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RIO DE JANEIRO, Brazil — This week Brazil is poised to start selling off its “gift from God.”

After billions of barrels of undersea oil were discovered off Brazil’s coast in 2007, President Luiz Inacio Lula da Silva credited divine providence. “God is Brazilan,” he said in a speech following the discovery, later calling the oil not only a divine gift, but his country’s “passport to the future.”

That future arrives in part on Thursday. That’s when Brazil’s state-run oil company, Petrobras, is set to announce the start of its commercial production of a small section of the Tupi field, one of several billion-barrel oil fields the company says are located offshore.

More than 200 miles out to sea and several miles below the surface, the pools of oil are thought to be among the biggest discovered on earth in recent decades. Properly tapped, they’ll make fortunes and catapult Brazil into the ranks of the world’s top oil-producing countries.

But as the crude begins to flow, some industry experts say Brazil has understated the risks involved — made apparent by the deep-sea drilling that caused this year’s disastrous spill in the Gulf of Mexico.

The fields are known as the "pre-salt" region, due to the fact that the bulk of the oil is locked beneath a formation of hardened salt as much as 6,500 feet thick. The closest of the new fields are about 200 miles southeast from the beaches of Rio de Janeiro on a patch of sea floor where the water is more than a mile deep.

No one knows for sure how many barrels of oil are down there but — based on the test wells drilled so far — the Brazilian government is betting tens of billions.

“If this plays out they’ll be more than doubling their production before 2035,” said Jonathan Cogan, a specialist with the United States Energy Information Administration, a government agency that monitors oil production worldwide.

Before the discovery in 2006, Brazil didn’t even rank among the world’s top 10 oil producers — pumping out about 1.9 million barrels of oil per day, by the agency’s own estimate. In 25 years, it predicts Brazil will produce 5.5 million barrels per day — just ahead of Kuwait and behind Iraq in the world’s top five.

The oil being tapped now is in shallower reservoirs, but the bulk of the discovery lies as much as 22,000 feet below the ocean’s surface.

Extracting it will require multiple feats of engineering. Shifting ocean currents, near-freezing temperatures and crushing pressure on the sea floor — not to mention miles of mud, sand, stone and salt — all stand between drillers and the oil.

Smashed beneath nearly 9,800 feet of rock and other sediment, the mile-thick salt layer poses a special challenge for drill bits, said Carlos Torres-Verdin, a petroleum engineering professor at the University of Texas, Austin, who has advised Petrobras.

“The hardness of salt at these conditions could be comparable to the hardness of granite,” he said. Nevertheless, Torres-Verdin said he believes the obstacles can be safely surmounted and are “comparable in difficulty” to those regularly overcome by deepwater drillers in the Gulf of Mexico.

That’s what worries his colleague, Tad Patzek, a professor of petroleum and geosystems engineering who testified before U.S. Congress on the causes of the Deepwater Horizon oil spill in the Gulf of Mexico earlier this year.

Patek said ultra-deep water oil rigs have become more complex and harder to control than some oil companies would prefer to admit.

Drilling remotely at depths that would crush a submarine requires technological systems so complex they’ve left behind their roots in mining and become something akin to space exploration.

“We’re really pushing the limits of human technology and human reach here into an environment that’s far less friendly than outer space,” he said. “When you think of the world down there in the ocean, this is sort of like going — not even to the moon — but to Mars.”

Petrobras insists it is using the very latest in safety technology, including sea floor blowout preventers several generations more advanced than the faulty piece of equipment that contributed to the Deepwater Horizon disaster.

“Petrobras maintains the highest technical standards with regard to equipment and the training of its personnel,” the company said in an emailed statement. “All offshore drilling units working for Petrobras are equipped with detection systems that can provide immediate and automatic closure of the well in the event of an emergency.”

But critics of the oil industry are skeptical. The pre-salt wells will be deeper and more difficult to drill than the disastrous gusher in the Gulf of Mexico and will therefore pose a greater risk for

similar spills, said David Hughes, a Canadian geoscientist with the Post Carbon Institute.

“Technologically the well in the Gulf was easy compared to the pre-salt,” Huges said. “I’m not saying they can’t engineer around it maybe 99 times out of a hundred, but there’s always that one chance out of a hundred. Mother nature’s unknowable down to the last detail, unfortunately.”