China's CNOOC and CNPC, Anglo-Dutch giant Royal Dutch Shell and France's Total joined Brazilian state operator Petrobras on Monday in winning production rights to the huge "Libra" Atlantic oilfield, AFP reports.
The five firms won 35-year concessions, with Petrobras taking a 40 percent stake, more than the minimum required by the terms of Brazil's offer, which has been controversial at home.
Shell and Total both earned a 20 percent stake, with CNOOC and CNPC securing 10 percent each.
Their consortium was the only bid to offer the Brazilian state the minimum 41.65 percent of oil to be extracted from the site, which holds an estimated eight to 12 billion barrels of oil.
To put that into context, Brazil currently has 15.3 billion barrels of proven reserves and is already the second-largest in South America after Venezuela.
Brazilian President Dilma Rousseff hailed the auction's outcome, calling it a "success" that would bring about a "small revolution" in the country.
Finance Minister Guido Mantega, meanwhile, said the government was "very satisfied."
"We are talking about ... 41 percent of a very big cake -- so we are very satisfied with this stake," he added.
The auction attracted 10 participants, but none from the United States. US firms saw too many strings attached, including major state intervention via Petrobras, which will enjoy sole operator status.
A further concern was the creation of a new state company, PPSA, to oversee offshore exploration.
Spain's Repsol pulled out just ahead of a decision, unveiled at a hotel in Rio.
Analysts had expected China National Petroleum Corporation (CNPC) and China National Offshore Oil Corporation (CNOOC) to land the lion's share of the deal.
Instead, they each had to settle for less than their Anglo-Dutch and French partners.
Even so, with Libra holding the equivalent of around three years worth of ever rising Chinese consumption, China's state firms were keen to come aboard.
Brazilian Energy Minister Edison Lobao indicated Libra will transform the country's energy scene and "more than duplicate its reserves inventory of proven oil reserves."
Magda Chambriard, head of oil regulator ANP, said that Libra would produce "300 billion reais ($150 billion) in royalties" alone.
The concessions are for developing huge so-called "pre-salt" oil deposits found six years ago in deep water off Brazil's Atlantic coast.
The winning consortium will have to pay a signing fee of 15 billion reais ($6.9 billion).
Ahead of the auction, ANP estimated the Brazilian government would receive around three quarters of overall Libra profits.
Clashes between workers and police
Earlier, five people were reported hurt as union workers opposed to the auctioning off of national assets to foreign companies clashed with police.
More than 1,000 police were called in and responded with tear gas and rubber bullets after some 200 protesters converged on the hotel hosting the action.
Analysts say the Libra field will be able to produce around 1.4 million barrels a day by 2017, according to ANP.
A further spinoff could come in the form of a doubling of Brazilian gas reserves, currently 459.3 billion cubic meters.
The reserves are what are known as pre-salt -- that is, they lie beneath a layer of salt deep below the Atlantic Ocean.
Losing out with their bids were Petronas of Malaysia, Japan's Mitsui & Co, Portugal's Petrogal, Colombia's Ecopetrol and ONGC Videsh of India.
Buried under layers of salt, the deposits cover 149,000 square kilometers (58,000 square miles). Libra lies 183 kilometers (112 miles) off the coast.
Currently, Brazil produces two million barrels a day but hopes to boost output to 4.3 million a day by 2020, thanks in large part to the pre-salt reserves.
Libra alone covers 1,548 square kilometers (597.3 square miles), equating to around ten percent of Brazil's overall pre-salt deposits.
Legislation passed earlier this year provide for Brazil's oil royalties to be poured into education and health.
But unions fear the auction constitutes a selloff of national assets and last week Petrobras workers began an indefinite strike in protest.
However, analysts estimate the deal could boost employment and raise Brazilian GDP by $1.7 trillion over 30 years, the O Globo daily quoted the Getulio Vargas Foundation as saying.
Lobao strongly rejects the labor union's thesis.
"We are not privatizing pre-salt oil. On the contrary, we are harnessing these immense riches lying undersea and in the ground," he said Saturday.