SAN FRANCISCO (MarketWatch) —Oil futures on Friday settled at their lowest in five years as the Organization of the Petroleum Exporting Countries’ decision to keep crude production the same heightened fears that the existing glut in the oil market would persist.

It was the first chance for U.S. markets to react to the cartel’s move, announced Thursday, and the flurry of selling intensified as closing time neared — adding to downward momentum that was already threatening to be severe.

On the New York Mercantile Exchange, light, sweet crude futures for delivery in January CLF25, was off $7.54, or 10%, to settle at $66.15 a barrel on Friday.

That was the lowest settlement for a front-month oil contract since Sept. 25, 2009, and it brought crude’s monthly losses to 18%, the largest one-month percentage decline since December 2008.

On the week, futures declined nearly 14%.

January Brent crude UK:LCOF5 on London’s ICE Futures exchange fell $2.43, or 3.4%, to finish at $70.15 a barrel. That was Brent’s lowest settlement since May 25, 2010.

Brent had fallen more than 6% on Thursday, when the Nymex floor trading was closed for Thanksgiving.

Brent, viewed as the global oil benchmark, also lost 18% on the month. Brent and WTI have been down for five straight months.

OPEC’s announcement on Thursday dashed hopes of an output cut that could boost prices, with the cartel showing it was willing to withstand the lower prices in order to defend its market share.

Market share has been under threat from growing production from countries outside OPEC, including shale-oil production from the U.S. and output from Latin American countries and from Russia.

Oil prices have lost nearly 40% of their value since a peak in June, and OPEC’s decision to maintain its current production ceiling of 30 million barrels a day does little to remove the glut that has kept oil prices low.

New York-traded oil is likely to stay around $65 a barrel for the next couple of months, said Darin Newsom, a senior commodity analyst with DTN.

“We may see pops (to above $70 a barrel) but it will not turn around quickly,” he said.

On Friday, analysts warned of an oil surplus around 1 million barrels a day for the next year.

“The oil price is likely to continue falling until rising demand and declining non-OPEC supply get rid of the oversupply. The key role in this looks set to be played by US shale oil producers who will probably face more and more problems at prices below $70 per barrel,” analysts at Commerzbank said in a note Friday.

Also read: 10 U.S. shale stocks that are getting crushed the most

Prices could hit bottom “fairly rapidly,” however, if markets start to see signs of adjusting, “notably in falling US drilling activity or rising demand estimates,” analysts at HSBC said in a note Friday.

Also read: How OPEC might get the last laugh

OPEC's Dramatic Move and the Impact on Consumers

Demand is also a relatively brighter spot. The last three oil price shocks that hit the world economy over a span of 16 years saw sharp drops in global oil demand, but this time oil demand, although slower, is still growing, analysts at RBC Capital Markets said in a report.

Low oil prices are good news for consumers, who are seeing low retail-gasoline prices, and for countries that are big oil importers, such as China, but the HSBC analysts cautioned against “getting too carried away” with that aspect: falling oil prices “disinflationary pressures” would work on wages, too.

The slide in oil also has sparked heavy selling of energy-related stocks.

Elsewhere in energy futures trading, Nymex gasoline for December US:RBZ4 delivery fell 13 cents, or 6.5%, to end at $1.9039 a gallon. That was the lowest settlement for gasoline since Sept. 22, 2010. On the month, gasoline futures lost 12%.

December heating oil US:HOZ4 receded 17 cents, or 6.9%, to settle at $2.2308 a gallon on Nymex. That was heating oil’s lowest settlement since Oct. 29, 2010, and one that brought monthly losses to 11%.

January natural gas US:NGF15 fell 27 cents, or 6.1%, to $4.0880 per million British thermal units. On the month, however, natural gas rose 5.6% as earlier in November expectations of cooler-than-normal temperatures at the start of heating season had lifted prices.