Headed by former Centrica boss Sam Laidlaw and backed by the Carlyle Group and CVC Capital partners, Neptune seeks to become a leading independent exploration and production (E&P) firm, it added.

The transaction follows a deal by Neptune last year to buy the oil and gas unit of France's Engie for $3.9 billion.

The deal is also a continuation of a trend among European utilities selling out of upstream oil and gas business as their core onshore activity is being challenged by renewable energy.

The size of the VNG transaction was not disclosed, but a source close to the deal said it was valued at around 400 million euros ($462.60 million).

Neptune declined to comment on the amount it paid.

VNG separately said it has decided to reorganise its portfolio over the next few years to help shape a "green, digital, gas-based future", including a sale of the E&P business it entered in 2006.

The acquisition gives Neptune access to over 50 million barrels of oil equivalent (boe) in reserves and resources, and production of about 4,000 barrels of oil equivalents per day (boepd), with a "significant" ramp-up potential by 2022 when new developments come on stream, the company said.

Via VNG's assets, Neptune will increase its stakes in Norway's Njord field, operated by Equinor, and acquire the operatorship and a 30 percent stake in the 100 million boe Fenja field, expected to start producing in 2021.

"We would like to buy more (assets in Norway), if we get an opportunity," Laidlaw told Reuters, adding Neptune was also looking at some exploration opportunities.

VNG Norge's portfolio includes a total of 42 licenses, five producing fields and three projects under development off Norway and Denmark.

Neptune expects the deal to close in the fourth quarter of 2018, pending approvals from VNG's supervisory board and national regulators.

(Reporting by Nerijus Adomaitis and Arno Schuetze, editing by Terje Solsvik and Alexandra Hudson)