Mr Locke, who made around £120m when the Aberdeen-based Abbot drilling operation he built was sold, and Patron have signed an agreement to buy the retail assets of Murco from America's Murphy Oil Corporation.
The acquisition will allow the partners to add 228 Murco-owned petrol stations to the 60-strong portfolio of outlets they operate through the Motor Fuel Group. They will acquire rights to supply fuel to 200 Murco-branded outlets owned by others.
The portfolio of Murco-branded stations includes 10 outlets in Scotland, where MFG has seven forecourts.
Stephen Green, senior partner at Patron Capital, said: "We are delighted with this exciting transaction. The signing of this agreement supports our stated objective to grow Motor Fuel Group into a significant force in the UK forecourt sector."
Mr Locke and Patron bought Motor Fuel Group for a reported £40m in 2012.
Motor Fuel Group said the Murco transaction is scheduled to close no later than 30 September 2014.
In July Murphy Oil agreed to sell the Milford Haven refinery in Wales to US entrepreneur Gary Klesch.
Aldershot-born Mr Locke started his career as an investment banker focused on oil and gas.
He founded Abbot in 1992 and sold it to private equity group First Reserve for £906m in 2007.
Patron Capital led a £210m buyout of Cala, the Edinburgh-based housebuilder, in March last year.