LONDON, May 29 (Reuters) - Shell and Exxon

Mobil ( XOM ) are nearing an agreement to sell their

jointly-owned gas fields in the southern North Sea to

independent British producer Viaro Energy, three industry and

banking sources said.

The potential deal is valued around $500 million, one of the

sources said.

The sale of the Clipper and Leman Alpha field clusters would

mark the latest step in a steady retreat of major oil and gas

companies from the ageing basin in recent decades as they focus

on newer and more profitable prospects.

For Texas-based Exxon, it would complete the exit from the

North Sea, where it has been present since 1964. It sold most of

its assets in the central and northern North Sea to Neo Energy

in 2021.

U.S. rival Chevron is also selling its last remaining assets

in the British North Sea.

The deal is close to being agreed but there are no

guarantees that it will be signed, one of the sources said.

Shell, Exxon and Viaro Energy declined to comment.

Viaro Energy acquired RockRose Energy in 2020 and has since

then made several other deals in the British and Dutch North

Sea. The company produces around 30,000 barrels of oil

equivalent per day and has interests in over 30 fields,

according to its website.

The sale of the Clipper and Leman Alpha fields would also

mark the dissolution of the Esso joint venture between Shell and

Exxon, which joined forces in the North Sea in 1965.

Shell remains one of the main producers in the North Sea,

operating several fields including the Penguins redevelopment

and holding a stake in the BP-operated Clair field.