Embodiments of the present invention relate to heat transfer fluids (e.g. molten-salt) as a thermal buffer for heating, by thermal energy derived from wind-generated electricity, at least one of (i) a subsurface hydrocarbon-containing formation or (ii) a bed of hydrocarbon-containing rocks. During times when 5 wind is plentiful, wind electricity is used to heat the heat transfer fluid - e.g. by means of an electrically resistive heater immersed in the heat transfer fluid. At any time, thermal energy from the wind electricity may be transferred to the hydrocarbon-containing rocks or subsurface formation by the heat transfer fluid. In some embodiments, the fluid is 10 heated both by wind-generated electricity and by solar radiation. Some embodiments relate to a subsurface molten salt heater (e.g. powered by wind-generated electricity) having a non-thermally insulation portion through which molten salt flows. The heater is configured to maintain a temperature of the circulating molten salt, throughout the substantially non-thermally-insulated portion, at a temperature significantly above a 15 melt temperature of the molten salt.