Estimation of Shear Wave Velocity, Ghadames Basin, Libya

Estimation of shear wave velocity (Vs) plays an crucial role in amplitude variation with offset (AVO) analysis studies and rock physics. This paper introduces a method for superior estimation of shear wave velocity (Vs) from compressional wave velocity (Vp), bulk density (ρb) and gamma ray (GR/γ) logs, using multi-linear regression technique. Previously, Vp-Vs and Vp- ρb relationships are commonly used in order to extract AVO attributes which are influenced by lithology and fluid saturations. Due to the shortage and the availability of the log data, empirical equations used to calculate approximately shear wave velocity and bulk density logs. Empirical equations, (such as Castagna’s relationship, 1985, which known as the Mudrock line equation), are derived from the linear relation between compressional wave velocity and shear wave velocity for worldwide rocks. The Ghadames basin is considered to be a Paleozoic basin, the main reservoirs' age are Paleozoic, thus applying aforementioned equations in order to estimate shear wave velocities provide relatively underestimated values at the reservoir zones while they predict overestimated values at the cap rock shales. An advantage of the new equations is that a new set of relationships to optimize the predicted shear wave velocity.