As graveyards for marine organisms as well as prehistoric plants, reserves can be viewed as mounds of ancient plants. Many bodies of water and oceans have contained this substance which has remained there for thousands of years. The resulting pressure and temperature were caused by the sediments that covered those remains over time. Once the oil had been formed, the pressure and temperature were important factors. Oil is currently being consumed more quickly than it. As a result, oil has been considered a non-renewable resource.

Oil reserves are quantities of crude oil that are believed to be recoverable from underground formations in a commercially viable manner. These reserves are identified through exploration activities such as geological mapping, seismic imaging, and test drilling. After a discovery is made, specialists assess the field to estimate how much oil can be extracted using available technology and at current market prices.

Reserves are commonly divided into categories including proven, probable, and possible reserves, depending on the confidence level of recovery. Proven reserves are considered the most certain and are expected to be produced under existing conditions.

Reserve estimates are important for oil companies, investors, and governments because they influence production planning and long-term energy strategies. While reserves decline as oil is pumped from the ground, they can also grow through discoveries or advances in extraction methods. As a result, oil reserves remain a key factor in the global energy industry.