Saudi Arabia's massive oil and gas reserves are even bigger than previously reported, according to an outside assessment commissioned by the kingdom.

The independent audit not only revised Saudi reserves higher, but may help put to rest skepticism over the nation's oil and gas wealth, which has persisted in some corners of the market for years. It also shows national oil giant Saudi Aramco is taking strides towards transparency as it continues to consider a stock market debut.

On Wednesday, Saudi Energy Minister and Aramco Chairman Khalid al-Falih said the kingdom expects the initial public offering for Aramco to take place in 2021, following a delay.

State-controlled Aramco had 263.1 billion barrels of oil waiting to be tapped at the end of 2017, according to Dallas-based petroleum consulting firm DeGolyer and MacNaughton. That is 2.2 billion barrels more than Aramco reported in its last annual review.

Aramco's natural gas reserves total 319.5 trillion cubic feet, according to the audit. The company, which is not a major player in the gas market, previously reported 302.3 trillion cubic feet of gas reserves.

Saudi Arabia has additional reserves in an area along the border with Kuwait that has sat idle due to a dispute between the neighbors. Including this so-called Neutral Zone, Saudi oil reserves total 268.5 billion barrels, DeGolyer and MacNaughton concluded. That compares with an earlier estimate of 266.3 billion barrels.

The Saudis commissioned the audit as part of their plan to publicly list shares of Aramco, the world's largest energy company. The kingdom hoped to attract a $2 trillion valuation and raise $100 billion to underwrite Crown Prince Mohammed bin Salman's economic transformation plan, Vision 2030.