Saudi state oil giant Aramco said on Wednesday it is buying a majority stake in petrochemicals behemoth SABIC for $69.1 billion, a deal that gives the crown prince's ambitious reform programme a massive cash boost.

The 70-percent stake purchase effectively merges the kingdom's two largest companies, handing its top sovereign wealth fund around the same amount it had expected from a much-delayed Aramco IPO.

The deal, mooted last year, lends Crown Prince Mohammed bin Salman greater fiscal flexibility to pursue his "Vision 2030" reforms agenda after journalist Jamal Khashoggi's murder and a sweeping crackdown against dissent left many investors spooked.

"Saudi Aramco today announced the signing of a share purchase agreement to acquire a 70 percent majority stake in Saudi Basic Industries Corporation (SABIC) from the Public Investment Fund of Saudi Arabia, in a private transaction for... $69.1 billion," Aramco said in a statement.

The company did not say how it would finance the deal, one of the biggest in the global industry.