One of the main factors that determines the viability of an investment in an oil and gas field is the volume of the hydrocarbons present in it. In simple terms, we would like to invest to produce oil and gas, if only the money that can be generated by producing the hydrocarbon from that field generates profits. As typical investments in a field run into billions of dollars in terms of facilities and operation costs, it becomes imperative to have a good idea about the hydrocarbons present.

Initially, when we set out to develop a field, we do not have much idea about the reservoir and its characteristics viz. porosity, permeability, areal extent, thickness etc. As we continue to drill wells and develop the field, we get more and more data about the reservoir.

Correspondingly, at the beginning we have very little idea about the crude volume in place at the reservoir and so we employ some relatively simple and unsophisticated methods for determining the volume. These methods have a high degree of uncertainity associated with it. This means that there is a high probability that the volume which we have estimated from these methods may have a wide variance from the actual volume in the reservoir. The methods employed at this stage are :

*Analogy*and*Volumetrics*.
Later , as we have more data from the reservoir , we can employ more sophisticated and reliable methods for generating the volume of hydrocarbon. The results that we generate from these methods have a higher degree of certainity and we can expect the results to be closer to the actual values.The methods employed are :

*Decline Curve Analysis*,*Material Balance*and*Numerical Reservoir Simulation.*