(Adds details)
July 21 (Reuters) - ExxonMobil Corp XOM.N said on Thursday it would buy InterOil Corp IOC.N for more than $2.5 billion in stock, adding a gas field to expand its exports from Papua New Guinea.
Exxon said it would pay InterOil shareholders $45 per share in stock and that it would also make an additional cash payment based on the size of the Elk-Antelope gas field.
Exxon said it would pay $7.07 per InterOil share for each trillion cubic feet equivalent (tcfe) of certified gross resource from the field above 6.2 tcfe and up to a maximum of 10 tcfe.
"InterOil's resources will enhance ExxonMobil's already successful business in Papua New Guinea and bolster the company's strong position in liquefied natural gas," Chief Executive Rex Tillerson said.
Australia's Oil Search Ltd (OSH) , backed by French giant Total SA TOTF.PA , had offered in May to buy InterOil in a $2.2 billion deal, but said on Thursday it would not raise its offer.
InterOil owns a 36.5 percent stake in the Elk-Antelope gas field.
The deal could lead Exxon and Total to tie together their competing gas interests in the South Pacific nation, cooperating to reduce costs as they battle cheap oil and liquefied natural gas (LNG) prices.
Oil Search said it and Total agreed that letting Exxon take over would help speed up development of the discovery.
