* Discovery may be one of largest in offshore Brazil-Barra CEO
* Carcará prospect may be as big as giant Lula field - geologist
* Barra Energia, Queiroz Galvao, Portugal's Galp are partners
By Jeb Blount and Sabrina Lorenzi
RIO DE JANEIRO, Aug 13 (Reuters) - Brazil's Petrobras has made "one of the most significant oil discoveries" in the country's offshore sub-salt area, a partner in the discovery said, raising expectations the state-led oil company has made another giant find in a hot exploration region.
Petrobras, which operates the BM-S-8 block south of Rio de Janeiro in the Santos Basin, said in a securities filing that the group's Carcará prospect has shown 400 meters (1,312 feet) of continuous and connected oil reserves with excellent porosity and permeability.
"This 400 meter column is among the biggest ever found in the subsalt," Joao Carlos de Luca, the chief executive of Barra Energia, a partner in the discovery, told Reuters in a phone interview.
"It's one of the most significant discoveries realized in the area."
The Carcará strike is the latest in a region that is home to several of the world's largest recent oil discoveries. The first discovery announced in 2007 was in a nearby block owned by Petrobras, BG Group Plc and Galp Energia SGPS, and was the biggest discovery in the Americas in three decades.
Now known as the Lula and Cernambi fields, that discovery holds an estimated 8.3 billion barrels of oil and natural gas, enough to supply total demand in the United States, the world's largest oil consumer, for more than 14 months.
"With an oil column that large, I wouldn't be surprised if Carcará was another Lula," said Cleveland Jones, a petroleum geologist and professor at the Brazilian Petroleum Institute at the State University of Rio de Janeiro who has been developing reserve estimates of the subsalt region.
"Since we started looking at the subsalt, we've been predicting that we would have another field of that size out there and this seems to fit the bill."
Petrobras owns 66 percent of the block and Portugal's Galp owns 14 percent. De Luca's Barra Energia and Queiroz Galvao Exploracao e Producao SA each own 10 percent.
The subsalt as a whole is a New York-state-sized area that runs along the coast of Brazil near Rio de Janeiro and Sao Paulo. The entire region, which extends throughout much of the Campos and Santos oil basins, may contain as much as 100 billion barrels of oil, Jones and his colleagues at the Brazilian Petroleum Institute estimate.
That's enough to supply the United States for about 14 years.
"The results obtained up to now have exceeded our initial expectations and make a positive impact on our portfolio of assets," Lincoln Rumenos Guardado, chief executive of Queiroz Galvao Exploracao e Producao said in a statement.
Barra's De Luca warned that it was probably still too early to estimate the total volume of oil in the Carcará prospect. De Luca is a former president of Repsol in Brazil and is president of the Brazilian Petroleum Institute, the country's main oil industry association.
The oil is of a light grade at 31 degrees on the American Petroleum Institute (API) scale, Petrobras said.
The prospect, about 300 kilometers (188 miles) south of Rio de Janeiro, was first announced in March. It sits in 2,027 meters of water.
The well where the oil was found has been drilled to a depth of 6,213 meters by the Sevan Driller, a semisubmersible platform owned by Norway's Sevan Drilling ASA and leased to Petrobras.
Petrobras began drilling the well on Dec. 15.