LONDON— Two years after dismissing North America's shale-oil boom as "marginal," OPEC changed its tune Thursday, acknowledging new extraction technology in the country could sharply cut the need for the group's own oil.
 
In its annual World Oil Outlook, the Organization of the Petroleum Exporting Countries said new oil supply from Canada and the U.S. would reach 4.9 million barrels a day within five years. That is more than double last year's forecast of 1.7 million barrels a day by 2018.
 
As a result, global needs for the group's own crude will be one million barrels a day less by 2018, OPEC said.
 
OPEC, which groups some of the world's biggest producers, has been slow to recognize the scale of North America's oil boom. In the same report in 2011, it said "shale oil should not be viewed as anything more than a source of marginal additions" to global supply. 
 
Yet this year alone, shale oil production in the U.S. and Canada is expected to jump by nearly a third, to 3.3 million barrels a day. That is because hydraulic fracturing and other technological innovations have revolutionized the industry, enabling drilling firms to access difficult-to-reach resources in complex rock formations in places like Texas and North Dakota.
 
The shift has worried OPEC members including Algeria and Nigeria, which have been hit by a steep drop in export shipments to the U.S. In many cases, those producers have found new markets, particularly in Asia.
 
But the U.S. market has long been an anchor for big oil producers. OPEC said Thursday that exports from African producers to the U.S. won't recover from the pickup in U.S. output.
 
OPEC's findings come after the group agreed on a study on shale oil at its last meeting in June. Members are set to discuss the matter further when they meet in technical meetings at the group's biannual gathering next month, according to OPEC officials.
 
Still, OPEC insisted that the shale oil bonanza in the U.S. may not make much difference in the long run, amid rising global demand for petroleum. The group argued that energy companies' success in exploiting shale oil was unlikely to be duplicated outside North America. It also said that high depletion rates will result in declining shale oil production from 2018.
 
During that period, "drilling efficiencies and [hydraulic fracturing] operations, impressive in the past, could plateau," OPEC said.
 
As such, OPEC said it sees its role as the world's most important supplier of crude regaining momentum by the end of the decade.
 
OPEC produces about a third of the world's oil.