Crude prices have plunged by two-thirds since mid-2014 as soaring output from the Organization of the Petroleum Exporting Countries, Russia and the United States led to a global surplus of between half a million and 2 million barrels per day.
More recently, a slowing demand outlook, especially in Asia but also Europe, has started dragging on prices.
Front-month U.S. West Texas Intermediate crude futures were trading at $37.18 per barrel at 0140 GMT, down 69 cents or 1.82 percent from their last settlement. Brent futures were down 47 cents, or 1.24 percent, to $37.32 a barrel.
Traders said the price falls were largely a result of a weak outlook for next year and the closing of 2015 trade books.
"The 2016 outlook is for lower prices, especially early next year. Many are closing their last long positions for the year today as nobody wants to come back in January and be surprised badly. Better start with a clean sheet," a trader said.
Forecasts that an upcoming cold weather in Europe will only be short-lived could also hurt crude prices.