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Oil reserves are the quantities of crude oil estimated to be commercially recoverable by application of development projects to known accumulations from a given date forward under defined conditions

To qualify as a reserve, they must be discovered, commercially recoverable, and still remaining. Reserves are further categorized by the level of certainty associated with the estimates

Reserves are those quantities of petroleum claimed to be commercially recoverable by application of development projects to known accumulations under defined conditions. Reserves must satisfy four criteria: They must be:

• discovered through one or more exploratory wells.

• recoverable using existing technology.

• commercially viable.

• remaining in the ground

1- Proved reservs

Proved reserves are those reserves claimed to have a reasonable certainty (normally at least 90% confidence) of being recoverable under existing economic and political conditions, with existing technology

2- Unproved reserves

Unproved reserves are based on geological and/or engineering data similar to that used in estimates of proved reserves, but technical, contractual, or regulatory uncertainties preclude such reserves being classified as proved.[10] Unproved reserves may be used internally by oil companies and government agencies for future planning purposes but are not routinely compiled. They are sub-classified as probable and possible

3- Strategic petroleum reserves

Many countries maintain government-controlled oil reserves for both economic and national security reasons. According to the United States Energy Information Administration, approximately 4.1 billion barrels (650,000,000 m3) of oil are held in strategic reserves, of which 1.4 billion is government-controlled. These reserves are generally not counted when computing a nation's oil reserves.

Another types of reserve depending on the type of reservoir source of energy

1- Primary reserve

That is the amount of oil that we can produce with natural forces or with primary recovery

a-dissolved gas in oil

b-gas cab drive

c-rock expansion

d- using artificial lifts or pumps

2-Secondery reserve

That is the amount of oil that we can produce with satisfaction of pressure differences between reservoir and surface pressures like

a-water injection

b-gas injection

enhanced reserve3-E

That is the amount of oil that we can produce with adding external forces like

) a-chemical adding( reducing surface tension

b-enhanced oil recovery methods (EOR)

c-microbial adding

d-surfactants

e-hot steam