The complete lithostratigraphic section of the geothermal wells in Rittershoffen (Upper Rhine Graben, eastern France): a key for future geothermal wells

Abstract

Between 2012 and 2014, in Rittershoffen, in the Upper Rhine Graben, two geothermal boreholes (GRT-1 and GRT-2) reached the granitic basement at a depth of around 2150 m. The wells crossed about 1160 m of Cenozoic and 1050 m of Mesozoic. In the Cenozoic, these wells crossed the greatest part of the Eocene, the lower part of the Oligocene and a thinner Pliocene. The Quaternary is poorly represented (less than 10 m). In the Mesozoic, the wells crossed the Lower Dogger, the entire Lias and the entire Triassic. A reduced thickness of about 50 m of Upper Permian terminates the sedimentary column before entering into the granitic basement. A major erosional unconformity separates the Middle Jurassic from the Tertiary units (the Upper Jurassic, entire Cretaceous and Paleocene are absent). The Rittershoffen drilled doublets were the subject of particular attention concerning the acquisition of a very precise stratigraphic profile. In this paper, we give the recognition criteria for the fifty-seven formations crossed by the GRT-1 well and their upper and lower boundaries as well as their specific gamma ray signatures. The data are presented in four figures: a general complete log displaying the main sets and three detailed, precise logs showing the sedimentary formations overhanging the granitic basement: the Tertiary, the Jurassic, the Triassic, the Permian and the basement.

Heat flow density estimates in the Upper Rhine Graben using laboratory measurements of thermal conductivity on sedimentary rocks

“…Above these Permian sedimentary rocks are the fluvial to fluvio-deltaic sandstones and conglomerates of the lower Triassic Buntsandstein (B) series, overlain by marine-to-lagoonal sediments of the middle Triassic Muschelkalk (M) series (Aichholzer et al 2016). The Muschelkalk formations are mainly composed of interbedded marlycalcareous rocks, shelly sandstones, evaporitic deposits, and dolostones (Duringer et al 2019;Ménillet 2015). The carbonate-shelf environment that formed the upper Muschelkalk series was followed by the evaporitic episode of the Keuper (K), upper Triassic.…”

“…The Keuper is characterized by massive clays, marly clays and dolomites that contain evaporitic deposits (gypsum and anhydrite). Some sandstone marker beds are also encountered in the lower, middle and upper Keuper formations (Aichholzer et al 2016;Duringer et al 2019;Ménillet 2015). The depositional environment became fully marine from the Jurassic, with marls/limestones alternations during Hettangian, followed by clays and clayey marls alternations with occasional calcareous banks up to the base of Bajocian.…”

Detailed descriptions of the lower-middle Triassic and Permian formations using cores and gamma-rays from the EPS-1 exploration geothermal borehole (Soultz-sous-Forêts, Upper Rhine Graben, France)

“…Then, the cuttings' examination during drilling was interpreted in terms of lithostratigraphical units. The precise depth of the casing shoe was decided on the basis of the daily geological profile produced and calibrated with the knowledge gained from the EPS-1 cores (Duringer et al 2019). The use of the lithostratigraphic column for setting the casing was, therefore, applied to the two successful Rittershoffen geothermal wells (Baujard et al 2017).…”

“…The top of the Grès Argileux (upper Grès à Voltzia) is characterized by a less radioactive peak followed seven meters below by a very strong radioactive peak, which is the highest value along the entire Triassic sedimentary column. This highest peak characterizes the summit of the Grès à Meules, which could be related to the presence of radioactive minerals (probably in zircon) in the sole of the transgressive phase of the Grès Argileux, which is often described as a transgressive series (see Duringer et al 2019). Progressing downwards through the entire Grès à Voltzia, the GR signal decreases progressively as the clayey sandstones disappear, marking the passage between the Grès Argileux and the massive Grès à Meules.…”

“…1. Duringer et al 2019) was a great help for the Soultz-sous-Forêts GPK wells reinterpretation (Aichholzer et al 2016). Thus, to be sure of the given lithostratigraphic limits, an assessment of the geological formations was conducted through the comparison of ESP-1 core analyses and field data on available outcrops.…”