New York's main contract, light sweet crude for delivery in October shed two cents to $94.60 a barrel in the afternoon and Brent North Sea crude for October delivery added eight cents to $112.73.
Volumes were thin and traders were "very cautious, very defensive" ahead of Bernanke's closely-watched speech later Friday at Jackson Hole, Wyoming, said market strategist for IG Markets Singapore Justin Harper.
"I just think we're caught in the crosshairs of Jackson Hole where people are being very cautious across the board," he told AFP.
"Volume is so thin you can change the market very quickly," he added.
Investors are watching if Bernanke would indicate a fresh stimulus package for the US economy.
IG Markets Singapore said in a commentary however it was unlikely that the US central bank chief would announce a new round of quantitative easing, known as QE3, because of signs the US economy was stablising.
"On the eve of the long-waited, highly-anticipated and much-hyped Jackson Hole symposium the watchword was defence as traders reduced their risks," it said.
"High hopes of QE3 have been edging down this week as US economic data continues to show mild strengthening and less need for central bank intervention," it added.
US economic data Thursday showed consumer spending rebounding in July while initial jobless claims held steady, indicating the economy is stabilising and strengthening the argument that a fresh stimulus is not needed for now.