Abstract The Hai Thach field is one of the biggest gas and condensate producing fields in Vietnam. Located in the Nam Con Son Basin, offshore southern Vietnam, the field consists of multiple, stacked reservoir units, for which petrophysical characterization is challenging and a site-adapted rock physics template (ARPT) was proposed to solve this task. As reported in many publications, rock physics templates (RPTs) have been successfully applied for the characterization of shallow unconsolidated and weakly cemented reservoirs with cementation of less than 5% like in the North Sea or some other sites. However, the construction of RPT is very site-dependent. In this study, an approach of rock physical modelling, named the adapted stiff sand model (ASSM), was proposed to construct a site-adapted RPT of Vp/Vs versus acoustic impedance (AIP) for characterizing the upper part of Middle Miocene Sand in the Nam Con Son Basin, which is a 3.5-km deep and strongly cemented gas sand with 10% up quartz-overgrowth and clayey cementation. A key point in this approach was the use of a modified Biot-Gassmann's equation and in situ well log data to calculate the dry moduli of reservoir rock frame. As a result, the elastic and petrophysical parameters of this upper MMS sand, named as MMF30, are well evaluated, i.e., AIp from 9000 to 11,000 m/s*g/cc, Vp/Vs from 1.65 to 1.8, porosity from 12 to 16%, and gas saturation from 50% up, which match well with the core data and were used further in a prestack elastic inversion of a seismic inline that could map well the multiple stacked gas zones of the study reservoir. The rock physical modelling approach in this study could help characterize successfully deep and highly-cemented clayey gas sands in the Nam Con Son Basin.