The original hole was drilled to a total measured depth of 22,890 feet from Green Canyon 826 to downdip targets in the adjacent Green Canyon 825 Block. The blocks are located in about 5,000 feet of water.

The well and its sidetracks were designed to test the previously undrilled south-west flank of the field. Individually the penetrations encountered up to about 300 feet of net oil pay in the early Miocene section and found hydrocarbons some 700 feet deeper on the west flank of the structure than previously encountered.

“These results significantly reduce the risk associated with our previously announced reserve estimate of 200 to 450 million barrels of oil equivalent,” said BHP Billiton Group President Energy, Philip Aiken.

“The resources associated with these wells could be produced either through spar development wells or future subsea tie-backs.”

Production at the Mad Dog field began in January 2005 from Green Canyon Block 782.

Gross field production is currently about 30,000 barrels of oil per day from two wells.

BHP Billiton holds a 23.9% working interest in the Mad Dog unit. BP is the Operator with 60.5% working interest and Unocal, a 15.6% interest holder in Mad Dog, was the operator for the drilling of the Southwest Ridge appraisal well, using the Discoverer Spirit drillship.