"Statoil has now transferred its 18.75 percent share in the West Qurna 2 field to the operator Lukoil following the Iraqi authorities' approval of the transaction," it said.

The financial terms of the deal were confidential, it added.

Statoil provided no details on why it was selling its stake, saying simply that "after thorough evaluation, we have decided to pursue other projects and opportunities."

Statoil has been a partner with Lukoil in the West Qurna 2 project since the two companies won a competitive bid in 2009.

West Qurna 2 is one of the world's biggest undeveloped oil fields, with known reserves of 12.9 billion barrels.

Statoil and Lukoil signed a 20-year deal in early 2010, under which they were aiming to raise production at the field to 1.8 million barrels per day.

The companies were to receive fees of $1.15 per barrel extracted.

Iraq has the world's third-biggest oil reserves, with an estimated 115 billion barrels.