New stratigraphic interpretation of the Soultz-sous-Forêts 30-year-old geothermal wells calibrated on the recent one from Rittershoffen (Upper Rhine Graben, France)
Résumé
The Soultz-sous-Forêts site (Alsace, France) is the first pilot geothermal project in
the world that furnished the proof of concept of enhanced geothermal systems by
producing energy from a deep-seated granite. The Alsace region, with its hundreds
of previous drillings and seismic history related to both oil and potash exploitation,
has been shown to have high subterranean temperatures. In the Soultz-sous-Forêts
geothermal project, most attention has focused on the basement and the sedimentary
cover/basement interface. Thus, the sedimentary portion of the well (approximately
1.4 km thick) has never been fully investigated. The only available data on the Soultzsous-
Forêts sedimentary cover are from an old masterlog (GPK-1) with interpretations
of the tops and bases of the main geological formations, and from some other
less well-documented well logs (EPS-1 and GPK-2). The main challenge of this work
is therefore to re-interpret the old well data to provide precise and detailed chronolithostratigraphic
logs for GPK-1 and GPK-2, especially in their sedimentary portions.
These new investigations of the GPK wells have been possible due to the new data collected
in the recent geothermal wells at Rittershoffen (GRT-1 and GRT-2, located 6.5 km
from Soultz-sous-Forêts), which are characterized by a quite complete stratigraphic
succession. Both sites have been explored by deep drilling operations aiming to exploit
the heat extracted from a deep granitic basement (Palaeozoic) covered by a stack of
1.4- and 2.2-km-thick sedimentary rocks (Mesozoic to Cenozoic) at Soultz-sous-Forêts
and Rittershoffen, respectively. Thus, the Rittershoffen chrono-lithostratigraphic logs
have been used as a baseline to interpret the sedimentary succession in GPK-1 and
GPK-2. In conclusion, all the well logs are compared for stratigraphic comparisons. With
approximately 1400 m of sedimentary cover at Soultz-sous-Forêts instead of 2200 m
at Rittershoffen, the correlation between the two sites showed many differences exist
in the sedimentary columns: layers missing due to erosion, lateral thickness variations
in formations, and—above all—the occurrence of at least four major fracture zones
affecting the units. In addition, a structural analysis was made to more precisely define
the limits of the geological formations observed and to present more arguments for
the presence of fracture zones or faults.
Domaines
Sciences de la TerreOrigine | Fichiers éditeurs autorisés sur une archive ouverte |
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