Heat flow and deep temperatures in the Southeast Basin of France: Implications for local rheological contrasts

Abstract

Triassic salt at 5-10 km depth may drive some of the recent tectonic features in southeastern France. We estimate the likely temperature range of the salt using two different approaches. The first of these, based on the extrapolation of deep temperatures obtained in oil exploration wells, predicts temperatures at a depth of 8 km to be in the range of 230-300 o C. However, this prediction could be biased by a lack of deep measurements and problems related to lateral heat transfer caused by thermal conductivity contrasts. The second approach can overcome these problems by modelling the actual heat transfer for appropriate basin geometry, including temperature-dependent thermal properties, and a mantle heat flow of 35 mW.m -2 . This latter value enables us to reproduce available temperature measurements and surface heat flow data. Here we evaluate the stationary temperature field along two sections constrained by seismic profiles, one at a local scale across the Vistrenque graben and the other at a more regional scale across the Southeast Basin. Our findings suggest that the temperatures in the deepest parts of the evaporitic layer (11 km depth) can reach up to 398 o C, but can be as low as 150 o C on the edge of the basin at the top of the salty layer. This temperature difference leads to important changes in salt viscosity. Results indicate that at a depth of 8 km, lateral viscosity contrasts within the evaporitic layer may reach 40. Such rheological contrasts might favour and amplify local subsidence, as seems to have been the case near the two Palaeogene half-grabens of Vistrenque and Valence, where deep hot zones are identified. Flux de chaleur et températures profondes dans le bassin du Sud-Est de la France : conséquences sur les contrastes rhéologiques locauxMots-clés. -Flux de chaleur, Cartes de températures, Modèles thermiques, Viscosité du sel.Résumé. -Le sel triasique à 5-10 km de profondeur peut guider la tectonique récente dans le Sud-Est de la France. Nous estimons ici, par le biais de deux approches différentes, le domaine des températures les plus probables dans cette couche salifère. La première approche est basée sur l'extrapolation de températures profondes mesurées en forages pé-troliers, et prédit des valeurs entre 230 et 300 o C à 8 km de profondeur. Cependant, ces estimations peuvent être biaisées par le faible nombre de températures profondes et par des problèmes reliés aux transferts latéraux de la chaleur, causés par des contrastes de conductivité thermique. La deuxième approche qui permet de contourner ces problèmes consiste à modéliser les transferts de chaleur dans un bassin dont la géométrie est connue, où les propriétés thermiques dépendent de la température, et où le flux de chaleur en provenance du manteau vaut 35 mW.m -2 . Cette valeur permet de reproduire les données de températures profondes et de flux de chaleur. Le champ de température est calculé le long de deux coupes contraintes par des profils sismiques, l'une à l'échelle du graben de Vistrenque et l'autre à une plus grande ...

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

“…Radiogenic heat production can contribute significantly to surface heat flow density in sedimentary basins (Frone et al 2015;Guillou-Frottier et al 2010;Schütz et al 2012;Waples 2002). In sediments, radiogenic heat production can be high in some shales especially in those rich in organic matter, like in the black shales of the Toarcian (Jurassic) encountered in the URG (Böcker et al 2017;Böcker and Littke 2016;Waples 2002).…”

“…Rittershoffen cross-section modified after Équipe du projet GeORG (2013) the GPK-1 well crosses a fracture zone at 639 m below surface with fractures with apertures on the order of millimeters (oil was detected within these fractures). The GPK-2 well crosses a fracture zone accompanied by a natural gas anomaly at 686 m and the Soultz-sous-Forêts major fault, also associated with a gas anomaly, at 708 m. If fluid migration (such as water or hydrocarbons) through these fractures occurred, or still occurs, and if the rates of migration exceed a few millimeters per year, the circulation could have induced a local thermal perturbation where the fractures cross the sedimentary formations, generating discontinuities in the heat flow density profiles (Andrews-Speed et al 1984;Guillou-Frottier et al 2010). If these fractures reach the basement (which is the case for the Kutzenhausen major fault), the thermal effect of fluid circulation in the faults could be non-negligible (Waples 2002).…”

Structure of hydrothermal convection in the Upper Rhine Graben as inferred from corrected temperature data and basin-scale numerical models

“…Presence of evaporites or salty layers in sedimentary basins may also create heat refraction processes leading to large temperature variations (case of the south-east basin in France, Guillou-Frottier et al, 2010). Around and within faulted zones, fluid circulation may also create lateral temperature differences greater than 20 °C (Clauser and Villinger, 1990;Smith, 1995, Fleming et al, 1998;Bächler et al, 2003;Wisian and Blackwell, 2004;Yang et al, 2004;Appold et al, 2007;Harcouët-Menou et al, 2009;Magri et al, 2010;Garibaldi et al, 2010).…”

“…Previous studies demonstrated that in the subsurface, lateral temperature variations of several tens of °C can be found within a distance of a few kilometres (Pribnow and Schellschmidt, 2000;Rühaak et al, 2009;Bonté et al, 2010;Garibaldi et al, 2010;Guillou-Frottier et al, 2010). Moreover, downward extrapolation of subsurface temperature measurements through the use of an averaged regional temperature gradient may result in significant errors and could thus induce wrong decisions in geothermal exploration strategies.…”

Hydrochemistry of Avène thermal waters (Avène‐les‐Bains, France)

“…According to uncertainties, we can estimate the temperature of the reservoir to be around 50°C. Considering surface water around 10°C and a regional geothermal gradient of 4°C/100m, 10 we could estimate that thermal waters flow down to 1000 m.…”