Petroleo Brasileiro, known also as Petrobras, said it discovered "good quality oil" at its deep-water Florim well below the salt layer of the lucrative Santos Basin.
Petrobras said, once drilling is completed below the salt layer, it would conduct tests to determine the reserve's productivity. The company said it expected to determine if the reserve is of commercial value by late 2014.
Earlier this week, Petrobras said it started oil production at the Bauna field in the southern section of the Santos Basin using a floating, production, storage and offloading vessel.
The FPSO at Bauna is connected to a well that can produce as much as 12,000 barrels of oil per day. The company said peak production from the site is expected by August.
Brazil ranks second behind Venezuela in terms of proven oil reserves in South America.
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