Seismic Reservoir Characterisation Using Vp-Vs Ratio in Tight Gas Sandstones

Behavior of Vp/Vs depends on a variety of reservoir conditions such as lithology, pressure or fluids. However, common practice in Vp/Vs interpretation focuses on the effect of only one parameter at the time. Although it is common, for instance, to associate presence of sands to lower values of Vp/Vs, other parameters such as pressure or fluids may produce changes in Vp/Vs that diminish the lithology effect we are looking for. <br><br>In this paper we quantify the effect of different reservoir parameters on Vp/Vs to help the interpretation of Vp/Vs maps derived from seismic data. We find that in tight gas sandstones reservoirs, low Vp/Vs anomalies can be useful in prospect identification since they are related to good quality rocks (sandstones with low clay content), presence of gas, and overpressure conditions. These three conditions act in the same directions and all produce a decrease in Vp/Vs. In tight gas sandstones, gas saturation lowers Vp/Vs and the effect is larger if the reservoir is overpressured. Water saturation, however, increases Vp/Vs and this effect is enhanced with decreasing differential pressure. Tight gas sandstones typically have a Vp/Vs lower than that in shales.<br>