BEIJING – A consortium led by oil giant PetroChina has formally signed a 20-year deal with Iraq to develop the Halfaya oil field, as China boosts its presence in the crude-rich region. PetroChina, France’s Total, Malaysia’s Petronas and Iraq’s South Oil will develop the field in southern Iraq, which has proven reserves of about 4.1 billion barrels, the Chinese company said in a statement late Wednesday.