Vitrinite reflectance measured in immersion oil (R_o) on kerogen extracted from hydrothermally altered mudstones in borehole M-84 at the Cerro Prieto geothermal field exhibit an increase in mean reflectance (R_o) from 0.12 per cent at 0.24 km depth to 4.1 per cent at 1.7 km depth. Downhole temperatures measured over this interval increase from about 60° to 340°C. These R_o data plotted against temperature fall along an exponential curve with a coefficient of determination of about 0.8. Other boreholes sampled in the field show similar relationships. A regression curve calculated for temperature and R_o in borehole M-105 correctly predicts temperatures in other boreholes within the central portion of the geothermal system. The correlation between the reflectance values and logged temperature, together with consistent temperature estimates from fluid inclusion and oxygen isotope geothermometry, indicates that changes in R_o are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used in this geothermal system to predict the undisturbed temperature in a geothermal borehole during drilling before it regains thermal equilibrium. Although existing theoretical functions which relate R_o to temperature and duration of heating are inaccurate, empirical temperature-R_o curves are still useful for geothermometry.

A comparison of temperature-R_o regression curves derived from nine boreholes within the Cerro Prieto system suggests that heating across the central portion of the field occurred penecontemporaneously, but varies near margins. Boreholes M-93 and M-94 appear to have cooled from their maximum temperatures, whereas M-3 and Prian-1 have only recently been heated.

Comparison of the temperature-R_o data from the Salton Sea, California, geothermal system indicates that the duration of heating has been longer there than at the Cerro Prieto field.