Resumen
The Upper Rhine Graben (URG) hosts thermal anomalies that account for the development of oil fields. Recently, a geothermal power plant has been installed in this area. Data obtained in this framework provide an insight into the temperature distribution in the URG. The present thermal gradient at Soultz-sous-Forêts is not linear: nearly 90 °C/km down to 1400 m depth, then about 12 °C/km from that depth down to 5000 m. The combination of temperature conditions and natural fluid circulation in fracture networks has led to the hydrothermal alteration of the granite into mineral assemblages such as those including illite, quartz and calcite. Illite is locally impregnated with organic matter of two kinds: a mature type derived from oil source rocks and a less mature type derived from surficial sedimentary layers indicating the km-scale of transfer. Newly formed crystals of quartz and calcite from around 2000 m depth record a fluid temperature range of 130 to 170 °C, consistent with modelling and the temperatures measured at present in the drill-holes at this depth. In such hydrothermally altered zones, local variations of temperature are encountered indicating current fluid flows that are being sought for geothermal purposes.