A gas/air compressor system used for compressing a gas/air stream of air, oxygen, oxygen-enriched air and other combustible gases and delivering a moderate-to-high temperature, moderate-to-high pressure fluid to a pipeline connected to one or more injection wells. Fuel with cooling water is injected downhole for combustion in the compressed gas/air stream to be used for in-situ-retorting of oil shale, volatile coal beds, tar sands, heavy oil and other hydrocarbon or carbonaceous deposits. A preferred embodiment of the compressor system includes a compressor housing with a male screw rotor having helical lobes received in helical grooves in a female screw rotor. The housing includes a gas/air inlet for receiving the gas/air stream therein and a series of water/mineral injectors for injecting a water/mineral slurry into the rotating screw rotors. The water/mineral slurry acts as a sealant, a lubricant and also a coolant for male and female screw rotors during the compression cycle. The slurry is non-combustible and non-reactive to combustion-supporting gases. Also, the housing includes a gas/air outlet for discharging the compressed gas/air stream into the pipeline connected to the injection wells used in the in-situ-retorting process.