New York's main contract, light sweet crude for August delivery, shed $1.09 to $80.36 a barrel in the afternoon of its first trading day and Brent North Sea crude for delivery in August retreated 47 cents to $92.22.
"The lack of fresh Fed stimulus, the downgrade to the US outlook and an unexpected rise in US inventories took their toll on the price," IG Markets said in a report.
The Fed on Wednesday announced that it was extending a programme designed to push down interest rates on long-term bonds, encouraging investors to move money into more neglected securities and lowering costs for borrowers.
But traders had been hoping for a third round of asset purchases, or quantitative easing, to boost growth in the world's largest oil consumer.
The Fed's prediction that US unemployment would remain above eight percent for at least the remainder of this year also added to the gloom.
Crude prices were also pressured by data showing a hike in US crude inventories, Phillip Futures said in a report.
Official data Wednesday showed stockpiles rose 2.86 million barrels last week against analyst forecasts of a 1.1 million barrel decline.
