The venture, expected to cost between $14 and 20bn, is expected to boost production at the southern Rumaila field from the current one million barrels a day (bpd) to around 2.8 million bpd over its 20-year duration.
Rumaila is already vital to Iraq's output, contributing almost half the nation's oil production of about 2.5m bpd.
BP and CNPC were the only companies to win a bid when Iraq offered eight contracts in a June auction. The companies agreed to receive only two dollars a barrel to operate Rumaila, which has known reserves of 17.7 billion barrels.
It is the first big upstream deal between Iraq and foreign oil companies since nationalisation of the country's industry about 40 years ago.
Under the contract, Iraq's State Oil Marketing Organisation will be allocated Baghdad's 25 per cent stake, while BP will take 38 per cent and CNPC hold the remaining 37 per cent.
The plan to increase production envisages a 10 per cent increase within a year and raising the field's output by 300,000-400,000 bpd within three to four years, the Middle East Economic Survey (MEES) reported last week.
The second round of bidding for Iraqi oil contracts is due in the first half of December, Hussein al-Shahristani, Iraq's oil minister, said last month.