Abstract: Pressure energy in a petroleum reservoir is not wasted, but used for cooling the gas/liquid mixture through expansion in turbo-expanders (5, 6). The power generated thereby is used for recompression of the gas or for other power needs. This saves Freon cooling apparatus, compressors for recompressing the cooling fluid, drive motors and power for driving them (e.g. fuel gas at an off-shore site), space and weight. In an alternative embodiment the process uses stripping-reboiling whereby again the amount of compression may be reduced as well as a higher yield of selected heavy hydrocarbons is obtained.