The temperature profiles performed in the deep geothermal wells drilled in the Upper Rhine Graben, and in particular in Northern Alsace, show a very linear shape in the sedimentary cover down to the top of the Muschelkalk formations. Below this depth, the gradient decreases sharply, indicating the transition between a conductive gradient in the upper part of the formation, and the convective reservoir below. The top of the Muschelkalk is defined as the Cap-Rock of the convection loop of the geothermal brine which induces an important geothermal gradient up to 10°C/100 m in this region. From this observation, a new exploration method emerged, which consists in drilling shallow wells of 200 m depth in the sedimentary part of the graben to measure an equilibrium temperature profile. From this temperature profile, a gradient is computed, allowing to extrapolate the temperature at the top of the Cap-Rock and to estimate the temperature at the geothermal target, in the convective reservoir. This study describes the results of a campaign of exploration composed of seven so called gradient wells drilled in zones of interest, previously identified by vintage 2D seismic retreatment and interpretation. Analysis of the cuttings of the boreholes, coupled with field observations of the area, allowed to establish a chrono-stratigraphic log containing information on the nature, depth, thickness and age of the geological formations in each zone. The analysis of the temperature and geothermal gradient curves showed a good correlation between the lithology and the local thermal variations. Indeed, a large number of variations observed on the curves can be explained by local facies changes. Similarly, a methodology was established using the thermal logs of the deep wells of Rittershoffen and Soultz-sous-Foręts (Alsace, France) and applied to the temperature profiles measured in the gradient wells in order to estimate the temperature at the top of the Cap-Rock and at the geothermal target. This method of exploration was found to be relevant because of its valuable contribution to the estimation of the temperatures at the geothermal target.

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