Stratigraphic, sedimentary and structural approach to characterize the cover-basement interface in the Rhine Graben. Thesis defended by Coralie Aichholzer

10 décembre 2019 par Stéphanie Robert
On October 10, 2019, Coralie Aichholzer defended her thesis in geology, entitled "Stratigraphic, sedimentary and structural approach to characterize the cover-basement interface in the Rhine Graben”. It was directed Philippe Duringer (EOST, IPGS) and co-financed by the LabEx G-eau-thermie Profonde and the University of Strasbourg.

“Geology is not paramount in deep geothermal energy since the target is essentially the deep fractured granitic reservoir. But the drilling, before reaching it, crosses most of the sedimentary cover. It is in this context that my thesis subject was developed.

The geological approach has been essential for the Rittershoffen boreholes and will be even more so for future projects, given the lack of geological knowledge of some deep areas of the Upper Rhine Graben (URG).

Indeed, while Rittershoffen has benefited from precise and detailed monitoring of the sedimentary cover drilled by GRT-1 and GRT-2, this is not the case for the other wells in the region. The first objective of my thesis was therefore to reinterpret the chrono-stratigraphy of all the sedimentary formations crossed by the Soultz-sous-Forêts wells (GPK-1 and GPK-2), located 7 km as the crow flies from Rittershoffen. Then, with the new projects under development in the Strasbourg area (Illkirch-Graffenstaden, Vendenheim-Reichstett, Hurtigheim, Eckbolsheim), a major new challenge emerged: to better limit the sedimentary cover of the Strasbourg region and above all to try to provide knowledge on certain geological areas that are still not well known.

During my thesis I reinterpreted the chrono-stratigraphy of fifteen deep wells going at least to the Triassic, distributed between the North and South of the Upper Rhine Graben. These reinterpretations were correlated across the entire basin, allowing the development of a complete stratigraphic column including the top and base of each formation. This highlighted the characteristic signal of gamma-ray log (GR) for each of the formations of the URG column. As a result, I proposed a catalogue of GR signatures in my manuscript.

The lithostratigraphic characterization of the the the sedimentary cover – basement transition was also an important focus of my research. I studied 120 km of 2D seismic lines to point the roof of the Permian seismic horizon, to characterize sedimentary filling and observe the lateral variation of Permian formations. In addition, I studied 8 quarries spread over the Vosges, the Black Forest and the South of the Palatinate in order to propose field analogues to calibrate the seismic observations. Finally, these results were discussed in order to better constrain the Permian and to respond to the problems it may pose by being at the interface between the Variscan crystalline basement and the Buntsandstein (Triassic), both naturally fractured, and geothermal reservoirs in the URG.”

Coralie Aichholzer