The commercial discovery of giant crude oil deposits at depths deeper than 10 km in various petroleum basins worldwide casts doubt on the validity of the theoretical calculations that have determined that the main zone of petroleum formation is at depths of 6–8 km (the ‘oil window’). However, the behavior of complex hydrocarbon systems at thermobaric conditions, which correspond to depths below 6–8 km, is poorly known. We experimentally investigated the thermal stability of a complex hydrocarbon system at the pressure-temperature conditions of Earth’s lower crust by means of Raman and Mössbauer spectroscopies. Our results demonstrated the chemical stability of the complex hydrocarbon system at thermobaric conditions corresponding to depths of 50 km, including the redox stability of the hydrocarbon system in a highly oxidative environment. The results of these experiments allowed us to revise the depth range in which petroleum deposits could occur.