Prior to oil production from a new reservoir, volumetric methods are used to estimate oil-in-place.
In this case, stock tank barrels refers to the volume of oil after production, at surface pressure and temperature (as opposed to reservoir conditions).
[2] The analogous term original gas-in-place (OGIP) is used to refer to the total natural gas in a reservoir.
Furthermore, there is a term called Hydrocarbons Initially in Place (HCIIP) that is used for either oil or gas.
The wide variance is due largely to the diversity of fluid and reservoir characteristics for different deposits.