Abstract: Exhaust heat from engines used in oil field operations can be utilized to (1) at least partially separate a water-containing wellstream fluid into a progressively enlarged hydrocarbon phase which floats on the remainder of the wellstream fluid and/or on a water phase and (2) vaporize lighter fractions of the hydrocarbon phase for use as fuels for such engines. The level of the wellstream fluid in the separation vessel and the depth of the hydrocarbon phase which forms a top of the remainder of the wellstream fluid and/or water phase are automatically adjusted so that a heater tube which directs exhaust heat through the separation vessel is predominantely in contact with the hydrocarbon phase. The lighter hydrocarbon vapors thus produced are collected and, preferably, superheated prior to being used as fuels.