- A rise in U.S. exports to a record level and rising production pulled oil lower.
- The U.S. rig count suggests a further rise in shale output.
- Analysts say oil markets have largely rebalanced.
Oil prices fell on Wednesday after a surprising jump in U.S. crude exports to a record 2 million barrels per day fanned worries about global oversupply.
U.S. exports have became more attractive to buyers because the price of U.S. West Texas Intermediate crude (WTI) futures has been trading at a steep discount to Brent.
Rising U.S. crude production has held down WTI prices, while Brent's price has been heavily influenced by output cuts led by the Organization of the Petroleum Exporting Countries.
U.S. West Texas Intermediate crude futures ended Wednesday's session down 44 cents, or nearly 1 percent, to $49.98 per barrel. Brent crude futures were down 19 cents to $55.81 a barrel at 2:29 p.m. ET (1829 GMT).
The spread between the two benchmark's December contracts , which had narrowed earlier in the day, widened out again, to $5.36 a barrel from $5.31 before the data.
The concern among traders is that the heavy increase in U.S. exports — while shale production continues to rise — will undermine the OPEC-led efforts to reduce supply.
"The U.S. oil-production profile has forced OPEC and some non-OPEC countries participating in the ongoing output-cap agreement to re-evaluate their strategy," said Abhishek Kumar, senior energy analyst at Interfax Energys Global Gas Analytics in London.
U.S. crude inventories fell 6 million barrels in the week to Sept. 29, a much bigger decline than the decrease of 756,000 barrels analysts had expected. Gulf Coast refineries have been using more crude as they resumed operations after weeks of shutdowns following Hurricane Harvey.
Strategists viewed Brent as pricey after a third-quarter rally lifted it to mid-2015 highs by late September. A resumption in output at Libya's Sharara oilfield fed the concerns.
"Fundamentals may not yet be strong enough to support a continued rally, especially in growth-dependent commodities such as oil," Ole Hansen, head of commodity strategy at Denmark's Saxo Bank, said in a quarterly outlook to investors.
The oil market has historically faced headwinds in the final three months of the year. Crude futures traded positive 40 percent of the time in the fourth quarter over the last 25 years, a CNBC study using trading analytics tool Kensho found.
Citigroup, however, forecasts WTI at $54 a barrel and Brent at $58 in the fourth quarter. Ed Morse, Cit's global head of commodities research, told CNBC the momentum in the physical and financial markets will boost oil prices.
Observers said a market rebalancing was well underway as a result of strong consumption and the OPEC-led output cuts.
On Wednesday, OPEC Secretary-General Mohammad Barkindo said he was confident his organization could restore sustainable stability to markets. Russian President Vladimir Putin said he did not exclude an extension of output cuts until the end of 2018. Russia is part of the supply agreement.
The Sharara oilfield restarted on Wednesday. It had been producing more than 230,000 bpd before armed brigades closed it on Sunday.
Rising oil production in the United States, which is not involved in the deal, has limited price gains. U.S. output hit 9.56 million bpd at the end of September, highest since July 2015. Drillers added six oil rigs in the week to Sept. 29, according to energy services firm Baker Hughes.
— CNBC's Tom DiChristopher contributed to this report.