A method and an assembly for heating and evaluating a formation of the Earth while drilling a wellbore filled with drilling mud are described. A first drilling mud temperature at a depth in the wellbore is received from a first sensor by a controller. The formation proximal to the depth is heated by a heat source mounted to the assembly to a temperature greater than a formation temperature as the drilling assembly drills the wellbore. A second drilling mud temperature is received from a second sensor by the controller. The heat source is positioned in between the first sensor and the second sensor. A difference between the first drilling mud temperature and the second drilling mud temperature is compared to a threshold drilling mud temperature difference value by the controller. Based on a result of the comparison, the drilling assembly is controlled and directed in the formation.