The Brent/WTI spread has historically ranged between $4/bbl and $8/bbl, but it can expand or contract based on factors related to U.S. and international supply and demand conditions. For example, the Brent/WTI spread hit nearly $14/bbl in April 2011 when protests sparked market fears of significant oil supply disruptions in the Middle East.

Oil prices fell more than 3% on Thursday on a Financial Times report that Saudi Arabia, the world's top crude exporter, will give up its $100 price target in preparation for raising output, along with OPEC members and allies in December.

Expectations that the OPEC and its allies will start unwinding voluntary output curbs of 2.2 million barrels a day starting from December is adding to market nervousness.