* Petrobras said found relatively light offshore oil
* Discovery comes a only a week after another major find
* Franco SW well has shown 295 meter oil column so far
RIO DE JANEIRO, Aug 21 (Reuters) - Brazil's state-led oil company Petrobras found "good quality" crude in a deep water field south of Rio de Janeiro, the latest in the series of large discoveries in one of the world's most-promising offshore oil frontiers, the company said Tuesday.
The Rio de Janeiro-based company found a 295-meter (968-foot) column of hydrocarbons in a prospect known as "Franco SW" in the Santos offshore basin about 210 kilometers south of Rio de Janeiro, according to a securities filing.
Petrobras has yet to determine the full extent of the find.
Franco SW was drilled 17 kilometers to the south of Petrobras "Franco" prospect. In 2010 Petrobras said an exploratory well at Franco found an estimated 6 billion barrels of oil and natural gas equivalent.
That's enough to supply all U.S. needs for 10 months according to the BP Statistical Review of World Energy and Reuters.
Petrobras bought the rights to drill in the area from the Brazilian government as part of a more than $70 billion stock sale in 2010. The sale was the largest stock offer in history.
Petrobras said the oil discovered is a relatively light grade of crude at 28 degrees on the American Petroleum Institute (API) scale. That's lighter than most of the oil Petrobras produces in Brazil, which means that crude should be easier and cheaper to refine that its current output.
Last week Petrobras said announced "a significant" oil discovery in another of the company's off-shore sub-salt area in the Santos Basin.
The Carcará prospect has found an oil column of at least 400 meters, one of the largest ever found in Brazil offshore. Petrobras operates the area with its partners Portugal's Galp Energia SGPS and Brazil's Barra Energia and QGEP Participacoes SA.