Goldman Sachs and Citigroup are poised to take a major role in the sale of a portion of Saudi Arabia's state oil giant, which is expected to be the biggest initial public offering ever, according to multiple reports.

Saudi Aramco aims to offer investors a 5 percent stake in the company this year. The IPO is central to Crown Prince Mohammed bin Salman's effort to create the world's largest sovereign wealth fund and diversify Saudi Arabia's economy.

Saudi officials have said Aramco could attract a $2 trillion valuation, though others forecast the offering will value the company at $1 trillion to $1.5 trillion. The kingdom aims to raise about $100 billion to underwrite Crown Prince Mohammed's "Vision 2030" economic transformation plan.

It also stands to enrich banks that secure a role in a complicated offering for a sprawling enterprise deeply intertwined with Saudi Arabia's state finances.

The slow pace of progress toward an Aramco IPO has fueled speculation that the sale could be delayed beyond 2018. But on Monday, Bloomberg News reported that Aramco is likely to name Goldman and Citi as lead managers for the offering.

Also on Monday, Reuters reported that the Saudis have invited Goldman and Citi, along with Deutsche Bank, to the kingdom to pitch for a role in the share sale in the coming weeks.

Three other banks that have advised Aramco on the pending share sale — J.P. Morgan Chase, HSBC and Morgan Stanley — are also expected to be global coordinators, according to news reports going back to last year.

Aramco and the banks declined to comment on the Bloomberg and Reuters reports.

In another recent sign that momentum toward an Aramco IPO is picking up steam, the Saudi government on Friday announced it had changed Aramco's status to a joint-stock company, a move that lets the company issue shares.