Do anomalously high Vp/Vs exist in porous reservoir rocks?

Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur, interpreted as highly pressurized heavily fractured zones. Assuming the rocks isotropic, Vp/Vs can be directly linked to rocks Poisson’s ratio in the elastic regimes relevant to both the field and laboratory measurements. From dedicated measurements across the frequency range it was shown that such insights hold, in agreement with a micromechanical model for isotropic micro-cracked rocks: Anomalously high Vp/Vs exist for any low porosity isotropic rocks of any mineral content, if heavily micro-fractured and at near-lithostatic fluid pressures, i.e. at very low Terzaghi effective pressure.

Extending that understanding, one could question if such anomalous Vp/Vs could also be observed and similarly explained in isotropic porous reservoir rocks. From the typical micromechanical inclusion models for predictions at the sample’s scale, such is unlikely as Poisson’s ratio should largely decrease with an increasing content of spherical pores. Yet, that is not what is measured in the relevant undrained elastic regime in well-cemented porous sandstones. For these, most rocks Poisson’s ratio remain anomalously high and comparable to that retrieved in low porosity rocks. Moreover, while models would then predict a dependence of Poison’s ratio to the liquid’s bulk modulus that is again not consistent with the measurements.

From comparing literature datasets reporting drained and undrained Poisson’s ratio and bulk modulus for sandstones of varying porosity, the aim of this work is to investigate and discuss (i) how the measured properties compare, (ii) if one property or the two deviate from existing models and why.