Well Log Data to Infer Geophysical Features of Shallow Crust: Some Examples in Italy

A detailed crustal velocity model is fundamental for different purposes especially in tectonically complex regions. Sonic log data have been used to better characterize in situ P-wave velocities in the shallow crust and to build a velocity model for the upper 5-6 km. Many deep wells drilled in Italy by oil companies, although not homogeneously distributed, are available and contribute to estimate reliable petrophysical parameters of the shallow crust in different geodynamic contexts. We have selected in Sicily about 30 deep wells, mainly located along the Gela foredeep - Hyblean foreland system and surroundings, and analysed sonic log data in comparison to the stratigraphic profiles. Indeed, from wave velocities, retrieved from sonic "slowness", we have inferred rock density values for homogeneous intervals, following an empirical relationship between sonic velocity and density in sedimentary rocks. These direct data can contribute to build a robust 3D model of the area, together with other geological and geophysical information inferred independently. In Sicily the present-day stress field orientations (IPSI 1.2 database, Mariucci and Montone 2018) delineate a complex tectonic picture evidencing adjacent areas characterized by distinct stress regimes: in the northern offshore of Sicily and in the Hyblean plateau the alignment of horizontal stress is consistent with the crustal motions, whereas different directions are observed along the belt and foredeep. In particular along the northern offshore of Sicily, several earthquakes, with prevalent reverse focal mechanisms, describe a clear NNW-SSE current compression. In southeastern Sicily, breakout data along the Hyblean foreland are all consistent with ENE-oriented minimum horizontal stress. Moving to the Gela foredeep, the minimum horizontal stress rotates spatially from ENE-WSW to NW-SE. Along the northern Sicilian belt, a diffuse area with N-S-extension has been recently well depicted; nearby the Tindari-Letojanni fault, the direction abruptly changes into WNW-ESE extension. In the offshore of eastern Sicily up to eastern Calabria, a strike-slip regime with NE-SW-oriented minimum horizontal stress is well defined.