The audit has so far confirmed the reserves figures earlier given by Saudi Arabia, the sources said, an important part of the state oil company's preparatory work for its planned initial public offering next year.
Saudi Arabia's reserves of easily recoverable oil have long been considered the world's largest. But there have also been questions about their volume and quality, and the audit seeks to provide internationally recognised figures for investors.
"It's a huge task," a source familiar with the matter said, commenting on the progress of the reserves audit. "They are about two-thirds of the way through. It's all going well and smoothly - no surprises."
For nearly 30 years - despite rising production, large swings in oil prices and improved technology - Riyadh has annually reported the same number for reserves of 261 billion barrels, according to BP's statistical review.
Dallas-based DeGolyer and MacNaughton, and Gaffney, Cline and Associates, part of Baker Hughes <BHI.N>, are involved in the auditing, sources have said. Baker Hughes declined to comment on the progress of the work, while DeGolyer did not respond to a request for comment.
Asked to comment, Aramco said: "We do not comment on rumours and speculation. Investors will receive relevant information in due course in connection with the IPO."
An industry source had told Reuters in March that Aramco aimed to have one of the two reserves auditors wrap up the review this year, long before the planned listing. But this now looks unlikely.
"Aramco wants to make sure the companies are shown full data to avoid sceptical remarks on its reserves later on," a second source said. "It's unlikely the audit will be finished this year."
A third source said auditing work on the major Aramco fields was completed, leaving smaller fields still to be audited.
A reserves total that is significantly above or below the 261 billion figure is likely to affect Aramco's potential value. Earlier phases of the audit have supported Aramco's statements on the total size of deposits.
The sale of around 5 percent of Aramco next year is a centrepiece of Vision 2030, an ambitious reform plan to diversify the Saudi economy beyond oil which is championed by Saudi Arabia's Crown Prince Mohammad bin Salman.
No decision about the location of the international share listing has been announced, fuelling market speculation the IPO could be delayed or shelved. But Prince Mohammad told Reuters in October it was still on track for 2018.
(Additonal reporting by David French in New York and Reem Shamseddine in Khobar, Saudi Arabia; editing by David Evans)
By Alex Lawler and Rania El Gamal