Two consortiums -- one comprising Brazil's state-owned Petrobras and US giant ExxonMobil, the other US multinational Chevron and China's CNPC -- bought the rights to 19 of 47 deepwater blocks on offer in an area considered vulnerable to environmental harm.

Brazil on Tuesday started auctioning the rights to 172 oil blocks, most of them offshore, with some 30 companies registered to take part.

Shortly after opening, rights to the first 19 blocks were taken, as dozens of protesters gathered outside under a banner reading: "Stop the doomsday auctions."

Green groups have expressed concern over 47 Atlantic deepwater exploration blocks in an area near the mouth of the Amazon River that flows through the world's largest carbon-capturing tropical rainforest.

If exploited, the 172 blocks would emit some 11.1 billion tons of CO2 equivalent into the atmosphere, according to Brazil's ClimaInfo research institute -- undermining the country's target to become a net zero emitter of planet-warming greenhouse gases.

Already Latin America's biggest oil and gas producer, Brazil is seeking to increase production from 4.68 million to 5.3 million barrels per day by 2030.

Under the 2015 Paris climate agreements, Brazil has pledged to reduce its emissions to 1.2 billion tons of CO2 equivalent by 2030, and to reach neutrality by 2050 -- meaning emissions do not exceed the amount captured, by forests for example.

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