Saudi Aramco has asked JP Morgan Chase and Morgan Stanley to assist with its upcoming mammoth IPO and could call on another bank with access to Chinese investors, a source with direct knowledge of the matter said, according to CNBC.

The U.S. banks have joined boutique investment bank Moelis in being tapped for coveted roles in what is expected to be the world’s biggest initial public offering worth some $100 billion.

HSBC has emerged as the leading contender for a role among a list of five banks that could provide a pipeline to Chinese investors — an important part of the offering, the source said, adding that the other four are Chinese banks.

The final lineup for banks could still be adjusted, the source said, declining to be identified due to the sensitivity of the matter.

The IPO is the centerpiece of the Saudi government’s ambitious plan, known as Vision 2030, to diversify the economy beyond oil. Up to 5 percent of the world’s largest oil producer is likely to be listed on both the Saudi stock exchange in Riyadh and on one or more international markets.

Aramco, formally known as Saudi Arabian Oil Co., declined to comment, as did JPMorgan, Morgan Stanley and HSBC. The Wall Street Journal reported earlier that JPMorgan, Morgan Stanley and HSBC had been selected as lead underwriters.