"While the Saudis and the Russians have been complying with their cuts we've seen Iraq and a number of other countries produce more than their share of the quota", Andrew Lipow, president of Lipow Oil Associates in Houston, said.
Citigroup Inc. says output cuts by the Organization of Petroleum Exporting Countries will be able to offset the response from American producers to higher prices.
Crude oil prices were down almost a full percentage point in overnight trading, but pulled back close to even after China report strong growth figures. At a total of 683, the current tally is the highest in two years.
Brent crude futures, the Global benchmark for oil, were at $54.84 per barrel at 0059 GMT, down 5 cents from their last close. Supply from the 11 Opec members with production targets fell to 29.761 million bpd, implying a compliance of 104 per cent.
U.S. WTI futures fell 24 cents, settling at $52.41 a barrel.
Cost of the United States light crude oil decreased $0.43 to stand at $52.75, while price of the Brent crude oil fell $0.46 to trade at $55.43.More news: North Korea condemns USS Carl Vinson deployment
Both benchmarks closed out the Easter holiday eve higher for a third consecutive week, with Brent adding 1.2 percent over the four days and WTI up 1.8 percent. "If I were OPEC, I'd be pretty anxious". Still, major disturbances in Chinese oil demand, Russian output, or other aspects of geopolitics can overpower the effects of the bloc's efforts. Investors are also pouring money into the industry, suggesting US output gains will continue.
On Monday, shortly before the Nymex settlement, a monthly report from the Energy Information Administration forecast a rise of 124,000 barrels a day in May to 5.193 million barrels a day for crude-oil production in seven major US shale-oil plays.
While Iran fueled hopes that OPEC and non-OPEC oil producers could extend their output cuts beyond the six-month agreement, Saudi energy minister Khalid al-Falih said it was too early to discuss an extension.
US oil rig activity increased for the 14th straight week last week, the highest since April 2015. As OPEC & co. cut, they cede market share to their American rivals, and as oil prices rise, those USA shale producers are predictably taking advantage and upping their own output.
China's March refinery throughput also rose to 11.19 million bpd, just shy of December's record, as margins remained attractive.