The demand prompted the world's largest oil firm to market the six-part deal - which will price on Tuesday and could see Aramco raise more than $10 billion - with a smaller than expected premium to the Saudi government that owns it.
People familiar with the matter told Reuters that Aramco's vast profits - nearly three times those of Apple last year - meant investors were willing to buy the bonds even if they end up getting a lower return than on Saudi sovereign debt.
Aramco last year postponed until 2021 an initial public offering (IPO) aimed at raising money for a government looking to cut its budget deficit and diversify its economy beyond oil.
"The success of this bond issue will be the litmus test and a crucial precursor for the anticipated Aramco IPO within the next two years," said Salah Shamma, head of investment, MENA equities, at Franklin Templeton.
Speaking at an event in Riyadh, Saudi Energy Minister Khalid al-Falih said he believed demand for the issue was "north of" $30 billion.
Aramco met investors last week in a global roadshow ahead of the issue. The bonds, which range in maturity from three to 30 years, are expected to attract demand from both emerging markets and investment-grade buyers.
WIDE RANGE OF INVESTORS
Sergey Dergachev, a manager of emerging market corporate debt at Germany-based Union Investment, said ahead of the issue that he expected demand for to reach $45 billion-$50 billion, as the issue's different maturities would attract a wide range of investors from different regions.
"The short part of the curve, three and five years, will be strongly bid by locals and some Asians. The 30-year will be heavily in demand by U.S. and Taiwanese investors," he said.
Pension funds and insurance companies in the United States and Taiwan are traditionally interested in investing in long-dated securities to match their own long-term commitments.
Fund managers said the initial price guidance was very close to the yields offered by Saudi government debt, particularly for shorter maturities, suggesting they may eventually be sold with a lower yield than Saudi government bonds.
The longer-dated tranches offer a small premium over Saudi sovereign debt, but this is expected to shrink as the notes are marketed.
The bond issue, announced last week, follows Aramco's agreement to buy a 70 percent stake in petrochemicals firm Saudi Basic Industries (SABIC) from Saudi Arabia's Public Investment Fund (PIF) in a deal worth $69.1 billion.
Al-Falih said on Monday he hoped Aramco's acquisition of SABIC would be completed within six months.
In a company presentation last week, however, the company said the deal will close in 2020.
The transaction will give PIF firepower to proceed with its plans to create jobs and diversify the largest Arab economy beyond oil exports.
(Reporting by Rania El Gamal and Stephen Kalin, additional reporting by Sylvia Westall, Tom Arnold, Virginia Furness; Writing by Davide Barbuscia; Editing by Susan Fenton and Mark Potter)
By Rania El Gamal, Stephen Kalin and Davide Barbuscia