* $676 mln stake to secure S.Korea’s biggest oilfield assets

* Asian, Western firms bid for UAE’s biggest oilfields

ABU DHABI/SEOUL May 13 (Reuters) - State-run Abu Dhabi National Oil Company (ADNOC) has signed a 40-year deal with GS Energy, selling the South Korean firm a 3 percent stake in an onshore oil concession to develop the United Arab Emirates’ (UAE) biggest oilfields.

The $676 million deal is the latest move by an import-dependant Asian country to secure energy supplies. The stake will be South Korea’s single biggest oil asset, providing around 800 million barrels over the 40-year period.

Nine Asian and Western companies have bid for stakes in the Abu Dhabi Company for Onshore Oil Operations (ADCO) concession after a deal with Western majors dating back to the 1970s expired in January 2014.

GS Energy joins Japan’s Inpex Corp and France’s Total in winning contracts so far to develop the oilfields, which produce 1.6 million barrels per day (bpd) and have a target of 1.8 million from 2017.

“By this agreement, Korea’s GS E&P Ptd Ltd, a wholly-owned subsidiary of GS Energy Corp, receives a 3 percent participating interest in the new concession with effect from 1 January 2015,” ADNOC said on Wednesday.

GS Energy said it was paying 743 billion won ($676 million) for the stake.

Four oil majors – ExxonMobil, Royal Dutch Shell, Total and BP – had each held 9.5 percent equity stakes in the ADCO concession since the 1970s.

After the deal expired last year, state-run ADNOC took 100 percent of the concession as political leaders in Abu Dhabi weighed up whether to bring in Asian firms or stick with old partners, industry and diplomatic sources said.

Shell and BP have also put in new bids, while Exxon has decided against bidding, sources have told Reuters.

Total became the first oil major to renew the concession, putting peers under pressure to improve terms after the local partner said the French firm made the best offer.

Other bidders for the concessions include Occidental Petroleum Corp, Italy’s ENI, China National Petroleum Corp, and Norway’s Statoil.

GS Energy, owned by GS Holdings Corp, has a 50 percent stake in South Korea’s second-largest crude oil refiner, GS Caltex Corp, with U.S oil major Chevron Corp holding the other half.

The ADCO purchase would “greatly contribute to stable energy supplies” in South Korea, GS Caltex said in a statement.

Abu Dhabi, capital of the UAE, a core Gulf OPEC member, is spending billions of dollars to boost crude oil production capacity to 3.5 million barrels per day by 2017 from around 3 million bpd currently.