Santos exits Stag field in sale to Malaysia's Sona Petroleum

* Sona Petroleum buys Stag field for $50 mln

* Santos says mature field no longer a core asset (Adds Sona's plan, Quadrant statement)

By Sonali Paul

MELBOURNE, Nov 2 (Reuters) - Santos Ltd, which rejected a $5 billion takeover offer last month in the hopes of getting better value from selling assets, made its first sale on Monday with a small deal to exit the Stag oil field off northwestern Australia.

Malaysian company Sona Petroleum Bhd said it had agreed to buy the ageing oil field from Santos and private firm Quadrant Energy for $50 million, in a statement.

Santos, scrambling to pay down A$8.8 billion ($6.3 billion) in net debt, had been looking to sell its two-thirds stake in Stag before it effectively put all its assets up for sale in August.

"Stag had delivered a strong production performance over the life of the field but it was now mature and no longer core to the company's strategy," Joe Ariyaratnman, Santos' general manager for Western Australia and Northern Territory said in an emailed comment.

Wood Mackenzie valued Santos' stake in the field, which as of June was producing 4,600 barrels a day, at $13 million.

Santos shares rose 2.6 percent on Monday, defying weakness in other energy company shares.

The company has declined to comment on reports it is in talks to sell down part of its coveted 13.5 percent stake in the Papua New Guinea liquefied natural gas (PNG LNG) project to Japan's Marubeni Corp.

That stake alone could be worth A$6.5 billion ($4.6 billion) based on the value implied in a recent bid by Woodside Petroleum for Oil Search Ltd, a bigger stakeholder in PNG LNG.

Sona plans to pay for the acquisition from cash it raised in its initial public offering two years ago.

It expects to extend the life of the Stag field, producing since 1998, by adding new wells to boost oil recovery and has agreed to take over environmental liabilities which kick in at the end of the field's life.

Sona in January scrapped plans to buy a stake in two oil and gas blocks in the Gulf of Thailand from Salamander Energy for $280 million after London-listed Ophir Energy Plc proposed a takeover of Salamander.

Quadrant Energy is owned by Brookfield Asset Management and Macquarie Capital which acquired oil and gas assets, including a one-third stake in the Stag field, from Apache Petroleum earlier this year.

"Today's announcement represents a notable step in Quadrant Energy's strategic portfolio repositioning as we seek to capitalise on new growth opportunities," Quadrant said.

($1 = 1.4000 Australian dollars) (Reporting by Sonali Paul; Editing by Richard Pullin)