Rock-physics diagnostic (RPD) analysis is widely used to describe the functional relationship between seismic velocity and porosity, which is consistent with local geology, for example, characterizing diagenetic pore-filling cement and contact cements. Rock-physics studies indicate that quantitative cement substitution is as important as fluid substitution in understanding the difference between the hydrocarbon effect and the cementation effect from seismic data. Cement quantification is also important in understanding the fluid flow and mechanical strength of reservoir rocks. A new method quantifies the amount of contact and pore-filling cement by using RPD analysis. This cement-quantification method combines soft-sand, stiff-sand, and intermediate stiff-sand models and is a physical-based approach to quantify the amount of contact and pore-filling cement. By using these rock-physics models, one can examine the relationship between elastic properties and porosity in laboratory data and can link rock-physics properties to diagenesis to quantify the amount of cement. Pore-filling berthierine cements determined from thin-section analyses agree well with pore-filling cement determined from RPD analysis. The amount of contact cements determined from RPD analysis also agrees well with the amount of quartz cement determined from thin-section analyses. This new approach can be used to quantify the amount of pore-filling and contact cement from seismic properties.