Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)
© Baillieux et al.; licensee Springer. 2014
Received: 13 June 2014
Accepted: 13 November 2014
Published: 19 December 2014
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.
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.
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.
Following the concept of negative anomalies revealing zones of increased fracture porosity, and in agreement with fluid-chemistry, our findings suggest infiltration of meteoric water through the graben boundary fault and along preferential flow pathways that merge at intermediate depth. Up-flow of thermal water mixed most likely with brine from the deeper eastern part of the graben occurs along W-dipping typically rather steep structures.
Enhanced geothermal system (EGS) technology is based on increasing the transmissivity of naturally fluid-bearing fracture zones by engineering measures. Worldwide, the first EGS has been developed at Soultz-sous-Forêts (Northern Alsace, France). The Soultz geothermal field is part of a series of thermal anomalies located in the western part of the central Upper Rhine Graben (URG) (Baillieux et al. ; Illies et al. ; Pribnow and Schellschmidt ).
Fluid-chemical analyses reveal equilibrium condition with a sedimentary environment of high salinity at temperatures around 250°C (Aquilina et al. ; Sanjuan et al. ). Geologically, such condition is observed on the eastern side of the URG, only.
Fluid inclusions suggest a more local circulation involving down-flow of cold fluid with sedimentary signature and up-flow from >5-km depth of high temperature and low salinity fluids probably of meteoric origin and most likely from the adjacent Vosges Mountains (Cathelineau and Boiron ; Dubois et al. ; Dubois et al. ).
Accordingly, in the western part of the URG, a fault-related discharge zone for the basin-wide circulation system was assumed to account for about 50% increase of the mean heat flux of about 80 ± 10 mW m−2 at the top crystalline basement (Clauser and Villinger ).
The aim of this study is to investigate the possible pathways for fluid inflow into the Soultz and adjacent geothermal fields (e.g. the Rittershoffen field). On a graben-wide scale, Illies and Greiner () stated that convective heat transport occurs along N-S-striking fracture zones, because they are favourably oriented with respect to the present-day stress field to be reactivated. On a local scale, Bächler et al. () have shown that these graben-parallel faults are capable of hosting hydrothermal convection organized into cells. Additionally, on intermediate scale between Speyer and Soultz, Schill et al. () pointed out that major thermal anomalies are related to horst structures. Internally in the Soultz horst, an electrical conductivity anomaly indicating hydrothermal circulation is observed at the western horst boundary fault, only (Geiermann and Schill ). Thus, it may be concluded that only part of the faults is capable of hosting fluid circulation.
Here, we present the re-evaluation of gravity and magnetic data in terms of properties of geological structures. To this end, a new 3D geological model of the Soultz area had been established (Baillieux et al. ) based on the re-interpretation of seismic lines (Dezayes et al. ). In this study, temperature anomalies were re-located using 3D interpolation and compared to the structural pattern of the 3D geological model. After reprocessing and reinterpretation of gravity and magnetic data and application of pseudo-tomography, these results were compared to temperature distribution and analysed for possible fracture porosity.
The interpretation of geophysical data in terms of basement structures and hydraulic properties requires a sound knowledge on possible lithological changes (Abdelfettah et al. ; Denlinger and Kovach ). In this respect, in the geological setting, we will focus on both the basement lithology and the structural setting of the area of investigation.
The late phase of the Variscan orogeny is characterized by large-scale extension causing mainly NNE-SSW-trending graben structures such as the Kraichgau or Schramberg troughs (Ziegler et al. ). The eroded massif provides depositional environment to Mesozoic platform sediments of Triassic (Buntsandstein, Muschelkalk and Keuper) and Jurassic (Lias and Dogger) times. The URG originates in Paleogene times from the Alpine collision due to the build-up of far-field intraplate compressional stresses (Ziegler ) and is described as a typical example of syn-orogenic, intra-continental foreland rifting affected by Variscan crustal pre-discontinuities (Cloetingh et al. ; Dèzes et al. ; cf. discussions on the topic in Schumacher ).
3D structural model of the Soultz area
In the following, we will shortly describe the geological model (Baillieux et al. ; Dezayes et al. ). The 3D structural model of 31 × 18.5 × 6 km around the Soultz EGS site incorporates a network of 26 faults. Often, strike-slip features such as negative flower structures are present as well (Beccaletto et al. ). The horst structure beneath the Soultz site is indicated in 3D geological models. Most of the major normal faults cross-cut the Cenozoic and Mesozoic sediments into the top of the crystalline basement, but can hardly be traced further in its deep part. For the geological model, those faults have been extrapolated down to the main boundary fault or the bottom of the model at −6,000 m below sea level. The orientation of the major faults is NNE-SSW. Inside the different tectonic units, secondary faults trend NNW-SSE to N-S predominantly dipping to the west. It should be mentioned that there is a remarkable correlation between the orientation of secondary faults obtained from triangulated fault surfaces and fracture orientation, the reservoir (Baillieux et al. ). This suggests at least partial coupling of deformation between the sedimentary overburden and the basement at Soultz. Additionally, most of these fractures are water-bearing prior to stimulation and form the main flow channels after stimulation and during circulation (Dezayes et al.  and references therein). The orientation of these fractures is consistent with the present-day stress field, for which measured and derived orientations of SHmax determined down to 5,000 m vary between N125°E and N185°E with a mean value of N175°E ±10° (Cornet et al. ; Klee and Rummel ; Valley and Evans ).
where T is temperature and Z the depth below sea-level, and ∆T/∆Z is the minimum observed geothermal gradient.
Geophysical data processing
Apart from the interpreted seismic lines incorporated into the 3D geological model, in this study, we have investigated existing magnetic and gravity data for physical properties of the seismically identified fault zones. Therefore, existing data were analysed with reference to the local structural setting. In the area of investigation, a number of 837 homogenously distributed, ground magnetic data (Edel et al. ; Papillon ) were reduced to pole. Gravity data were taken from (Rotstein et al. ) database. To avoid physical inconsistency in the gravity data, redundant datasets with offsets were removed and a total of 1,011 measurements were selected. The dataset provides a complete Bouguer anomaly with a reference density of 2,670 kg m−3 and a terrain correction using a digital elevation model with a grid spacing of 75 m, to a maximum radius of 22 km.
Density (kg m−3)
Porphyric monzogranite/two-mica granite
Results and discussion
In the following, we are discussing the distribution of thermal anomalies with respect to structural and mechanical aspects. The geophysical results are discussed with respect to the observed link between thermal, magnetic and gravity anomalies on a graben-wide scale (Baillieux et al. ).
Temperature distribution at top basement
Comparing the location of the thermal anomalies to the structural setting obtained from the 3D geological model, we find that all three anomalies are located directly east of N-S-striking and W-dipping faults that in the case of Soultz and Rittershoffen represents the western limit of a horst structure (Figure 3). Interestingly, the observation of a relation to horst structures holds also for the thermal anomalies of Landau, Speyer (Schill et al. ) and Leopoldshafen (Illies ). Further indication for the relation between west-dipping faults and thermal anomalies is provided by the analysis of fracture families in the deep part of the wells (4 to 5 km) in Soultz, where the largest number of fractures with natural or enhanced transmissivity are dipping to the west (Baillieux et al. ; Dezayes et al. ; Sausse et al. ). As mentioned above, the main hydrothermal alteration interpreted from magnetotelluric data is observed across the W-dipping Soultz and Kutzenhausen faults (Geiermann and Schill ). Analysing fluid inclusions, Cathelineau and Boiron () have found indication for a paleo circulation characterized by upward flow of thermal water with meteoric signature and interpreted this as deep circulation originating from the Vosges Mountains.
Results from potential field methods
It should be mentioned here that a direct correlation between the thickness of the Quaternary deposits (Bartz ) and negative gravity anomalies was not observed. Furthermore, Bouguer anomaly obtained from forward modelling (Baillieux et al. ) using a homogenous basement density reveals gradually decreasing gravity from about −44 mgal at the western boundary fault to about −55 mgal in the southeast of the study area (not shown).This is not compatible with the important negative anomalies observed in the centre of the investigation area. Stripping was carried out on the measured data based on the 3D geological model obtained from seismic data (Baillieux et al. ) to investigate the influence of the sedimentary cover on this prominent anomaly. No significant changes with respect to this anomaly were observed. This is in line with earlier results from gravity inversion that have shown that introducing a basement inhomogeneity of 250 kg m−3 density variation reduces the misfit between the forward modelled and the measured Bouguer anomaly (Schill et al. ). These findings confirm earlier interpretations of gravity in terms of basement lithology (Rotstein et al. ). However, an alternative possibility may be variation in bulk density due to fracture porosity as suggested in other fractured reservoirs (Denlinger and Kovach ).
A final conclusion on the contribution of lithology and fracture porosity on gravity anomalies cannot be drawn from the potential field methods only. Taking the lithological observations in the 5,000 m wells at Soultz as a reference, we may develop a scenario for a strong lithological contribution. There is the positive magnetic anomaly coinciding with the AN1-AN3 gravity anomalies indicating a granitic intrusion of the so-called magnetite series (Ishihara ) with about 1 wt.% of primary magnetite as the main carrier of susceptibility (Just et al. ) and a mean density of about 2,660 kg m−3 (Rummel ). It may be speculated that the increasing extension of AN3 with increasing low cut-off wavelengths might be related to an increasing contribution of the underlying two-mica granite revealing typically lower susceptibility (Meller et al. ), but differences between the two in susceptibility are not very high (between about 1 · 10−3 and 7 · 10−3 m3 kg). Density measurements in the two granites show that there is no significant change between them throughout the well lengths in Soultz (Grecksch et al. ). Cuttings from the geothermal well in Rittershoffen suggest that the petrography of the top crystalline in the centre of the anomaly is comparable to Soultz (Genter, pers. comm.).
This study has shown that there is a strong link between structural features observed typically at the top of the crystalline basement and extrapolated to depth on the basis of the Soultz wells and the localisation of temperature anomalies east of W-dipping fault zone within horst structures. Furthermore, Butterworth filtering of Bouguer anomalies has revealed fault parallel negative anomalies in a pseudo-tomography. These anomalies show a relatively shallow eastward and steep westward dip in the west and east, respectively, and they join at a depth of about 6 to 8 km forming a V-shape structure.
Although gravity anomalies in the URG may originate from both lithological changes and fracture porosity, the interpretation of negative anomalies in terms of possible fluid pathways is supported by different independent methods such as fluid chemistry and magnetotellurics. Our comprehensive approach may be generally applied to visualise flow pathways in deep basins. The occurrence and asymmetric connection of faults in the basement is investigated more in details using mechanical and geodynamic modelling in a forthcoming paper.
Upper Rhine Graben
corrected temperature anomaly
The authors would like to thank LIAG-Hannover, BRGM and EEIG Soultz and EOST Strasbourg for providing input data: borehole temperatures, seismic profiles and borehole geology for the creation of the 3D geological model, and geophysical data, respectively. J.-B. Edel (EOST) kindly provided the magnetic and gravity data; together with A. Genter (EEIG Soultz), we would like to thank them also for fruitful discussions. We would also like to thank B. Valley (ETHZ-UNINE) who provided necessary computing scripts, P. Renard (UNINE), P. Altwegg (UNINE) and L. Guglielmetti (UNINE) for their technical support and the three anonymous reviewers who helped improving the manuscript.
This study was mainly conducted in the framework of the PhD of Paul Baillieux financed by Canton of Neuchâtel (Switzerland). The contribution of E. Schill and Y. Abdelfettah has been co-financed by the HGF Portfolio project ‘Geoenergy’ of the Helmholtz Association at KIT.
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