BAGHDAD, Oct. 29  Guided by American legal advisers, the Iraqi government has canceled a controversial development contract with the Russian company Lukoil for a vast oil field in Iraq’s southern desert, freeing it up for potential international investment in the future.

In response, Russian authorities have threatened to revoke a 2004 deal under the Paris Club of creditor nations to forgive $13 billion in Iraqi debt, a senior Iraqi official said.

The field, West Qurna, has estimated reserves of 11 billion barrels, the equivalent of the worldwide proven oil reserves of Exxon Mobil, America’s largest oil company. Hussain al-Shahristani, the Iraqi oil minister, said in an interview that the field would be opened to new bidders, perhaps as early as next year.

The contract, which had been signed and later canceled by the Saddam Hussein government, had been in legal limbo since the American invasion. But the Kremlin remained hopeful it could be salvaged until this September, when Mr. Shahristani traveled to Moscow to inform officials there that the decision to cancel it was final, he said.