British oil giant BP announced Wednesday it had a major oil discovery in the Gulf of Mexico.An appraisal well the company drilled in a previously untested northern segment of the Mad Dog field in the Gulf of Mexico found "a significant resource extension" for the Mad Dog Field complex that includes the existing field, BP said in a statement. Pending confirmation through future appraisal drilling, the total hydrocarbons initially in place in the Mad Dog field complex are now estimated to be up to four billion barrels of oil equivalent, said the London-based company.According to BP, the well is located on Gulf of Mexico Green Canyon block 738 approximately 225 kilometers south of Grand Isle, Louisiana, in about 1,371 meters of water."With these additional hydrocarbon resources north of the main field, Mad Dog has been firmly established as a giant field in BP' s Gulf of Mexico portfolio, rivaling Thunder Horse in size of resource," said Bob Dudley, BP's chief executive.BP maintains a 60.5-percent working interest in Mad Dog. BHP Billiton has a 23.9-percent interest, while Chevron Corporation, through its subsidiary Union Oil Company of California, has a 15.6 percent interest, said the BP statement.