Saudi Arabia’s state oil company has hired two US companies - energy consulting services unit Gaffney, Cline & Associates, as well as Dallas-based reserves auditing group DeGolyer and MacNaughton - to assess its crude oil reserves, the Wall Street Journal reported, citing sources.

The audit is a prerequisite for listing Saudi Aramco in the capital markets, which could happen as soon as next year, according to the Journal.

The assessment will play a critical role in determining the value of the company. Aramco is said to be worth between $2 trillion and $3 trillion, according to Saudi Deputy Crown Prince Mohammed bin Salman.

That would put the company well above tech giant Apple's $642 billion market capitalisation. It would also be roughly three times larger than the market cap of the three U.S. supermajors ExxonMobil, Chevron and ConocoPhillips combined.

Gaffney, Cline declined to comment on the matter. Saudi Aramco did not immediately return our request for comment.

Saudi Aramco says it has 261.1 billion barrels of crude oil and condensate reserves. But considering that Opec's leading producer pumps out more than 10 million bpd of crude oil a day, there is uncertainty about the actual size of the reserves.

By comparison, ExxonMobil had about 8 billion barrels of proved crude oil reserves along with approximately 25 billion barrels of proved oil-equivalent reserves, according to its 2015 annual filing with the SEC.