It has been shown that accurate predictions of compressional and shear-wave velocities can be derived from measurements of clay content, porosity, and bulk density. In this paper, we test the efficacy of using a sequence of established petrophysical formulations to go a step further and predict porosity and bulk density as well as velocity from clay content alone. Other variables, like grain and pore-fluid properties, are assumed to be constant, while empirical relationships are used to model such behaviors as grain packing and compaction.
The sequence of formulations, referred to as the “model,” is evaluated using well-log and core data from the Amazon Fan collected during Ocean Drilling Program Leg 155. Clay contents are estimated from the borehole gamma-ray logs. These are input to the model, which in turn predicts observed porosities to a rms error of < 20%, and bulk densities and compressional velocities to rms errors of ≤ 10%. The results suggest that with better measurements of clay content and improvements to the model, even more accurate predictions of velocity might be achieved for unconsolidated sediments.