The week's trading was defined by bearish data about Chinese oil demand and rising crude production in the United States.

Benchmark Brent crude futures were down 5 cents at $50.98 by 11:01 a.m. EDT (1401 GMT). US West Texas Intermediate (WTI) crude futures were down 14 cents at $46.95 a barrel.

"The main question is whether we will continue to see the kind of inventory draws that may show the supply-demand balance is tightening over the next few weeks," said Gene McGillian, director of market research at Tradition Energy.

Prices for future months of Brent are now lower than the front-month contract, meaning the Brent forward curve has moved from contango into backwardation. A backwardated market is considered a bullish sign for prices since it indicates demand is outpacing supply.

Signs of supply tightness have started appearing in the United States, the world's biggest oil consumer.

Despite a 13 percent jump in production since mid-2016 to 9.5 million bpd, the country's commercial crude inventories  have fallen 13 percent from their March records to below 2016 levels.