ABSTRACT

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.

INTRODUCTION

Italy is one of the richest countries in terms of hydrocarbon reservoirs in southern Europe. Gas and oil accumulations in the Italian territory and in the off-shore area are mainly located along the Apennine chain, within the associated foredeep basins and along the Adriatic foreland. The gas fields are mainly concentrated in the Po Valley and within the northern and central Adriatic basin, while the oil fields are located in the Western Po Valley, Southern Apennines and Sicily [1], [2]. The current geological structure of the Italian territory is the result of a complex tectonic and sedimentary evolution that began at the end of the Palaeozoic period and lasted until the Pleistocene. The Tertiary, the Alpine and Apennine orogenic events produced a chain-foreland-foredeep system whose evolution created the necessary conditions for the formation of hydrocarbon bearing deposits both on-shore and off-shore. The analysis of data from more than 250 hydrocarbon fields shows large amounts of hydrocarbon were produced in the Italian peninsula as well as extensive drilling activity for exploration purposes in a broader area [3]. Over the last decades, more than 7000 wells were drilled in the entire Italian territory [4] reaching, on average, a depth of 3-4 km inside the earth's crust. In this work we focus on the Po plain and the northern part of the Adriatic Sea where more than 3500 wells provide a large amount of data suitable for the study of the geological characteristics of the sedimentary deposits present in the area.

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