A method for upgrading of a petroleum feedstock, comprising the steps of: providing a pressurized and heated petroleum feedstock, wherein said petroleum feedstock is maintained at a temperature of between 10°C and 250°C and a pressure of at least 22.06 MPa; providing a pressurized and heated water feed, wherein said water feed is maintained at a temperature of between 250°C and 650°C and a pressure of at least 22.06 MPa; combining said pressurized and heated petroleum feedstock and said pressurized and heated water feed to form a combined petroleum and water feed stream; supplying the combined petroleum and water feed stream to a hydrothermal reactor to produce a first product stream, wherein said reactor is maintained at a temperature of between 380°C and 550°C and at a temperature and pressure such that the water is in a supercritical state, the combined petroleum and water feed stream being maintained within the hydrothermal reactor for a residence time of between 1 second and 120 minutes to crack hydrocarbons present in the combined petroleum and water feed stream, wherein the first product stream includes lighter hydrocarbons than the hydrocarbons present in the petroleum feedstock, as well as water; transferring the first product stream to a catalytic post-treatment process to produce a second product stream, wherein said post-treatment process is maintained at a temperature of between 50°C and 350°C and at a temperature and pressure such that water is in a sub-critical state; collecting the second product stream from the post treatment process, the second product stream comprising hydrocarbon product and water, wherein the hydrocarbon product has a reduced sulfur content relative to the petroleum feedstock; wherein the term "petroleum feedstock" includes any hydrocarbon crude that includes impurities, such as elemental sulfur, compounds containing sulfur, nitrogen and metals, and combinations thereof, and/or hydrocarbons having a boiling point of greater than 360°C, including aromatic hydrocarbons, alkanes and alkenes. providing a pressurized and heated petroleum feedstock, wherein said petroleum feedstock is maintained at a temperature of between 10°C and 250°C and a pressure of at least 22.06 MPa; providing a pressurized and heated water feed, wherein said water feed is maintained at a temperature of between 250°C and 650°C and a pressure of at least 22.06 MPa; combining said pressurized and heated petroleum feedstock and said pressurized and heated water feed to form a combined petroleum and water feed stream; supplying the combined petroleum and water feed stream to a hydrothermal reactor to produce a first product stream, wherein said reactor is maintained at a temperature of between 380°C and 550°C and at a temperature and pressure such that the water is in a supercritical state, the combined petroleum and water feed stream being maintained within the hydrothermal reactor for a residence time of between 1 second and 120 minutes to crack hydrocarbons present in the combined petroleum and water feed stream, wherein the first product stream includes lighter hydrocarbons than the hydrocarbons present in the petroleum feedstock, as well as water; transferring the first product stream to a catalytic post-treatment process to produce a second product stream, wherein said post-treatment process is maintained at a temperature of between 50°C and 350°C and at a temperature and pressure such that water is in a sub-critical state; collecting the second product stream from the post treatment process, the second product stream comprising hydrocarbon product and water, wherein the hydrocarbon product has a reduced sulfur content relative to the petroleum feedstock;