Brazil's state-run producer Petrobras is sitting on recoverable reserves of between 5 billion and 8 billion barrels of oil and natural gas at the Tupi field, it said today.
"It's a giant one, without a hint of doubt. It's about twice the size of Roncador, Brazil's biggest discovery so far," Caio Carvalhao, research associate with Cambridge Energy Research Association in Rio de Janeiro, told Reuters.
Roncador has around 3 billion barrels of recoverable heavy oil reserves.
Brazil's total proven reserves are around 12 billion barrels.
Petrobras also said it made a regional evaluation of subsalt potential in southern and south-eastern basins and the outlook was very promising.
"If confirmed, the recoverable volumes of oil and gas will lift significantly the quantity of existing oil in Brazilian basins, putting Brazil among the countries with big reserves of oil and gas," the company said in a statement.
It said the wells drilled to subsalt layers in the Espirito Santo, Santos and Campos basins have until now shown high productivity.
Petrobras partner in Tupi, BG Group, said in February the field could contain between 1.7 billion and 10 billion barrels of in-place reserves. BG has a 25% stake in the Tupi project and Portugal's Petrogal has 10%.
The announced recoverable reserve estimate is above expectations by most sector analysts who said recoverable reserves in the area should make up around 30% of in-place.
Geologists have long been talking of possible large reserves underneath Brazil's main production base in the offshore Campos basin and in the promising Santos basin.