Oil prices slipped on Wednesday after an unexpectedly big build in US crude inventories, further evidence of an oversupply that has helped halve global spot prices over the last year.
US crude oil stockpiles rose 4.6 million barrels in the week to Sept. 25, the American Petroleum Institute (API) said, well above a modest increase of 100,000 barrels that analysts polled by Reuters had forecast.
Investors awaited official weekly inventory figures from the U.S. government's Energy Information Administration (EIA) due later on Wednesday to see if they confirmed the API data.
US crude, also known as West Texas Intermediate or WTI, was 15 cents lower at $45.08 a barrel by 1040 GMT, on course to end September down 11 per cent.
Brent crude oil was 9 cents lower at $48.14 a barrel, heading for a near 9 per cent fall this month.
Brent traded in a very narrow 60-cent range in early trade on Wednesday, partly reflecting low volume ahead of the week-long Chinese National Day holiday starting on Thursday.
If this range were to be maintained for the rest of the session, it would be the narrowest daily range since May 2014.
"The downward pressure is coming from ongoing high OPEC crude production, led by Saudi Arabia and Iraq, and expectations of global stockbuilds for an extended period," said Societe Generale oil analyst Michael Wittner.
Analysts polled by Reuters said oil prices would remain depressed, forecasting an average Brent price of $58.60 a barrel in 2016, well below $62.30 expected last month.
Inaction by the world's largest crude exporter Saudi Arabia to prop up prices has helped it build market share, a Reuters analysis shows. Saudi exports to Asian and European consumers reached multi-year highs in the first half of the year.
Saudi Arabia is banking on a rise in world oil demand and slower growth in non-OPEC oil supply, meaning it is unlikely to change its stance on not cutting production any time soon.
The chief executive of commodity trader Vitol, Ian Taylor, said on Wednesday he was seeing signs that global oil supply and demand were beginning to balance out.
"This is beginning to happen ... Demand is very strong, you can see US production beginning to come down," he said, declining to give a forecast for oil prices.