Hydrothermal Mineralogy and Isotopic Geochemistry in the Cerro Prieto Geothermal Field, Mexico, II: Isotopic Geochemistry

Abstract

Stable isotope measurements on borehole samples from Cerro Prieto provide estimates of temperature and water rock ratios (W/R) in the geothermal system. Shales normally have lower W/R in some wells. The best samples for temperature calculations are hydrothermal vein minerals. Our isotopic measurements have aided current exploration at Cerro Prieto. For examples, no temperature log could be run deeper than 1800 meters in exploration well M92, but isotopic temperatures indicate only 200°C at 2500 meters. In addition isotopic temperatures and wide W/R ranges in wells M91 and M92 suggest that they lie at the probable margin of the productive field. In the center of the field light oxygen del values for the produced fluid appear to delineate extensive drawdown. Similar studies could assist geothermal exploration elsewhere , and provide baseline information to monitor the effects of production.