Since the 1970s, Nigeria has sent a steady stream of high-quality crude oil to North American refineries. As recently as 2010, tankers delivered a million barrels a day. Then came the U.S. energy boom. By July of this year, oil imports from Nigeria had fallen to zero. Displaced by surging U.S. oil production, millions of barrels of Nigerian crude now head to India, Indonesia and China. But Middle Eastern nations are trying to entice the same buyers. This has set up a battle for market share that could reshape the Organization of the Petroleum Exporting Countries and fundamentally change the global market for oil. On Friday, crude prices dropped to their lowest level in five years after the International Energy Agency cut its forecast for global oil demand for the fifth time in six months. That signaled to investors that the world economy would struggle to grow in the coming year, sending the Dow Jones Industrial Average tumbling by 315.51 points, or 1.8%, to 17280.83. That’s the Dow’s biggest weekly percentage loss in three years. Since June, the IEA has cut its demand forecast for 2015 by 800,000 barrels, while it says U.S. oil output will rise next year by 1.3 million barrels a day.