Brent, the global benchmark, slid 0.14%, to $56.13 a barrel on ICE Futures Europe.
Both benchmarks fell for the third straight quarter on expectations of a final deal on Iran’s nuclear program in exchange for lifting international sanctions on Tehran. Market analysts say additional oil supplies from Iran would flood an already-oversupplied market and push prices even lower.
Crude prices have dropped more than 50% from their mid-June high due to growing production and lackluster demand.
Talks between Iran and six world powers to settle a dispute around Tehran's nuclear program extended beyond a Tuesday deadline, as the parties moved towards a deal but failed to agree to crucial details such as the lifting of UN sanctions.
Efforts to reach a framework deal were scheduled to continue on Wednesday in the Swiss city of Lausanne.
Russian Foreign Minister Sergei Lavrov said a general agreement had been reached on all key aspects of a future deal.
"If the flood gates to Iranian crude open, (prices) will probably test this year's lows again," Phillip Futures analyst Daniel Ang told Reuters.
Iran's oil production is expected to rise by around 500,000 barrels per day within six months if sanctions are removed, and by a further 700,000bpd within another year.
