By Ahmed Aboulenein
LONDON (Reuters) - Brent crude fell to a fresh four-year low under $72 a barrel on Friday after OPEC decided not to cut output, a move investors said would leave oil markets heavily oversupplied.
Saudi Arabia blocked calls from poorer members of the Organization of the Petroleum Exporting Countries for output reductions on Thursday, triggering a rout in oil markets.
The sell-off continued on Friday with Brent reaching a low of $71.12, its weakest since July 2010. By 0945 GMT, Brent had recovered a little to trade around $71.90, down 68 cents.
U.S. crude oil hit a low of $67.75 a barrel on Friday, its lowest since May 2010 and down from Wednesday's close of $73.69. U.S. financial markets were closed on Thursday for Thanksgiving.
"Welcome to the new world of oil," said Michael Wittner, senior oil analyst at French bank Societe Generale. "Saudi Arabia and OPEC will no longer be the mechanism to balance the market, they have relinquished that role."
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"Instead, the market itself - prices, in other words - will be the mechanism to rebalance the market. We cannot overstate what a dramatic and fundamental change this is for the oil market," he added.
Brent was headed for its steepest monthly decline since November 2008, after falling more than 15 percent this month.
It has lost more than a third of its value since June, falling from above $115 a barrel as increasing shale output in North America has helped create an oil glut at a time of sluggish global economic growth.
"The market is looking for a new paradigm, a new range to settle into. Where that is, is anybody's guess. It could be $60, $70, or maybe even $80 a barrel," said Eugen Weinberg, head of commodities research at Commerzbank in Frankfurt.
"We think somewhere between $70 and $80 a barrel is probably about right. But we shall see."
Russia's most powerful oil official Igor Sechin said oil prices could fall to $60 or below by the end of the first half of next year. If prices remained low, Russia had the potential to cut between 200,000 and 300,000 barrels per day of production, Sechin said.
A further blow to global oil demand could come on Monday when China releases official Purchasing Managers' Index (PMI) data for November that could show slower growth, according to a Reuters poll.
(Additional reporting by Keith Wallis; Editing by Christopher Johnson)