Analisi della fratturazione in pozzo all'interno delle recenti perforazioni eseguite al Monte Amiata

Exploratory wells drilled by the Tuscan regional authority in order to gather data for defining a better hydrogeological model of the area provided also new data on the state of fracturing of the volcanic units. In this contribution, we present the study of core samples and in-hole data of 4 wells drilled in Monte Amiata volcano. The Monte Amiata volcanic rocks are characterized by very low matrix permeability; in spite of this they host an important aquifer. We can therefore deduce that within these rocks groundwater flow and storage take place mainly through fractures. As typical in this kind of rocks, generally groundwater flows through single faults and related damage zones. These discontinuities and, in particular, the fracture intersections are thus potential high permeability zones and sites for groundwater circulation and storage. In order to verify the role played by fractures and better characterize the hydrological features of the reservoir, the fractures have been identified and their strike and dip directions measured using oriented log image probe. The (not oriented) core samples have been studied for fracture dip measurements and fracture density evaluation (number of fracture/meter). In the cores, it was possible to observe the morphology of fracture surfaces, the occurrence of secondary minerals, and rare slickenside kinematic indicators. The fracture density observed in core samples and in hole image logs is generally comparable. Usually, core samples show a higher density fracture than in-hole images due to the presence of both natural and mechanically induced fractures. The discrimination between natural and induced fractures was based on characteristic fracture surface morphology, fracture geometry and occurrence of secondary minerals, and by comparing the in-hole image log and the core samples. Coring induced fractures were normally characterized by torsional and core-plug or unloading related fractures. Typically, the natural fractures show secondary minerals coatings on the fracture surface and are characterized by high dip angles (60-80°), whereas the mechanical fractures are not mineralized and characterized by a lower dip angle (20-40°). The measured fracture frequency per meter in the image log does not show a random distribution. The fracture distribution with depth is indeed well clustered, with high fracture densities alternating with less fractured portions. Taking into account both cores and image log data, where available, we observed intense fracturing located at defined depth intervals. The structural data measured by oriented hole image log have been used to get insights into the fracture orientations throughout the wells. The pole fracture distribution shows two main clusters quite orthogonally oriented and representative of two main sets of fractures. Set I consist of ENE-WSW oriented fractures, and Set II, more dispersed, is characterized by fractures oriented NNW-SSE. Moreover, considering the distribution of fracture orientations with depth, we note that ENE-WSW striking fractures become more frequent with increasing depth. Kinematic evidence is very rare and was found in the core samples rarely, indicating a mostly dip-slip movement, probably related to major normal faults. We suggest that the high fracture density zones, locally characterized also by the presence of cavities detected in the image log, could correspond to zones of localized deformation as main faults. Fracture dip and orientations collected within the wells are comparable with those measured at surface in the SW sectors of the Monte Amiata volcanic edifice. This confirms that the data collected at depth in the analyzed wells could be taken as representative of the deformation style characterizing this area. No evidence of a similar fracture pattern occurs in the core interval interesting the flysch substratum, which is instead characterized by a ductile pervasive cleavage and folding typical of Alpine regional deformation as observed in the Argille a Palombini outcrops. The study of the core-samples within the basal volcanic units provided also new insights on the modalities of magma emplacement, by the recognition of peculiar structures supposed to be genetically related to the magma emplacement phases, during the Monte Amiata volcanic edifice construction.