Quantitative Interpretation of Sonic Compressional and Shear Logs for Gas Saturation in Medium Porosity Sandstone

Kozlowski, Maciej (Halliburton) | Quirein, John (Halliburton) | Engelman, Bob (Halliburton) | Wolanski, Krzysztof (PGNiG SA) | Ochalik, Slawomir (PGNiG SA)

OnePetro 

ABSTRACT

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.