Petrobras, the world’s biggest crude producer in ultra-deep waters, estimates its Franco prospect and two other fields off Brazil’s southeastern coast hold about 3.6 Bbbl of commercial reserves.

Franco holds 3.06 Bboe, Carioca has 459 MMboe, and Sul de Tupi has 128 MMboe, the company said in regulatory filings, declaring the fields commercial. The three fields hold “good quality” oil, according to the filings.

Franco is “confirmed as a giant,” Bank of America Corp. analysts Frank McGann and Vicente Falanga Neto said in a report to clients today. The field’s “total potential could be substantially larger, though any additional barrels currently belong to the government.”

After declaring the reserves commercial, Petrobras and the government must negotiate a revision of the value of the transferred reserves set at $8.51 per boe in 2010, McGann and Falanga Neto said. While the Carioca field is smaller than originally expected a few years back, it’s in line with recent expectations, the analysts said.

Brazilian offshore oil fields must be given new names after they are declared to be commercial. Petrobras said it suggested to Brazil’s regulatory agency to change Carioca’s name to Lapa, Franco’s to Buzios, and Sul de Tupi to Sul de Lula. The new names refer to Portuguese-language names of fish or other types of sea life.

Carioca is located 270 km (170 miles) from the coast of Sao Paulo state. Franco is 200 km (124 miles) and Sul de Tupi is 300 km (186 miles) from Rio de Janeiro state.

Petrobras owns 45% of Carioca, BG owns 30%, and the Brazilian joint venture between Spain’s Repsol SA and China Petroleum and Chemical Corp. holds the rest.