitch Group, predicted the glut will persist until 2018.
"The return of Iranian oil to market, coupled with strong project pipelines in North America, the Middle East, west Africa and Kazakhstan, will see global supply expansion outstrip the growth in global consumption for the next two years," it said.
Punishing Western sanctions that have restricted Iran's oil exports for years are expected to be lifted once it is verified that Tehran is complying with a deal to curb its nuclear ambitions.
Analysts say the return of Iranian oil will add to the current excess, further dampening prices.