Mining magnate Gina Rinehart has chosen former foreign minister Alexander Downer as her second representative on the board of oil and gas explorer Lakes Oil.

Mrs Rinehart's Hancock Prospecting company snared an 18.6 per cent stake in Lakes Oil earlier this week.

As part of the deal, Lakes Oil agreed to appoint climate change sceptic and mining geologist Professor Ian Plimer to its board.

Lakes Oil said Mr Downer had also been nominated by Hancock Prospecting to join the board as a non-executive director.

Mr Downer was Australia's longest-serving foreign minister between 1996 and 2007. He also served as opposition leader between 1994 and 1995.

He has recently been touted as the next leader of the South Australian Liberal Party, but says he has no intention of running for the job.