Oil reserves are an estimate of how much oil can ultimately be recovered. This broad definition is also called oil in place. It includes undiscovered or "yet to find" reserves. It's based on the probability of finding reserves in certain geological areas. It also assumes new types of technology will make it economically feasible to extract the oil.

A more precise definition is discovered oil reserves. There are three categories. These are based on how likely it is the oil can be recovered using current technology.

Proven Reserves - There is a greater than 90% chance that the oil will be recovered. Probable Reserves - The chance of actually getting the oil out is greater than 50%. Possible Reserves - The likelihood of recovering the oil in place is significant, but less than 50%.

Keep in mind that part of an oil field's probable and possible reserves get converted into proved reserves over time. These discovered reserves are just a small part of the oil in place. It's just not technically feasible to get most of the oil out in any given field.

Don't believe it when someone says the world will run out of oil on a certain date. Instead, oil will become too expensive to use long before it runs out.

Proven Reserves

Of the three categories, the most commonly used is proven oil reserves. That's where analysis of geological and engineering data demonstrates with reasonable certainty to be recoverable from known reservoirs. Only the oil that is commercially viable under current economic conditions is counted. If oil prices rise or new technology makes costs lower, then more fields become viable.

Reasonable certainty means that either actual production or conclusive testing has occurred. The testing includes drilling. If not, then the site must be adjacent and similar to areas that have been drilled. The size of the field is determined by the edges where the oil contacts adjacent gas or water formations.

Oil is not counted as proven if engineers are uncertain if it can be recovered under current economic conditions. Engineers don't count it if it's in completely untested areas.

Oil Sands Reserves

The ability to extract oil sands for a reasonable cost has increased the amount of proven reserves. Most of it, totaling about 166 billion barrels, is in Alberta, Canada. The United States imported 1.236 billion barrels from these fields in 2014.

Oil sands are sand mixed with a thick substance called bitumen. The bitumen must be heated before it can be used as oil. Two tons of sand must be mined, using three barrels of water, to get one barrel of oil. The process is controversial because it uses a lot of energy and water and leaves a scar on the environment that can be seen from space. Companies are required to restore the area to its original condition after extraction.

World Reserves

In 2018, there were 1.73 trillion barrels of oil in the world. That's enough to last another 50 years since the world uses 95 million barrels per day. Only proven reserves are counted in the total world reserves. This number changes only slightly every year.

Largest Reserves in 2018

The world's largest proven reserves are in just a few geologically unique areas. Reserves are the graveyards of prehistoric plants and tiny marine organisms. Their remains settled at the bottoms of ancient oceans and lakes 300 million to 400 million years ago. Layers of sediment covered them, increasing the pressure and temperature. That changed the chemical composition into oil. Since we use it faster than its being created, oil is considered a non-renewable resource.

Most of the big fields in the proved oil reserves are in the Middle East, Venezuela, Canada, and Russia. Here's the number of barrels of proven oil reserves in 2018 for the top 15 countries according to the BP Statistical Review.