Dear Gitta,
In the estimation of GIIP or OOIP, a combination of methods helps reduce the risk as when you are attempting to calculate the GIIP or OOIP via two independent methods your risk is reduced as compared to placing all your faith in single deterministic answer.
As Vinomarky says the range of possible answers is far more important that the a single deterministic answer. The range is a measure of uncertainty you are dealing with.
Typically GIIP or OOIP can be calculated using production data (i.e. the field is producing and production and reservoir pressure is available) using a Material Balance to solve for the OOIP.
This is then contrasted with the GIIP or OOIP obtained from volumetric analysis and the two should be in reasonable agreement. What the Material Balance brings is an GIIP or OOIP estimation under dynamic conditions as compared to volumetrics static conditions.
See the attached slide on what techniques are used at what point in the hydrocarbon cycle to estimate OOIP and EUR.
This is the simple answer, but in practice it take experience and ability to do this.
Rgrds
Chee Koh Peh
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