Data of chemical and isotopic composition of fluids from Los Azufres wells, collected over a two year period, provide evidence of a process of upward flow and partial condensation of steam in the reservoir, which explains part of the previously reported heterogeneity in isotopic composition of the liquid phase (Nieva et a l, 1983). For the southern part of the field, a direct correlation is found between chloride and oxygen- 18 concentrations, and an inverse correlat i o n between these parameters and deuterium and carbon dioxide concentrations in the l i q - uid phase of the reservoir. Chloride ion conc e n t ration s increase gradually from = 600 ppm in the production zones of shallow wells to E 1900 ppm in the case of deep wells. This observations agree with predictions based on the well-known model for vapor dominated systems of White et a1 (1971). For the northern part of the field the same correlations are observed, except for the deuterium concentration which in this case correl at e s directly with oxygen-18. It is concluded that the same process of steam upflow occurs in the northern section but that some other process - perhaps a mixture with other hydrothermal fluid - is also occuring.

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