Rock physics model for interpreting elastic properties of soft sedimentary rocks

Abstract

To study the rock physics model for soft sedimentary rocks widely distributed in Japan, the bimodal sand/shale mixture model, which has been proposed for sedimentary rocks, is applied to the seismic P- and S-wave logging velocity, density and porosity data. This model consists of the mixture of two different-size grains. Macroscopic elastic properties are derived from the Hashin-Strikman lower bounds, Hertz-Mindlin contact model and Gassmann’s equation. The changes of P-wave and shear moduli as functions of porosity, clay contents and confining pressure are calculated by the model and compared with the data obtained in three different soft sedimentary rocks in Japan. The model thus obtained is used for predicting porosity and clay content of the soft sedimentary rock using S-wave velocity and density logs. These applications reveal that the model can reproduce the remarkable features of soft sedimentary rocks, in which their elastic properties strongly depend on the grain size distribution and confining pressure.