During their ascent toward the land surface, oil and gas sometimes seep into reservoir rocks, where they can accumulate. A reservoir forms when the hydrocarbons reach impermeable rock. Their progress halted, they build up in the porous reservoir rocks, forming what are known as “conventional” reservoirs. Hydrocarbons that remain trapped in the source rock, which is highly compacted and hardly permeable at all, are called “source-rock hydrocarbons”, or more commonly “shale hydrocarbons”. These are “unconventional” oil and gas resources.

In order for a hydrocarbon reservoir to be exploitable, it must hold significant quantities of oil or gas trapped in its reservoir rock. This configuration, notably, protects the hydrocarbons from external factors (e.g. air, bacteria) that could alter them.

The units of measurement for oil quantities are barrels (roughly 159 liters) and tons.

Estimates of proved reserves are calculated based on the recoverable quantity of hydrocarbons, with reasonable certainty, based on the economic conditions and the technologies of the time. These reserves fluctuate from year to year, and are still growing thanks to the discovery of new fields, and to the development of new recovery technologies that improve well yield.

The prices per barrel of oil are set on the international markets. As an example, Brent (the name of a reservoir in the North Sea) is the benchmark for crude oil, especially on the markets in parts of Europe, Africa and the Middle East.