In-situ oil recovery by thermal methods entails introducing heat into the reservoir to enhance productivity and ultimate recovery. Clearly, proper design and management of thermal floods require adequate insights into the temperature distribution in the reservoir. This paper interrogates the robustness of the traditional approach of describing steam-flooded reservoirs with two isothermal zones. Employing a rigorous one-dimensional reservoir-heating model, the spatial distribution of the heat-transfer mechanisms during each of conduction, convection (non-condensing) and multiple-contact condensation processes has been investigated. After a sufficient period of heating, the prevailing temperature profiles are analysed and the reservoir delineated into zones according to the temperature responses. From the simulation results, the thermal front is mapped, and the dominant heat-transport mechanisms at its leading and trailing edges are analysed, both in space and time. It is found that a maximum of four zones vis-à-vis condensation, convection, conduction and quiescent, can be identified at any instant of a thermal flood. However, the presence and relative thickness of the zones are controlled by the maturity of the thermal flood and the dominant process (conduction or convection) at the time of interest. In practice, these findings should find applications in the design and management of thermal floods, on both laboratory and field scales.