Early on October 15, a group of international investment bankers delivered some unwelcome news to top executives of Saudi Arabia's giant oil company, Saudi Aramco.

The bankers, gathered at Aramco's headquarters in Dhahran, reported that global investors weren't as bullish on the company's initial public offering of stock as the officials had expected, said two people who were in the room and three who were briefed on the meeting. That meant Aramco appeared unlikely to reach the $US2 trillion ($2.9 trillion) valuation wanted by Saudi Arabia's crown prince, Mohammed bin Salman.