"The acquisition is part of Chrysaor's strategy to prolong asset life, maximise recovery and deliver value from the UK North Sea," Chief Executive Officer Phil Kirk said in a statement.

The company will soon begin surveys in preparation of drilling new wells in the fields, he said.

Private equity-backed Chrysaor will become the sole owner of the Armada, Maria and Seymour fields, it said. It expects the deal to close in the second half of 2018.

The value of the deal was not disclosed.

Spirit, an oil and gas joint venture created last year between Centrica and Bayerngas Norge, will retain costs linked to future decommissioning - plugging and dismantling - of the fields, Chrysaor said.

Analysts at consultancy Wood Mackenzie said the fields produced 8,300 barrels per day (bpd) in 2017 and that previous plans had called for production to end this year.

Smaller, privately owned companies have entered the North Sea in recent years by acquiring fields from longstanding players such as Shell, Engie and OMV.

Backed by Harbour Energy, an investment vehicle of EIG Global Energy Partners, Chrysaor has stakes in 10 fields and produces around 130,000 barrels per day of oil and gas, making it the largest independent North Sea producer.

It aims to increase production to 200,000 bpd, Chairman Linda Cook has told Reuters.

"We expect the company will still look to grow its portfolio both organically and inorganically. This private equity player has a declining asset base, with production dropping below 100,000 bpd by 2020," WoodMac analyst Fiona Legate said.

"Our view is that larger acquisitions will be needed if the company is to monetise this portfolio in the near term."

(Reporting by Ron Bousso; editing by Robin Pomeroy and Jason Neely)

By Ron Bousso