Effects of porosity and clay content on P- and S-wave velocities in Cooper Basin Sandstones

Abstract

Laboratory velocity measurements on cores along with petrographic and core porosity data have been used to investigate the effect of porosity and clay content on acoustic wave velocities in 22 core samples from the Cooper Basin, South Australia. Empirical equations are determined for Vp and Vs as a function of porosity and clay content for air- and water-saturated samples under elevated effective stress. Results are compared with other studies. At elevated effective stress, porosity is the rock property best correlated with velocity. Statistical analysis of experimental data shows that considering a clay content term improve the velocity-porosity transform for the studied samples. This observation is consistent with previous works. However the influence of clay content on velocities in Cooper Basin samples appears to be weaker than has been reported in the literature for other sandstones. The lack of correlation between clay content and velocity in those Cooper Basin sandstones studied is initially attributed to the textural characteristics and the type of clay distribution within the studied samples. Considering the effect of clay and fines on velocities and velocity-porosity transforms in shaly sandstones is necessary, but this effect may not adequately be explained if only the volume fraction of clay is taken into account. Other factors including pore geometry, textural pattern and microstructure of the rock such as grain or matrix supported, type and the position of clay particles within the rock skeleton should also be considered. The clay models of Minear (1982) and the velocity-porosity model of Vernik (1994) for "wackes" provide an analog for the observed velocity-porosity-clay content relationship in the Cooper Basin samples. The existence of non-potassic clay minerals such as kaolinite complicate the interpretation of shaly sandstone formations and the application of 'standard' velocity-porosity-clay content equations to well logs.