Carbonate rocks cover over 20% of the earth crust; they contain over 20% of the world's endowment of viscous oil estimated at over 2 trillion barrels mainly reported in Canada, the United States and the Middle East. To date, only steam injection processes look promising in accessing this immense resource commercially. High T and p production processes have a profound impact on geomechanical behavior of viscous oil carbonate reservoirs. Massive geomechanical effects occur in carbonates when subjected to thermal stimulation; dilating natural fractures can show changes in flow capacity of several orders of magnitude from wedging or shear dilation around the thermally stimulated zone. In this article, the importance of thermal geomechanics effects during viscous oil production from carbonate rocks is emphasized. Approaches to calculating thermally-induced stresses are described. Thermal, physical and geomechanical behavior of carbonate rocks under elevated temperature and pressure is also investigated based on the available experimental data and field evidences. Finally, a practical example of thermal geomechanics effects during thermal oil production operations into viscous oil naturally fracture reservoir is demonstrated. Thermal viscous oil production changes reservoir behavior, generally leading to production enhancement although thermal stimulation can generate operational issues such as thermally induced casing shear and seal breaching.