Conoco said ONGC Videsh Ltd. agreed to buy Conoco's 8.4 percent stake in the Kashagan field in the northern waters of the Caspian Sea.
Kashagan is one of the largest oil fields in the world, with as much as 16 billion barrels of estimated commercial reserves.
Kamlesh Kotak, vice president of research at Asian Markets Securities, told Bloomberg News that ONGC may have its work cut out for it in Kazakhstan.
"Running the field at full potential is going to be a challenge," he said.
Bloomberg reports that the budget for the first phase of operations at Kashagan could nearly double to $46 billion by the time first oil is exported.
The deal is expected to close in the first half of next year.
In September, Hess Corp. announced it agreed to sell its 2.72 percent stake in the Azeri-Chirag-Guneshli fields in the Caspian and its 2.36 percent interest in the associated Baku-Tbilisi-Ceyhan oil pipeline to ONGC for $1 billion.
ACG, operated by BP, is producing around 700,000 barrels of crude oil per day.