Even in the days of frequent bumper oil finds, billion-barrel discoveries were something special. Today, in a world without cheap and plentiful oil, such a find sends markets into excited spasms. Witness the reaction to Thursday's announcement from Petrobras, the Brazilian oil major, that it is poised to recover as much as 8bn barrels of light crude from Tupi, an ultra-deep field off Rio de Janeiro. That is double initial estimates and confirms Tupi as the world's largest new oil source since Kashagan in Kazakhstan in 2000.

Little wonder shares surged. BG, with a 25 per cent interest in the field, has put on about 14 per cent since the announcement. Although the first oil may not flow from Tupi for at least another five years, the find - one of five concessions BG has in the area - could boost its reserves by two-fifths. Galp, a Portuguese explorer with a small stake, has shot up 41 per cent on a reassessment of its takeover allure. Petrobras, with the remaining 65 per cent, has gained 20 per cent.