Estimation of initial hydrocarbon in place is a critical step for any investment in the oil and gas industry, based on which revenues and further developments are planned and designed. The conventional ways of estimating the hydrocarbon initially in place are: Material Balance Equation, Volumetric Methods, and Numerical Simulation Models. These approaches could require a considerable time of production and some tests to collect data about the average reservoir pressure profile in time and apply the conventional Material Balance Equation or to build the numerical simulation model. However, for some economic reasons, we require to have a first indication about the reserves within the first year of production to evaluate the development strategies for the reservoir; considering the cost of well testing, the time down when the well being shut in, the loss of production, the time needed to stabilize the pressure in the reservoir, the conventional material balance method becomes ineffective.