watch out for the reaction to this!

LONDON (MarketWatch) -- Companies led by Petrobras encountered fresh success in a field offshore of Brazil, saying a recent oil find in the Santos Basin contains up to 4 billion barrels of oil.

Petrobras /quotes/comstock/13*!pbr/quotes/nls/pbr (PBR 47.28, +0.11, +0.24%) , BG Group /quotes/comstock/23s!a:bg. (UK:BG. 1,086, -22.00, -1.99%) and Galp Energia /quotes/comstock/24s!e:galp (PT:GALP 11.93, -0.02, -0.17%) said a find announced a month ago contains an estimated 3 billion to 4 billion barrels of oil equivalent.

It's the third find in what's called the BM-S-11 concession, the first being the high-profile Tupi discovery that contains up to 8 billion barrels of oil.

The companies said the most recent discovery was better than initially expected. The well is 230 kilometers (143 miles) off the coast of Rio de Janiero at a water depth of 2,230 meters.

Petrobras holds 65% of BM-S-11; BG controls 25% and Galp holds 10%.

"Brazil will become a very major production center for our company and will sustain a major earnings contribution over at least three decades," said BG CEO Frank Chapman in a statement.

Shares of Galp rallied 5% in Lisbon, and BG rose 2.8% in London. See London Markets.

Petrobras shares advanced 2% in New York trade.

Deutsche Bank analysts estimated the most recent discovery is worth between $15.6 billion and $20.8 billion, or $10.1 billion and $13.5 billion to Petrobras alone.

Steve Goldstein is MarketWatch's London bureau chief.