Monitoring changes with time of the isotopes of water (I8O and D) in wellhead fluids is an effective way of indicating reservoir changes and processes. Because 180 concentrations in water are altered by high-temperature exchange with rock oxygen and because both 180 and D are fractionated in vapor-liquid separation processes at the surface (separators and cooling towers), these isotopes are excellent indicators of inflow and distribution of fluids from outside the reservoir, either natural or injected. Studies of the isotopic compositions of fluids from the Cerro Prieto field in Baja California, Mexico show that pressure drawdown in the major p (beta) reservoir has caused intense boiling followed by inflow of water from outside the reservoir. A method of field exploitation based on this behavior is discussed.

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