Doubts over OPEC deal: Goldman Sachs sees oil in low $40s

"The lack of progress on implementing production quotas and the growing discord between OPEC producers suggests a declining probability of reaching a deal on November 30," Goldman analysts including Damien Courvalin wrote in a note dated Oct. 31.

The Organization of Petroleum Exporting Countries is due to meet in Vienna to implement the first supply cuts in eight years and get other producers from outside of the group to join in the action, including Russia. Brent crude extended losses below $50 a barrel on Monday after weekend talks failed to yield concrete details on an accord to reduce the global crude surplus and stabilize prices.

"The lack of an agreement so far has pushed oil prices sharply lower, with weakening oil fundamentals warranting oil prices in the low $40s a barrel in our view if OPEC is unable to deliver a convincing agreement,” the Goldman analysts wrote.