Kashagan, the biggest world oilfield discovery since 1968, holds an estimated 30 billion barrels of oil-in-place, of which 8-12 billion are potentially recoverable.
ConocoPhillips said the carrying value of the assets related to its Kashagan interest was about $5.5 billion as of September 30.
The company said it will take an after-tax impairment of about $400 million in the fourth quarter to reduce the carrying value. The deal is expected to close in the first half of 2013.
ConocoPhillips said it notified government authorities in Kazakhstan, and its partners in the North Caspian Sea production-sharing agreement of its intention to sell the stake.
Kazakh Oil and Gas Minister Sauat Mynbayev last month disclosed ConocoPhillips' plans to sell its stake in the Kashagan oilfield.
The field is jointly controlled by state-run KazMunaiGas and six international companies, including Eni Spa
ConocoPhillips, which is looking to generate $8 billion to $10 billion from divestments by the end of 2013, has raised $2.1 billion from asset sales this year.
(Reporting by Swetha Gopinath in Bangalore; Editing by Maju Samuel)
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