LONDON—Two years after dismissing North America’s shale-oil boom as “marginal,” OPEC changed its tune Thursday, acknowledging new extraction technology 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 […]