Vitrinite Reflectance Geothermometry in the Cerro Prieto Geothermal Field Baja California, Mexico

Abstract

Measurements of vitrinite reflectance in oil (Ro) on phytoclasts extracted from hydrothermally altered mudstones in Well M84 at Cerro Prieto exhibit an increase in Ro from 0.12% (peat) to 0.24 km depth to 4.1% (anthracite) at 1.7 km depth. Temperatures increase from about 60° to 340°C over this interval. Three other wells sampled in the field show similar relationships. The median values of Ro plotted against temperature fall along a smooth exponential curve with a correlation coefficient great that 0.9. The correlation between reflectance profiles and temperature logs, together with consistent temperature estimates from fluid inclusion and oxygen isotope goethermometry, both indicate that changes in Ro are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used to predict the undistributed temperature in a geothermal well during drilling before it regains thermal equilibrium. Although existing theoretical models which relate Ro values to temperature an duration of heating are inadequate, empirical Ro/temperature curves are still useful for geothermometry.