Shear velocity estimation based on rock physics modeling of limestone gas reservoir in the Pannonian basin

In the Pannonian Basin, especially in Croatia, there are a small number of wells with acquired shear velocities. Often, quantitative interpretation must rely only on compressional velocity data, and shear velocity must be modelled. Shear wave velocity estimation in combination with other petrophysical data is essential for detailed reservoir characterization. Compressional and shear wave velocity allow the seismic modelling of different saturation states in a reservoir. This paper demonstrates a workflow for S-wave velocity estimation where shear velocity data is absent in the gas field and neighboring fields with the same lithology, based on the Kuster-Toksöz and Xu-Payne models applied to the Pannonian basin limestone reservoir. The results are calibrated with the P-wave velocity obtained from borehole data and the VSP S-wave interval velocity. Although rock physics models are idealized analogues of real rocks, a very good correlation was obtained between the modelled and measured P-wave velocity, as well as between the modelled S-wave velocity and the VSP interval velocity. The study also illustrates the problem of defining the pore aspect ratio in zones of increasing shale content. Due to the limited research on the limestones of the Pannonian Basin, these results enable a better understanding of the seismic parameters of the Pannonian Basin limestones. The results indicate that the proposed workflow gives an adequate estimation of S-wave velocities.