rock physics modeling (RPM)

Rock Physics Modeling (RPM) is the discipline that establishes the quantitative link between the geological and petrophysical properties of rocks and their elastic properties, which ultimately control the seismic response. In seismic reservoir characterization, seismic data provide information only about acoustic impedance contrasts, which are functions of compressional velocity, shear velocity, and density. However, the main exploration and production objectives are lithology, porosity, and fluid content. RPM serves as the physical bridge that translates between these domains.

The foundation of RPM lies in describing how mineral composition, porosity, fluid saturation, pressure, and temperature affect elastic parameters such as VpV_pVp​, VsV_sVs​, and density. These elastic parameters are then used to predict seismic attributes such as acoustic impedance, shear impedance, Vp/Vs ratio, and Poisson’s ratio. The modeling process involves both empirical and theoretical frameworks. Empirical relations include Gardner’s density–velocity relationship and Castagna’s mudrock line, while theoretical approaches rely on Gassmann’s equations for fluid substitution and effective medium theories for pore structure, cementation, and fractures.

In practice, RPM begins with well log and core data, which provide the necessary calibration for elastic property prediction. Once calibrated, RPM allows the simulation of fluid substitution scenarios, for example, predicting how the seismic response changes when brine is replaced by oil or gas. It also enables the forward modeling of synthetic seismic and amplitude variation with offset (AVO) responses, which can then be compared with field seismic gathers. A key application of RPM is the construction of Rock Physics Templates (RPTs), which are crossplots of elastic properties under varying lithology and fluid conditions, and which serve as diagnostic tools for lithology and fluid discrimination.

Ultimately, RPM provides the essential framework for linking petrophysical properties to seismic observations, thereby enabling robust seismic inversion, AVO analysis, and quantitative reservoir characterization. Without this physics-based connection, seismic amplitudes could not be reliably interpreted in terms of subsurface geology or fluid content.

Rock Physics Modeling (RPM) is the science and practice of linking geological and petrophysical properties of rocks (porosity, mineral composition, fluid content, pressure, temperature) to their elastic properties (P-wave velocity, S-wave velocity, and density), which in turn control the seismic response.

In other words:     👉 RPM is the bridge between rocks in the subsurface and seismic data at the surface.

•  Synthetic AVO curves for different lithology-fluid combinations.
• Crossplots (like AVO class crossplots, RPTs).
• Feasibility analysis: Can seismic actually detect fluid/lithology change?
• AVO Class Prediction: Gas sands → often Class III, tight sands → Class I/II.