An unusual spate of major refinery glitches across the U.S. East and Midwest added to the concerns, threatening to accelerate a buildup of surplus crude.
Brent fell $2.68, or more than 5 percent, to settle at $47.43 a barrel, its third-largest one-day decline since 2011 and its lowest close since March 2009. The decline was the 10th in the past 12 sessions.
U.S. crude settled down $2.29 at $46.07, leading losses across the complex. Gasoline and ultra-low sulphur diesel (ULSD) futures fell by around 3 percent as refinery outages spurred some prompt buying.
"I figured we'd see $40 in the near term, but everything seems to be happening quicker than expected," Tariq Zahir of Tyche Capital Advisors.
Prices were relatively stable last week, but that respite ended abruptly when Goldman slashed its three-month forecasts for Brent to $42 a barrel from $80. It cut its outlook for the U.S. futures contract to $41 from $70.
The unrelenting rout, which has wiped nearly 60 percent off prices since June, shows no sign of letting up, with many traders giving up attempts to predict a bottom even amid growing signs that U.S. shale drillers are hitting the brakes.
The number of rigs drilling for oil in North Dakota fell by eight to the lowest since 2010.
"I think from a technical viewpoint there's no reason to try pick a bottom right now," said Phil Flynn, an analyst with Price Futures Group.
Four U.S. refineries with a combined capacity of more than 1 million barrels per day were recovering from disruptions at the weekend caused by either cold weather or unexplained fires. Three were restarting on Monday, while the fourth, in Lima, Ohio, was expected to be offline for a week.
"So not only do you have the macro influences on world crude prices, but now you have some refinery outages, so that will put some pressure on U.S. crude," said Richard Hastings, macro strategist at Global Hunter Securities.
Even with oil plumbing new lows, Saudi Arabia and its Gulf allies appeared no less resolved to maintain their market share, resisting a diplomatic push by Venezuela and Iran to begin cutting output.
(Copyright Thomson Reuters 2015)