Gassmann equations (Gassmann, 1951) are used to calculate seismic velocity changes that result from variations in reservoir fluid saturation. These equations became predominant in the analysis of a direct hydrocarbon indication from seismic data through their use in analyzing the compressional to shear velocity ratio, Vp/Vs. This Vp/Vs ratio is used in many industry analyses, such as the amplitude variation with offset (AVO) analysis developed by Castagna et al. (1993). Multiple authors have since published a variety of Vp/Vs seismic interpretation techniques that use empirical relationships with Vp, Vs, and porosity terms. Unfortunately, however, there is a gap in the use of Vp/Vs relationships in petrophysical interpretation.
The Vp/Vs ratio analysis was expanded in 1995 when Brie et al. proposed the application of a Vp/Vs vs. Vp crossplot for gas trend indication and included a correction for shale effect. The crossplot of Vp/Vs vs. Vp was published in 2015 by Quirein et al. and was applied to organic shale reservoirs for kerogen volume and anisotropy trend indications.
This paper explores the use of a crossplot of Vp/Vs vs. Vs for quantitative petrophysical interpretation. A relationship developed in the paper is used to describe water-wet and gas-saturated sandstone trends, and to independently calculate water saturation from a proposed crossplot in low and medium porosity isotropic sandstones. These proposed Vp/Vs vs. Vs crossplot water saturation results are compared to traditional resistivity-based results. This proposed simplified method provides a suitable approach for determining gas saturation when resistivity logs yield inadequate results in, for example, medium porosity or low-resistivity pay formations.