Brent crude was on track for its lowest weekly close since 2008 on Friday as the International Energy Agency (IEA) warned global oil oversupply could worsen in the new year.
Output in the Middle East continued to rise, figures showed this week, despite an already huge global glut and the IEA, which advises developed nations on energy, warned that rocketing demand growth for fuels could ebb.
"Consumption is likely to have peaked in the third quarter and demand growth is expected to slow to a still-healthy 1.2 million bpd in 2016, as support from sharply falling oil prices begins to fade," the IEA said in its monthly report.
Brent crude futures were down 63 cents at $39.10 a barrel, not far off almost seven-year lows hit earlier in the session.
U.S. crude futures were at $36.3 per barrel, down 43 cents. Earlier in the session, U.S. futures touched their lowest since 2009.
Oil prices have tumbled this month after to OPEC failed to impose a ceiling on its output. OPEC producers pumped more oil in November than in any month since late 2008, some 31.7 million barrels per day.
Should sanctions on Iran be lifted, its exports could rise, adding to the market's oversupply.
"The next quarter is going to be particularly tough as we go from a high-demand to a low-demand quarter," said Richard Gorry, director of consultancy JBC Energy Asia.
"Can you rule out $20 per barrel? No, you can't," he said, although adding that prices would not likely fall that far.
Gorry said he expected a slow rebalancing of the market towards the end of next year, with production remaining stubbornly high despite low benchmark prices.