Spanish oil company Repsol says an offshore block near Brazil's coast contains at least 1.2 billion barrels of oil equivalent, one of the world's largest finds this year.
Testing confirms the potential of the well, according to a company statement on Thursday.
Repsol Sinopec Brasil is the operator of the well. Norway's Statoil has a 35 per cent stake and Brazil's Petrobras owns 30 per cent.
Repsol says the total estimated resources in the block amount to more than 700 million barrels of light crude and 3 trillion cubic feet of gas, the latter being equivalent to 545 million barrels of oil.
Brazil's offshore oil fields contain some of the world's biggest finds in the last 30 years, with estimates of at least 50 billion barrels of oil.