A simple to use spreadsheet model has been developed that estimates original gas in place (OGIP), layer productivity and recoverable reserves for wells with commingled production completed in multilayered tight gas reservoirs. Predicting productivity of multiple reservoir layers is old technology. However, this model replicates the observed material-balance trend while also honoring total well production data by varying layer properties. The model maps the material-balance (pressure/compressibility factor, p/z) trend of the higher-permeability and lower-permeability layers to bracket a productivity-index (PI) -weighted p/z Curve that is used to match historical data.