Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)

Abstract

Background: This study aims on investigating the regional flow field of the Soultz and adjacent geothermal fields located on the western side of the central Upper Rhine Graben and thus to provide insight into the origin of the 70% of the geothermal fluid coming from the regional inflow in the deep reservoir of the Soultz site. In an integrative approach, we consolidate conceptual models on fluid flow in the central Upper Rhine Graben. Methods: Based on a 3D geological model and a new 3D temperature interpolation, we tackle the relation between tectonic structures and the occurrence of advection/ convection along favourably oriented fault zones. Using sequential Butterworth filters, we study the distribution of negative residual anomalies in a pseudo-tomography down to a depth of about 6 to 8 km. Results: We derived N-S-striking V-shaped negative anomalies that are consistent with the orientation of fault zones revealing major temperature anomalies to their east.

How do the geological and geophysical signatures of permeable fractures in granitic basement evolve after long periods of natural circulation? Insights from the Rittershoffen geothermal wells (France)

“…The crystalline granitic basement is composed of muscovite, biotite, K-feldspar, plagioclase, and quartz, and is interpreted to be equivalent to the two-mica granite encountered in the deep reservoir at Soultz (Stussi et al 2002;Traineau et al 1992). The formation of the granitic basement has been dated to the mid-Carboniferous, and it has since undergone pervasive alteration and exhibits several grades of hydrothermal alteration (Cocherie et al Baillieux et al (2014). b Geological cross section through the Rittershoffen and Soultz-sous-Forêts geothermal sites with the Rittershoffen well trajectories, after the GeOrg Team (2017).…”

Structure and nature of the Palaeozoic basement based on magnetic, gravimetric and seismic investigations in the central Upper Rhinegraben

“…Such bodies can be considered as continuous thermal conductors from the top of the lower sheeted crust up to the bottom of the sedimentary cover. In addition, the contribution of the natural radioactivity of such volumes of granitoids to the heat flow is significant (Jolivet et al 1989;Baillieux et al 2014;Ielsch et al 2016). In Table 1 are listed the heat-production values of the granitoids of the northern Vosges or from the Soultz geothermal site as well as petrophysical data collected from Variscan rocks in Europe (Correia 2015;Forster and Forster 2000;Grecksch et al 2000;Guillou-Frottier et al 2013;Lexa et al 2011;Tabaud et al 2014;Vilà et al 2010).…”

“…It is therefore anticipated that in the present stress field, the Eschau N-S faulted zone at the interface between the deepest sediments and the Palaeozoic crystalline basement could be characterized by a high concentration of closely spaced fractures acting as permeable drains. Just as at Soultz and Rittershoffen, where magnetic and heat-productive granites were drilled (Baillieux et al 2014 and references therein), magnetic and heat-productive granodiorites of the southern champ du Feu type are expected.…”

“…The Soultz site is located on the western margin of a magnetic body consisting of granitoids, whereas the Rittershoffen site is located more on its core (e.g. Baillieux et al 2014).…”

The Cornubian geothermal province: heat production and flow in SW England: estimates from boreholes and airborne gamma-ray measurements

“…Aeromagnetic data have been used in geothermal investigations to estimate basal depths of the Curie point magnetic layer (Abraham et al 2014) and to assess concealed structure (Maystrenko et al 2014). Baillieux et al (2014) report on the use of gravity pseudotomography to study the behaviour of density contrasts, related to increased fracture porosity, at depth. Here we utilise modern high-resolution airborne gamma-ray data which is effectively a shallow technique but which has, in theory, a direct relationship to heat production through the estimation of the radioelement concentrations.…”