DEEP TEMPERATURE EXTRAPOLATION IN THE SOULTZ-SOUS-FORÊTS GEOTHERMAL AREA USING MAGNETOTELLURIC DATA

An indirect electromagnetic geothermometer is used for the deep temperature estimations in the Soultzsous-Forêts (France) geothermal area using magnetotelluric (MT) sounding data collected along West-East profile in 2007-2008. Validation of the temperature assessment using MT data is fulfilled by retro-modeling, i.e. by comparison of the forecasted temperature profile with real temperature logs from GPK2 geothermal borehole for two depth ranges 2000-3878m and 3878-5046m as if it was done before appropriate drilling. The relative forecast error for the extrapolation from the upper 2km to the depth range up to 3878m was 1.8 %, while in the latter case it was equal to only 0.4%. Deep temperature forecast is provided for the well GPK2 up to the depth 8175m. The analysis of the profile beneath 5000m indicates that its behavior continues to be of the conductive type (as in the depth range 3500-5000m) up to the depth 6000m, while manifesting mixed conductive/convective type in the depth range 6000-8000m, which, in turn, may indicate the localization of the fluid circulation paths at these depths. The temperature forecast up to the depth 2500m is carried out for defining the best location for the new geothermal well planned to be drilled in the Rittershoffen area. It is shown that application of the indirect electromagnetic geothermometer constraints the decision on the location of the new borehole. In particular, it indicates that deepening of the available borehole RT3 or drilling in the SE direction from RT3 could be the best alternative from the temperature point of view. Finally, deep vertical temperature cross-section is constructed up to the depth 5000km for the profile starting from the borehole 4616 in the NW and ending by the boreholes RT1 and RT3 in the SE. It manifests two local temperature maxima at the depths larger than 5km to the NW from the GPK2 and to the SE from RT3 boreholes the latter being bigger.