Saudi Aramco may be worth as little as $1.5 trillion or even less, well below the target set by the kingdom, according to research sent Sunday to potential investors by the banks involved in the company's initial public offering.

The research, according to multiple investors who reviewed it, suggests the banks are struggling to pinpoint a precise valuation for Aramco. Some banks offered a huge range — as much as $1 trillion in the case of Bank of America — between their low and high estimates.

The pre-IPO research reports, which the banks use to drum up interest on share sales, suggest Riyadh may struggle to achieve the $2 trillion valuation that Saudi Crown Prince Mohammed bin Salman has publicly desired over the last three years.