Oil prices have fallen 15 per cent in May and the reason is not only slowing economic growth in China and Europe.

OPEC is churning out the most barrels since 2008, with output at almost 2 million barrels a day (bpd) above the official target rate.

In May, supply from the 12-member Organization of the Petroleum Exporting Countries has averaged 31.80 bpd, up from 31.75 million bpd in April, according to a Reuters survey of sources at oil companies, OPEC officials and analysts.

The reality is far from the ugly scenario envisaged by investors in March, when prices soared on concern of disruption to global supply.

The biggest contributor to the excess has been Saudi Arabia, which has boosted output to 10.1 million bpd, its highest level in decades. The Saudi government has been trying to stabilize prices at close to $100 (U.S.) a barrel for Brent crude, Reuters reports.

The price of West Texas Intermediate crude rose one penny to $90.87 on Tuesday.

Supply from both Libya and Iraq dropped in May. OPEC members are scheduled to meet in Vienna on June 14 to review output policy.