SAO PAULO – The exploration and production of Brazil’s massive offshore oil reserves is the driving force behind the globe’s biggest-ever share offering.

Petroleo Brasileiro SA, or Petrobras, the South American nation’s state-run energy company, raised $70 billion Thursday through the sale of more than 4 billion common and preferred shares in Brazil and the United States. Investors saw an opportunity to invest in one of the world’s fastest-growing regions for energy production.

The money will fund Petrobras’ ambitious $224 billion, five-year plan to develop offshore fields that Petrobras discovered in the last three years, holding more than 50 billion barrels of recoverable oil.

Petrobras could vault itself and Brazil into the top tiers of oil and gas producers around the globe. Yet some analysts say that the cost, and an increased role for the Brazilian government, could dampen eventual returns for shareholders.

Petrobras forecasts that by 2014 it will produce the equivalent of 3.9 billion barrels of oil per day when natural gas output is included. That’s equal to what Exxon Mobil Corp., the world’s largest publicly traded oil and gas company, produces today. The increase in market value, to about $220 billion, already moves Petrobras past Royal Dutch Shell PLC and Chevron into second place behind Exxon’s $311.36 billion.

According to the U.S. Energy Department, Brazil is the No. 12 oil producer in the world. If Petrobras hits its targets, Brazil would jump to No. 6, behind Saudi Arabia, Russia, the U.S., Iran and China.

Brazil’s government received $43 billion in shares in exchange for allowing Petrobras to drill for 5 billion barrels in reserves, hiking the government’s stake to 48 percent from 40 percent.

Brazilian President Luiz Inacio Lula da Silva hails the oil discoveries as an economic savior for 57 million Brazilians living in poverty — 30 percent of the population. The military wants new submarines and jets to protect the crude and leftist groups want it all nationalized.

“Brazil’s oil sector is already 10 percent of the country’s GDP,” said one industry insider. “When you look at what will be the engines of growth going forward, it will be increasingly from the oil industry.”