GNGTS 2018 - 37° Convegno Nazionale

594 GNGTS 2018 S essione 3.1 applied the methodological approaches proposed by Horner (1951), Zschocke (2005) and Pasquale et al. (2013). In case at least two measurements at the same depth were available, the former method is likely the most appropriate. For the estimate of the geothermal gradient ( K ) and the heat flux ( q ), based on the Fourier law, we considered three reference intervals: S 0 , S 1 and S 3 , respectively corresponding to i) the silicoclastic sedimentary succession down to the base of the marine Quaternary; ii) the stratigraphic units underlying the marine Quaternary and overlying the Scaglia formation; iii) from the top of the Scaglia formation and the bottomof the borehole analysed. For each interval both the geothermal gradient ( K 0 , K 1 and K 3 ; Fig. 2) and the heat flux ( q 0 , q 1 and q 3 ) have been calculated, while for calculating the overall heat flux of each borehole the thermal resistance method proposed by Pasquale et al. (2012) has been applied. The results of all the investigated boreholes have been plotted to generate thematic maps thus emphasizing first order geothermal anomalies. The calculation of the thermal conductivity was initially based on literature values (e.g. Robertson et al., 1988; Viganò et al., 2011) and subsequently corrected based on the porosity and compaction coefficient of each lithotype (Pasquale et al. , 2013). The analysis of the obtained geothermal gradients clearly suggests that the thermal conductivity ( viz. conductive flux) alone could not justify such vertical variations. On the other hand, these variations could be attributed to the occurrence of convective motions affecting the Mesozoic carbonate rocks, therefore generating a strong geothermal gradient decrease within these units and an increase in the overlying deposits. This thermal model could obviously work only in case a heat source is present at depth and a sufficient permeability allows the upwards circulation of the deeper fluids. Accordingly, we focused our attention on the carbonate sedimentary successions representing the geothermal reservoirs. In particular, the heat transmission characterizing these Fig. 2 - Example of geothermal gradients calculated for the three reference intervals. Fig. 3 - Example of thermal gradient distribution showing both vertical and horizontal variations. The seismic profile is from Mistroni (2016).