A method of treating a hydrocarbon formation includes providing heat to the hydrocarbon formation from a plurality of heaters. The heat transfers from the heaters so that at least a section the formation reaches a selected temperature. The section includes hydrocarbons having an API gravity below 10°. A solution including water is provided to the section. A temperature of the solution is at least 250° C. A pressure of the formation is maintained such that the water remains a liquid at 250° C. At least a some of the hydrocarbons in the section having an API gravity below 10° are contacted to produce hydrocarbon fluids. The hydrocarbon fluids in the section are mobilized. The hydrocarbon fluids include hydrocarbons having an API gravity of at least 10°.