Here is a discussion from June 2019 about various measures of proven oil reserves:
Global oil reserves data is muddled, but does it really matter?
Reserves vary markedly based on methodology employed:
But not all reserves are created equal. The data masks large disparity in production costs for the economic recovery of the oil, and a divergence in reserve accounting standards.
According to BP’s review, Saudi Arabia’s proved oil reserves are now 11% higher than previously thought, at close to 300 billion barrels, after the world’s top oil exporter reclassified some of its gas reserves as oil.
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A separate report by Norwegian consultancy Rystad Energy, however, takes a more rigorous approach to reserves by applying Society of Petroleum Engineers (SPE) standards.
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On a strictly SPE proven reserves basis, Saudi oil reserves stood at just 95 billion barrels last year, still well ahead of the US’ 32 billion barrels, Rystad believes. Canada’s massive but costly oil sands deposits suffer a similar fate under the tougher rules, shrinking to 24 billion barrels.
Proven reserves grow as the price of oil increases, which makes it economical to extract the best marginal unproven resources. The reverse is also true.
Oil reserves denote the amount of crude oil that can be technically recovered at a cost that is financially feasible at the present price of oil.[1] Hence reserves will change with the price, unlike oil resources, which include all oil that can be technically recovered at any price.
Oil reserves - Wikipedia
A low oil price not only crimps revenue of highly leveraged E&P companies, it also shrinks their proven reserves, so asset values take a hit. These assets are the basis for credit. Even if the assets are recycled through bankruptcy to other/new firms, their value still isn't the old value before the oil price drop. Only a rise in the price of oil can restore or improve that value.
Low oil prices, should they continue, will impair oil production unless greater E&P efficiencies are somehow realized. In recent years, costs of extraction (for shale oil, for example) have gone up, not down. We shall see what happens to the price of oil as the pandemic recedes.