Vp-Vs Ratio as a Lithological Indicator for Shallow Reservoir

The way how the compressional and shear wave sense the characteristics of geological material, makes compressional (Vp) and shear wave velocity (Vs) ratio is an important tool to predict lithology and pore fluid using seismic and/or sonic log data with varying degree of uncertainty. In this work, a set of sand-clay and their variable mixtures tested under uniaxial setup used to investigate Vp-Vs as lithological indicator under stress. The plot of Vp-Vs ratio with clay content under stress is found to be more obscure to predict lithology. However, in Vp-Vs plot different lithologies can be categorized. The changes of Vp-Vs ratio with stress are different for different lithologies. To predict shear wave velocity from compressional wave velocity for sand and clay saturated with 3.5wt% brine two models are suggested which can be useful for shallow subsurface reservoir.