Riverstone Holdings LLC and Blackstone Group LP-backed Siccar Point Energy Ltd. are considering bidding for oil and gas fields in the North Sea that are being sold by Russian billionaire Mikhail Fridman’s LetterOne, people with knowledge of the situation said.

The firms, as well as Rio de Janeiro-based oil producer PetroRio SA, are expected to bid for the assets by the end of July in a deal that may be valued at about $1 billion, the people said, asking not to be identified because negotiations are private. A buyer will likely be picked in August, they said.

LetterOne, which acquired its North Sea assets as part of a deal agreed last year to buy an oil and gas division of Germany utility RWE AG, is being forced to divest the fields by the U.K. government. The company, which counts ex-BP Plc Chief Executive Officer Lord Browne as executive chairman, has until mid-October to change control of its Dea U.K. gas fields or have its operational licenses revoked, Energy Secretary Ed Davey said in a statement on April 20.

Representatives for LetterOne, PetroRio, Siccar Point and Riverstone declined to comment.

A slump in oil prices and rising costs of production have raised concerns about the future of North Sea developments, with several large explorers seeking to sell assets in the region. OMV AG, Austria’s biggest oil and gas company, is working with Barclays Plc as it seeks to sell part of its stake in a deepwater project in the U.K.’s North Sea, people with knowledge of the matter said last month.

PetroRio, backed by Brazilian magnate Nelson Tanure, earlier this year sold control of an exploration venture in the Brazilian Amazon to OAO Rosneft.