Oil reserves
The total estimated amount of oil in an oil reservoir
Oil reservoir
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of hydrocarbons contained in porous or fractured rock formations. The naturally occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying rock formations with lower permeability...

, including both producible and non-producible oil, is called oil in place
Oil In Place
Oil in place is the total hydrocarbon content of an oil reservoir and is often abbreviated STOOIP, which stands for Stock Tank Original Oil In Place, or STOIIP for Stock Tank Oil Initially In Place, referring to the oil in place before the commencement of production...

. However, because of reservoir characteristics
Reservoir engineering
Reservoir engineering is a branch of petroleum engineering that applies scientific principles to the drainage problems arising during the development and production of oil and gas reservoirs so as to obtain a high economic recovery....

 and limitations in petroleum extraction technologies
Extraction of petroleum
The extraction of petroleum is the process by which usable petroleum is extracted and removed from the earth.-Locating the oil field:Geologists use seismic surveys to search for geological structures that may form oil reservoirs...

, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of producible oil reserves to total oil in place for a given field is often referred to as the recovery factor.