Reservoir “X” produced since 2009, it has a very high level of complexity that is not found in other reservoirs in Indonesia at this time. This level of complexity can be seen in the dynamics/behavior aspect of the reservoir and the integrated dual pressure maintenance system. The update of subsurface data completes the level of confidence in calculating the estimated reserves of the reservoir. Havlena-Odeh’s Classic Material Balance approach has not been able to accommodate the uncertain aquifer (We) behavior (changing with time, complex unsteady state). This study intends to evaluate the applicability of the Havlena-Odeh material balance model in predicting original oil in place (OOIP) of reservoir with dual pressure maintenance. Based on the calculation results, Havlena-Odeh method was modified for this reservoir types to determine OOIP. This modified We formula is successful to represent in place counting of Havlenah Odeh Rules. A convergent original oil in-place estimate of 1,299,002,483.24 barrels of Oil is obtained