There have been many studies carried out to determine changes in the permeability of unconsolidated reservoirs during production, but limited success has been reported to the application of the methodologies proposed so far due to the complex relationship between the petrophysical properties and changes in the reservoir pressure. Furthermore, many of the attempts made to date are principally based on core data analyses in the laboratory with a limited application on the field scale. In this study, an attempt was made to propose a Compaction Based Permeability Index (CBPI) in order to determine changes in the permeability of unconsolidated reservoirs on the field scale. To do this, a series of mathematical equations were derived to determine the CBPI by analyzing the literature data and performing a series of permeability tests on core samples of an unconsolidated gas reservoir from Western Australia. A numerical model of this field was also constructed in order to evaluate the application of the proposed approach. The results obtained indicated that the stresses relaxation may not be observed during the production period and the constant flow rate approach ensures that the permeability reduction is only induced by the compaction of the reservoir. It was found that the CBPI can be determined with a sufficient accuracy during production using the bottom-hole pressure and the average reservoir pressure if proper well testing and/or production data are recorded. An extensive worldwide evaluation of the application of the CBPI is recommended in other unconsolidated reservoirs for further validation. © 2019 Elsevier B.V.