Thanks to thermal conductivity maps, obtained from Optical Scanning method, and porosity maps, inferred from thermal conductivity maps, we have studied petrophysical heterogeneities commonly present in a granitic and sandstone geothermal reservoir (fault zone and permeable layers, respectively). The maps allowed determination of thermal conductivity and porosity variation to millimeter resolution , at a core scale. They permitted precise quantification and determination of the size of petrophysical heterogeneities (thermal conductivity and porosity) induced by rock variability.