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. After a predetermined amount of time, injection of the oxidizing gas is terminated and the injection well is shut-in for a predetermined period of time to permit the petroleum reservoir to undergo a soak period in order to increase the temperature and decrease the viscosity of the viscous heavy hydrocarbonaceous crude oil. As the viscosity of the heavy hydrocarbonaceous crude oil decreases, the oil flows downwardly into the combustion zone steam and/or brine is injected which is at supercritical conditions in order to upgrade the heavy hydrocarbonaceous crude oil into lighter fractions. The reaction products from a reactor or an in situ operation are also lower in obnoxious constituents such as sulfur, nitrogen and heavy metals.