When it comes to reserves of petrolium we tend to imagine a number carved in stone, objective, certified. In reality the estimates of oil reserves are the result of a mix complex between geology, engineering, mathematics, economy and, surprisingly, geopolitics.
The country with the most declared reserves in the world today is not theSaudi Arabiabut the Venezuelawith approx 303 billion barrels. Yet it is one of the poorest countries on the planet. But why? The answer lies precisely in the way these numbers are produced.
Oil resources and reserves: what is the difference
The first fundamental distinction is that between resources And reserves. The resources are all quantity of oil and gas which, according to geological models, they should exist underground, even those not yet discovered. The reserves, however, are just those quantities already discoveredphysically recoverable with current technology and economically viable to mine today.
The international reference standard is lo SPE-PRMSThe Petroleum Resources Management System developed by the Society of Petroleum Engineers. If the price of crude oil collapses, a part of the oil classified as “reserve” ceases to be so: physically it is still there, but extracting it it’s no longer worth it.
How are oil reserves calculated? The volumetric method
The first step to estimate a deposit is to calculate the so-called OIP (Original Oil-In-Place) or OGIP for gas, i.e. the total quantity of hydrocarbons present in the rock. We use the volumetric method: the reservoir rock works like a sponge, with many microscopic pores that house the fluid.
To calculate the volume you need five parameters:
- The area of deposit;
- The net thickness of the rock useful;
- There porosityi.e. the percentage of void;
- There saturationhow much of that void is really occupied by oil;
- The volume factorwhich converts the volume of the fluid at depth to what we will have at the surface.
Having a high OIP does not mean having as many extractable reserves. A must be applied to this number recovery factortypically between 20% and 50% for oil, higher for gas. To estimate it we use i dynamic methods: material balance (analogous to measuring the air coming out of a balloon), pressure drop analysis (mostly used for gas fields), and the production decline curve, applied in the final stages of a field’s life.
Proven, probable and possible reserves: degrees of certainty
The estimates are then divided into categories depending on the probability of extraction. The proven reserves (1P) have a probability of being drawn at least 90%the probable (2P) of 50%the possible (3P) of 10%. Since 2013, Russia and the countries of the former USSR have been using a parallel system based on categories A, B1, B2, C1, C2 depending on geological maturity and development, while categories D indicate resources not yet discovered.
How many oil reserves are there in the world today and why are many “political”
According to the OPEC Annual Statistical Bulletin 2025, at the end of 2024 the world reserves tried amounted to approx 1,567 billion barrels. The Oil & Gas Journal estimates a slightly higher value, around 1,770 billion. The podium: Venezuela with 303 billion, Saudi Arabia with 267, Iran with 209. They follow Canada (168) e Iraq (145).
In the 1980s, after the introduction of the OPEC production quota system in 1982, several member countries their reserves increased declared suddenly and sensationally: Kuwait +40% in 1983, Venezuela +100% in 1984, United Arab Emirates tripled in 1985, Iran, Iraq and Saudi Arabia followed in 1988. In total, OPEC reserves passed from 425 to 763 billion barrels in a decade, without equivalent discoveries.
In 1999, Canada reclassified Alberta’s oil sands from “resources” to “reserves” thanks to higher prices and more mature technologies. Between 2007 and 2011, Venezuela did the same with extra-heavy Orinoco crude, tripling its reserves from 99 to nearly 300 billion barrels. Furthermore, dozens of Gulf countries have been declaring for decades reserve numbers practically real estatea stability that from an engineering point of view is statistically impossible.
Who really controls the numbers of world reserves?
About the 90% of oil reserves world cup is in the hands of state companies (Saudi Aramco, Rosneft, PDVSA, NIOC, ADNOC), which they do not have transparency obligations of companies listed on the stock exchange. In the United States the Securities and Exchange Commission forces oil companies to declare reserves following very rigorous public standards. State companies, however, can declare what they want without having to justify the calculations.
For this reason, every time you read a ranking of world reserves, it is important to remember that behind those numbers there is science, but there is also a lot of politics.
