BP announced on Wednesday the discovery of what it characterized as a giant oil field several miles under the Gulf of Mexico, but it may take years to assess how much crude can actually be recovered. The discovery should have no immediate effect on world oil or gasoline prices because it could take three years or more to begin extracting oil. Because the oil is so deep underwater and difficult to extract, the price of oil will need to be above $70 a barrel to make drilling profitable, energy analysts said. Nevertheless, the discovery was another indication that the deep waters of the Gulf of Mexico are probably the most promising area in United States-controlled territory to bolster domestic oil production. The rise in gulf production in recent years, in large part because of BP