P- and S-wave velocities and attenuations of Naparima Hill Formation mudstones at elevated pressures

Abstract

Velocity and attenuation (the inverse of the Q factor, Q⁻¹) are fundamental elastic wave attributes used to understand subsurface physical properties and characterize unconventional reservoirs. Mudstone, an abundant sedimentary rock, is commonly regarded as a source rock with great promise for unconventional exploration. Despite this, velocity and attenuation studies are lacking on mudstones and have failed to address the link between these elastic wave attributes and physical rock properties. In this study, we measured the P- and S-wave velocities (Vp and Vs) and Q factors (Qp and Qs) of the Naparima Hill organic-rich mudstone at effective pressures up to 130 MPa. The main aims are to investigate the effects of mineralogy, bulk density, porosity, and saturation on these elastic wave attributes and to evaluate if a relationship exists between velocity and attenuation. Vp and Vs were obtained using the trough transmission method, while the spectral-ratio and rise-time techniques were used to determine Qp and Qs in the frequency range of 0.8 to 1.7 MHz. The results show that the velocities and Q factors rose with increasing carbonate, bulk density, and effective pressure, and decreasing clay, silica, and porosity. The saturated mudstones have lower velocities and Q factors than the dry specimens. Strong, linear relationships (R² > 0.85) were established between Vp and Vs under both dry and saturated conditions. Moderate to strong linear relationships (R² > 0.5) were established between Qs and Qp, Qp and Vp, Qs and Vs, and Qs and Vp under saturated conditions. These relationships create a framework for predicting the elastic wave attributes (Vs, Qp, and Qs), from the readily extractable Vp in mudstones.