The firms' Brazilian joint venture has struck oil in the Campos Basin off the coast of the South American country, it said on Monday.

"The well ... drilled into a hydrocarbons column of 500 metres, one of the thickest discovered in Brazil to date," Repsol said in a statement.

Repsol did not provide an estimate for the size of the find, but one of its partners, Norway's Statoil, said it was a "high-impact" one: it could hold more than 250 million barrels of oil equivalent (boe) or provide 100 million boe net to Statoil.

One analyst said the discovery could be even bigger and had the potential to be an elephant when compared with a recent discovery made by Petrobras in the same area.

"Using the same ratio between the oil column size and the recoverable recourses, the estimated size of (the find) is in excess of 500 million boe," Teodor Sveen Nilsen, an analyst at Oslo-based firm Swedbank First Securities, wrote in a note to clients.

The partners in the Brazilian discovery, called Pao de Acucar or Sugar Bread, are Repsol Sinopec Brasil, the well's operator with a 35% stake, Brazil's Petrobras with 30% and Statoil with 35%. Repsol has a 60% stake in the Brazilian joint venture, while Sinopec holds 40%.

The Pao de Acucar well in the Campos Basin about 195 kilometres off the coast of Rio de Janeiro in 2,800 meters of water and is the latest find in the so-called pre-salt area.

Statoil said the Brazilian find was the sixth high-impact find it had made over the past 12 months after it struck big offshore Tanzania, Brazil, in the North Sea and in the Arctic.

The find also confirmed the potential for finding large oil and gas resources on the other side of the South Atlantic Ocean, in Angola's offshore deepwater blocks, where the Norwegian firm believes it can make billion-barrel discoveries.

These areas were formed millions of years ago when the African and South American continents were still connected.

"This discovery ... improves our confidence in the recently acquired acreage position in the pre-salt Kwanza basin of Angola," Statoil's head of exploration, Tim Dodson, said in a statement.