Thermophysical properties of the Po Basin rocks

We present an analysis on thermal properties, density and porosity of clastic, chemical/biochemical and intrasedimentary volcanic rocks collected from petroleum exploration wells of the Po Basin (Northern Italy). Moreover, we investigate the applicability of the Hashin-Shtrikman's model for a mineral aggregate in combination with the Zimmerman's model that takes pore shape into account, to calculate the bulk thermal conductivity. In caseof macroscopically isotropic rocks, deviations between predicted and measured values range from -2.2 per cent to 6.9 per cent, and significantly decrease if a proper pore aspect ratio is chosen. Regarding the volumetric heat capacity, approximate estimates were obtained by means of a weighted average of the volumetric heat capacity of the mineral grains and that of the pore-filling water. The differences between the computed and measured values range from -6.2 per cent to 4.9 per cent and, on average, the computed volumetric heat capacity is lower by 1.6 per cent. The water loss during compaction and the temperature increase with depth are main factors controlling thermal properties. An anisotropy effect occurs in the case of rocks rich in sheet silicates. Due to rotation of these minerals, the vertical thermal conductivity of sheet silicates decreases exponentially with burial depth from 2.13 W m-1 K-1 at the surface to 0.52 W m-1 K-1 at 4.5 km. The laboratory data allow the formulation of compaction curves for the different sedimentary rock types. Examples of estimations of in situ vertical thermal conductivity and heat flow are finally given for two petroleum wells at which lithostratigraphic information is known in good detail.