Abstract This work presents application of a new material balance method to detect aquifer influence and calculate water influx and original gas in place for four over-pressured reservoirs. Calculation of water influx needs to satisfy a set of three equations as opposed to the existing method of one equation of unit slope. In each application, the presence of aquifer influence was identified first, and then material balance was used to determine original gas in place and water influx. The overpressure effect was handled by integrating rock compressibility over operating reservoir pressure. Compositional effects were modeled with Rv (volatile oil/gas ratio) by matching PVT data using Walsh-Towler algorithm or an Equation-of-State. This new method is internally consistent, which avoids potential pitfalls of existing methods. Comparison with other methods in analyzing overpressure reservoirs shows this new method is more robust and comprehensive.