Tight gas sands: rock physics and seismic applications

Today, tight gas sandstone reservoirs are a vast resource. Due to their complexity and our poor understanding of these unconventional petroleum systems, new technologies are necessary to successfully exploit them. Some of the geological challenges present in tight gas sandstone reservoirs are: high reservoir heterogeneity, very low porosity and permeability, possible presence of natural fractures, uncertainty in gas/water contact and high possibility of overpressures. In this work, we have collected ultrasonic data on tight gas sandstone cores. We also analyzed cross-dipole sonic log data to understand the relations between elastic properties (e.g., Vp/Vs, P- and S-impedance) and petrophysical properties (e.g., porosity, lithology). We show the effects of pressure, lithology, and pore fluids on Vp/Vs. Finally, we quantify Vp/Vs variations due to changes in reservoir properties of tight gas sandstones with the potential to apply this information to interpret Vp/Vs extracted from AVO analysis or multicomponent reflection data. The results show that low Vp/Vs anomalies in tight gas sandstones can assist in prospect identification, because they are related to good quality rocks (sandstones with low clay content), presence of gas, and overpressure conditions.