Calculation of lithology-specific p-wave velocity relations from sonic well logs for the po-plain area and the northern adriatic sea

One of the most useful petrophysical parameters in hydrocarbon reservoir studies is the velocity of the seismic waves propagating in the Earth’s subsurface. Seismic velocities have multiple applications in geophysical exploration, well log interpretation and petrophysical and geomechanical characterization. In this study we used publicly available well data (VIDEPI database) covering the Po Plain and the northern Adriatic areas to calculate the P-wave sonic velocity from the analysis of well profiles (1:1000 scale). Data were collected from 134 wells located inside the region of interest that included sonic log registrations. From each of the wells the cuttings interpretation log, the available spontaneous potential or gamma ray logs and the sonic log were digitized from existing profiles whereas the hydrocarbon-bearing-marker (resistivity log readings) and the geological formation log were constructed. The lithological and the geological formation logs were used to analyse the regional stratigraphy while the resistivity log was used to identify and exclude the hydrocarbon bearing intervals affecting the sonic log readings. The various lithologies reported on the well profiles were combined to characterize 9 main lithological groups (6 clastic, 1 marly, 2 carbonatic). For each group a linear regression was applied to extract the relation of velocity versus depth. The results show a gradual velocity increase with depth for most of the lithologies, while limestones and dolomites present constant velocities independently of the depth. Furthermore, at approximately 3.5-4 km the velocities of all lithologies tend to stabilise at a value that remains relatively constant even for larger depths. The results of this study can prove helpful for the construction and calibration of velocity models and for the calculation of dynamic geomechanical parameters (e.g. Young’s modulus), which are crucial for the mechanical characterization of the rock during geomechanical studies.