Diluted wet combustion forms a hot process fluid or VASTgas comprising carbon dioxide (CO.sub.2) and fluid water which is delivered geologic formations and/or from surface mined materials to reduce the viscosity and/or increase hydrocarbon extraction. This may improve thermal efficiency and/or increases heat delivery for a given combustor or per capital investment. High water and/or CO.sub.2 content is achieved by reducing non-aqueous diluent and/or adding or recycling CO.sub.2. Power recovered from expanding the VASTgas may be pressurize the VASTgas for delivery by partial expansion through a Direct VAST cycle, and/or by diverting compressed oxidant through a parallel thermogenerator in a Diverted VAST cycle. Pressurized VASTgas may be injected into well within the hydrocarbon formation or with mined material into a heavy hydrocarbon separator vessel to heat, mobilize, solubilize and/or extract heavy hydrocarbons. Light hydrocarbons may be mixed in with the hot process fluid to enhance hydrocarbon mobilization and recovery. Microwaves may further heat the VASTgas and/or hydrocarbon. Sulfur oxidation, calcining limestone and/or recycling may increase CO.sub.2. Oxygen enrichment may increase the specific power. VASTgas may be delivered through and back injection wells and/or production wells, and/or between sequential injection wells in alternating and/or paired zigzag formations with multiple wells per VAST combined heat and power recovery system.