Abstract: A majority amount of a heavy hydrocarbon crude is reacted with a minor amount of brine, at supercritical temperature and pressure for the brine, for a predetermined period of time in order to upgrade and convert the heavy hydrocarbon crude into a lighter hydrocarbon crude of higher API gravity. The upgrading and conversion of a viscous heavy hydrocarbonaceous crude oil into lighter hydrocarbons is accomplished in a continuous reactor system and may be accomplished in a subterranean petroleum reservoir at supercritical temperature and pressure. The overall heat of reaction is neutral, i.e., neither exothermic nor endothermic. In order to provide the necessary temperature, heat is added to the system prior to the reaction. For an in situ application, a combustion operation may be utilized to provide the necessary temperature, and is initiated using an oxidizing gas injected through an injection well.