A process for upgrading heavy oil in an environment free of an externally supplied catalyst or externally supplied hydrogen source, the process comprising the steps of: combining a heated heavy oil stream with a heated water feed in a mixing zone with mixing to form a heavy oil/water mixture and allowing the heavy oil/water mixture to become well mixed, wherein: the heated heavy oil stream has an oil temperature in the range of 10°C to 250°C at a pressure at or exceeding the critical pressure of water; the heated water feed has a water temperature in the range of 300°C to 550°C at a pressure at or exceeding the critical pressure of water; the volumetric flow rate of the heated heavy oil stream to the heated water feed is 1 to 5, the heavy oil/water mixture is at a temperature and pressure that exceeds the critical temperature and pressure of water; and the heavy oil/water mixture has a residence time within the mixing zone in the range of 10 to 120 minutes.; adding a heated oxidant stream to the heavy oil/water mixture to form a reaction mixture, wherein the heated oxidant stream is at a temperature and pressure that exceeds the critical temperature and pressure of water, wherein the heated oxidant stream comprises an oxygen-containing species and water, wherein the oxygen-containing species is selected from the group consisting of oxygen gas, air, hydrogen peroxide, organic peroxide, inorganic peroxide, inorganic superoxide, sulfuric acid, nitric acid, and combinations thereof, wherein the heated oxidant stream has an oxygen-containing species concentration of 0.1 weight percent to 75 weight percent.; introducing the reaction mixture into a reaction zone, wherein the reaction mixture has a residence time within the reaction zone in the range of 1 minute to 60 minutes, wherein the reaction mixture is subjected to operating conditions that are at or exceed the supercritical conditions of water, such that at least a portion of hydrocarbons in the reaction mixture undergo cracking to form an upgraded mixture, the reaction zone being essentially free of an externally-provided catalyst; removing the upgraded mixture from the reaction zone and cooling and depressurizing the upgraded mixture to form a cooled upgraded-mixture; separating the cooled upgraded-mixture into a gas stream and a liquid stream; and separating the liquid stream into upgraded oil and recovered water, wherein the upgraded oil has reduced amounts of asphaltene, sulfur, nitrogen or metal containing substances and an increased API gravity as compared to the heated heavy oil stream. the heated heavy oil stream has an oil temperature in the range of 10°C to 250°C at a pressure at or exceeding the critical pressure of water; the heated water feed has a water temperature in the range of 300°C to 550°C at a pressure at or exceeding the critical pressure of water; the volumetric flow rate of the heated heavy oil stream to the heated water feed is 1 to 5, the heavy oil/water mixture is at a temperature and pressure that exceeds the critical temperature and pressure of water; and the heavy oil/water mixture has a residence time within the mixing zone in the range of 10 to 120 minutes.; adding a heated oxidant stream to the heavy oil/water mixture to form a reaction mixture, wherein the heated oxidant stream is at a temperature and pressure that exceeds the critical temperature and pressure of water, wherein the heated oxidant stream comprises an oxygen-containing species and water, wherein the oxygen-containing species is selected from the group consisting of oxygen gas, air, hydrogen peroxide, organic peroxide, inorganic peroxide, inorganic superoxide, sulfuric acid, nitric acid, and combinations thereof, wherein the heated oxidant stream has an oxygen-containing species concentration of 0.1 weight percent to 75 weight percent.; introducing the reaction mixture into a reaction zone, wherein the reaction mixture has a residence time within the reaction zone in the range of 1 minute to 60 minutes, wherein the reaction mixture is subjected to operating conditions that are at or exceed the supercritical conditions of water, such that at least a portion of hydrocarbons in the reaction mixture undergo cracking to form an upgraded mixture, the reaction zone being essentially free of an externally-provided catalyst; removing the upgraded mixture from the reaction zone and cooling and depressurizing the upgraded mixture to form a cooled upgraded-mixture; separating the cooled upgraded-mixture into a gas stream and a liquid stream; and separating the liquid stream into upgraded oil and recovered water, wherein the upgraded oil has reduced amounts of asphaltene, sulfur, nitrogen or metal containing substances and an increased API gravity as compared to the heated heavy oil stream.