Publisher Summary The temperature field of reservoirs, the heat exchange among boreholes, and surrounding formations are very much dependent on the thermal properties of formations. Three macroscopic quantities—pressure (p), density (or specific volume, v) and temperature (T) define the state of a system in thermal and mechanical equilibrium. The relation f(p, v, T)= 0 is called the equation of state. A number of thermodynamic properties of a material can be determined from the equation of state. The temperature regime of sedimentary formations is influenced by many topographic and geological factors (constant terrain, sedimentation, erosion, lateral conductivity contrasts, and underground water movement), past climate, and by the heat flow from the Earth's interior–terrestrial heat flow. The time dependence of the temperature is mainly caused by changes in the Earth's surface temperature conditions (past climate) or by the changes in the value of the terrestrial heat flow. The major sources of thermal energy in the Earth's crust are radioactive isotopes of uranium, thorium, and potassium. Thus the thickness of the crust and the distribution of radioactive isotopes with depth to a large degree effect the heat flow from Earth's interior.