BP announced Wednesday the drilling of a successful appraisal well in a previously untested northern segment of the Mad Dog field in the U.S. Gulf of Mexico.

According to BP, the well results confirm a significant resource extension for the Mad Dog Field complex, which includes the existing field, in production since 2005, and appraisal drilling of the Mad Dog South field in 2008 and 2009. 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.

The well, drilled by BHP Billiton on behalf of the unit operator BP, is located on Gulf of Mexico Green Canyon block 738 approximately 140 miles (225 kilometers) south of Grand Isle, La., in about 4,500 feet (1,371 meters) of water. The well encountered about 166 net feet (50 meters) of hydrocarbons in the objective Miocene hydrocarbon-bearing sands and discovered an oil column of more than 300 feet (91 meters).

“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 group chief executive, in a prepared statement.

BP maintains a 60.5 percent working interest in Mad Dog. BHP Billiton has a 23.9 percent interest, Chevron Corporation, through its subsidiary Union Oil Company of California, has a 15.6 percent interest. Due to the materiality of the Mad Dog South finds in 2009, BP has been advancing development options to increase production from Mad Dog by adding another spar production facility with a production capacity of 120,000–140,000 barrels of oil equivalent per day.

“Coupled with the recent exploration success at the discovery at the Moccasin prospect, located in Keathley Canyon, the Mad Dog result re-emphasizes the exploration and development potential of the Gulf of Mexico and the region’s ability to continue to deliver material projects for BP,” Dudley said.