Most people seem to think that oil "reserves" are tangible and measurable, akin to putting a measuring stick in a tank. I have spent the better part of 40 years as a petroleum engineer putting together oil and gas reserves estimates, or working with other engineer's estimates of reserves.

In fact, all reserve calculations are estimates only, and may or may not be accurate within an error range, which may itself be quite wide.

IMO, no international agency or organisation has sufficient data to make other than a reasonably informed estimate as to the most likely reserves from each country. For various technical reasons, even if the most perfect information was available, it is doubtful whether OPEC's most likely oil reserves could be computed within +/- 10%.

Because of the scarcity of data and the suspect reserve estimates provided by most state-owned oil companies, it is highly unlikely that any estimate of OPEC's total oil reserves could be made within an error range of +/- 20%.

I don't believe that the oil markets factor in any reliance on what OPEC's reserves actually are, in setting today's, next month's or next year's oil price. Whether or not the IEA changes its estimates of reserves for Saudi Arabia or any other country will likely have no impact on OPEC's production quotas, on actual OPEC production, or on the price of oil.