A catalytic combustor for use in heavy or viscous oil production. Specific heavy oil products under development include 8 and 50 MM BTU/hr devices. Downhole steam generator offers substantial energy savings compared to current surface steam generators, delivering the entire BTU content of the fuel (including the higher heating value, and avoiding boiler losses as well as surface distribution and wellbore losses). In addition, the combustion products (CO2 and nitrogen, in addition to water) are injected into the reservoir rather than being released to the atmosphere, and additional CO2 could be added for sequestration purposes. Surface facilities deliver air, fuel and water to the downhole steam generator.