Exxon Mobil Corp has joined Shell Plc in seeking to divest their Aera joint venture, and the duo are in advanced talks with a buyer for the California oil and gas producer, three people with knowledge of the matter said.

Demand for oil assets is rising with this year's surge in energy prices and as publicly traded companies shed older operations to reduce greenhouse gas emissions. Exxon and Shell have signaled a desire to focus on larger-scale oil fields and liquefied natural gas projects.

Aera is one of California's largest oil producers, pumping 125,000 barrels of oil and 32 million cubic feet of natural gas per day, according to its website. The business generates about $1 billion a year in cash, putting the value of any deal at several billion dollars.

Exxon and Shell declined to comment.

Reuters reported a year ago that Shell was looking to exit the venture, which operates primarily in central California's San Joaquin Valley.

Exxon joined the effort, and the duo - assisted by financial adviser JPMorgan Chase - have been in talks with several potential buyers, some bidding solo and others in consortia, according to sources.

JPMorgan declined to comment.

Shell and Exxon have been selling older properties and using the proceeds to repay debt and investing in new areas. Shell this month launched a sale of its 30% stake in the Cambo oil prospect in the British North Sea, Reuters reported, while Exxon has cut holdings around the world.

Any deal potentially could allow Exxon to hit a target of selling $15 billion in assets that it set four years ago. The company has said it wants to focus on Guyana, Brazil offshore and liquefied natural gas (LNG) projects.

Exxon disclosed divestments valued at more than $3 billion in the second quarter of this year, including in Texas, Canada and Romania. This quarter, it told Reuters it agreed to sell shale-gas properties in Arkansas.

The U.S. oil producer has put on the market assets in the UK North Sea and in Africa, and last year completed a $1.1 billion sale of a U.S. chemical business.