Private equity-backed oil firm Chrysaor Holdings Ltd. has hired advisers ahead of talks to buy Chevron Corp.'s (NYSE: CVX) British North Sea oil and gas fields for over $2.5 billion, industry and banking sources said.

Chrysaor has signed up investment banks Jefferies and BMO Capital Markets for the discussions as a deadline for bids approaches before Christmas, the four sources said.

Chevron said earlier this year it was selling its assets in the central North Sea, which are expected to fetch around $1.5 billion.

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Chrysaor and Chevron are also discussing the sale of part or all of the U.S. group's 19.4% stake in the BP Plc (NYSE: BP)-operated Clair Field, the largest oil field in the British North Sea, which was not originally up for grabs, the sources said.

The Clair stake could easily fetch $1 billion, they said.

A Chevron spokeswoman said the company does not comment on industry speculation or commercial matters.

When asked about interest in its Clair stake, she said: "While we continue to evaluate our portfolio we are progressing the Clair projects."

A spokesman for Chrysaor declined to comment. Jefferies declined to comment. A BMO spokesman was not immediately reachable.

Chevron's net daily production in the North Sea in 2017 averaged 50,000 barrels of oil and 155 million cubic feet of natural gas, according to its website.

The San Ramon, Calif.-based company kicked off in July the sale of its central North Sea oil and gas fields Alba, Alder, Captain, Elgin/Franklin, Erskine and Jade as well as the Britannia platform and its satellites.

On Dec. 11, Chevron said that Britain's Oil and Gas Authority approved its development plans for the Captain Field.

In October, Chevron sold its 40% stake in the Rosebank development to Norway's Equinor ASA (NYSE: EQNR).

Selling the central North Sea assets as well as the Clair stake would effectively mark Chevron's full exit from the aging U.K. North Sea basin after over 50 years.

Chevron has in recent years focused its efforts on rapidly growing its shale production in the Permian Basin in Texas as well as the giant Tengiz Field in Kazakhstan.

Chrysaor, backed by Harbour Energy Ltd., an investment vehicle of EIG Global Energy Partners, became one of the largest North Sea producers after buying $3 billion of oil and gas fields from Royal Dutch Shell Plc (NYSE: RDS.A) in 2017.

With its current portfolio, Chrysaor expects daily production averaging 120,000 to 130,000 barrels of oil equivalent until 2020.