BAGHDAD  Iraq’s government sold development rights to two of its largest untapped oil fields at a public auction held under extraordinary security on Friday, three days after a series of coordinated bombings killed more than 100 people here in the capital.

A partnership of Royal Dutch Shell and Petronas, a state-owned Malaysian company, won the larger field, Majnoon, in southern Iraq, which contains an estimated 12.6 billion barrels of recoverable oil.

The second field, Halfiya, also in the south, was won by a consortium led by China National Petroleum Company that included Petronas and Total of France. It is believed to hold some 4.1 billion barrels.

The auction, the second since the 2003 invasion, attracted representatives of dozens of the world’s largest oil companies, including ExxonMobil, British Petroleum and Russia’s Lukoil. “I was nervous when I was sitting there,” said Mounir Bouaziz, a vice president at Shell, after he submitted the winning bid for the Majnoon field. “We are pleased and relieved to have won this. It has taken a lot of work to get this, including months and months of study and discussions.”