Operational responsibility for Azadegan, believed to be one of the world's largest untapped oil reserves, is being transferred to Iran after the cut in the stake, the company said.
Inpex, Japan's top oil explorer, had a 75% share in Azadegan when it reached a development agreement in 2004. The cost of the project was then estimated at $2 billion.
Talks on the project had stalled over problems including US opposition to the project, disputes over the size of the investment amid escalating raw material prices, doubts about the safety of the area that needed to be cleared of mines from the 1980-1988 Iran-Iraq War, and more recently international tensions surrounding Tehran's nuclear programme.
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