Slip- and dilation tendency analysis: Implications for geothermal exploration in the Upper Rhine Graben

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1 Slip- and dilation tendency analysis: Implications for geothermal exploration in the Upper Rhine Graben Jörg Meixner a, Emmanuel Gaucher a, Thomas Kohl a, Jens C. Grimmer a, Eva Schill b a : Division of Geothermal Research, Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT); Adenauerring 20b, Geb.50.40, Karlsruhe, Germany b : GEIE Exploitation Minière de la Chaleur, route de Soultz, BP 40038, Kutzenhausen, France Introduction Worldwide geothermal utilizations are bound to favourable tectonic settings producing thermal and hydraulic anomalies. Structural controls of geothermal systems are commonly associated with extensionrelated tectonics and an intensively fractured subsurface. It is widely accepted that fault zone permeability and thus potential fluid flow is closely linked to the state of stress resolved along these zones (e.g. Barton et al., 1995; Gudmundsson, 2000; Gudmundsson et al., 2002; Ito & Zoback, 2000; Townend & Zoback, 2000; Wiprut & Zoback, 2000). Fault orientation with respect to a given in-situ stress field controls the reactivation potential and therewith creates anisotropic permeability patterns. The complexity of possible fluid-flow paths in fractured and faulted geothermal reservoir is difficult to predict but can have a strong influence on the appraisal and development of a geothermal reservoir and can significantly affect the efficiency and thus the life-cycle of a geothermal project. We investigate the geomechanical behaviour of complex fault patterns at reservoir scale for the two geothermal sites of Bruchsal (SW Germany) and Riehen (NW Switzerland). Both fields are located at the eastern margin of the Upper Rhine Graben (URG) and produce geothermal energy out of highly fractured and faulted reservoirs. For our study we apply a slip and dilation tendency analysis to evaluate the geomechanical behavior of regional fault patterns in ambient tectonic stress fields. Slip tendency, T S, is the ratio between the effective shear stress and the effective normal stress (Morris et al., 1996). The dilation tendency (T D ) is the relative probability for a fracture or a fault plane to dilate under the local stress field (Ferrill et al., 1999). It is defined as the difference between the maximum principal compressive stress (σ 1 ) and the normal stress (σ n ), normalized by the differential stress (σ 1 -σ 3 ). To compute both values we determined the in-situ stress states and created 3D fault models for both locations. Geological settings Bruchsal is located in the central segment of the URG about 20 km NE of Karlsruhe. Available data for the construction of a 3D fault model include five analogue, migrated post-stack seismic sections from the early 1980s. A number of four wells out of the ten available ones are deeper than 1000 m. Lithological and stratigraphic logs of all ten wells were correlated and unified. The m deep Permo-Triassic reservoir is characterized by an intense tectonic shearing and fragmentation. Several fault zones dissect these formations into several fault compartments and tilted fault blocks. The interpreted fault pattern is characterized by three major sets of faults. The first set includes NNE-SSW striking faults with an average dip to W-NW. These faults outlining an en-echelon geometry along strike are subparallel and synthetic to the EMBF with an average dip of A second set of faults comprises NE-SW to N-S striking antithetic normal faults. In the southern part of the model these faults are limited to the Mesozoic sedimentary successions. In the northern part several antithetic normal faults are present in younger Tertiary (Lower Miocene). A third set of NW-SE striking faults are postulated because significant vertical displacement of the Mesozoic sedimentary successions between the northern and the southern part of the model is documented in the seismic sections and the stratigraphic well logs. A structural

2 separation of both parts of the model caused by extension-related transfer faults or preexisting and reactivated strike-slip faults can explain these discrepancies. Riehen is located at the southern end of the URG about 3 km NE of Basel (CH). The hydrothermal reservoir is located in the Upper Muschelkalk, a Middle Triassic aquifer composed of fractured and karstified limestone. The geothermal doublet is aligned parallel to the EMBF. The geological and tectonic settings of that area have been previously described by three separate 3D models. They were combined and extended by Klingler (2010). These models were created from 2D seismic reflection surveys, geological cross-sections, and structural maps (Fischer et al., 1971; Gürler et al., 1987; Hauber, 1993). 13 deep wells in this area provide additional information about depths and thicknesses of the sedimentary successions and fault orientations. Predominant fault trends constitute a distinctive fault pattern, which can be subdivided, again, into three different fault sets. A first fault set is characterized by an average NW-SE strike and was established in Palaeozoic times in the course of the Variscan orogeny (Schumacher, 2002). A second fault set contains Late Palaeozoic E-W to ENE-WSW striking, high-angle basement faults (Ustaszewski, 2004). This trend is related to post-variscan wrench tectonics that formed a system of E-W to ENE-WSW striking Permo-Carboniferous troughs and highs, which followed the general structural (E)NE-(W)SW trend of the Variscan orogen (Ziegler, 1990). Late Eocene to late Oligocene E-W to WSW-ENE extension (Larroque & Laurent, 1988; Ustaszewski, 2004) caused reactivation of the pre-existing crustal discontinuities and established a third fault set of graben-parallel N-S to NNE-SSE striking faults. Stress Field Models The stress field orientation in Bruchsal was determined in the 1980 s by analysis of oriented caliper logs and borehole breakouts in the wells GB1 and GB2. The vertical stress S v at a certain depth is, in general, equal to the weight of the overburden and was calculated for an average rock density of 2.43 g/cm³. The pore pressure P P was calculated for a brine density of 1.07 g/cm³ and an average free water table of 60 m below ground level. To estimate S hmin magnitude, we used leak-off test data acquired in 1984 in GB2 at 2245 m depth in the Middle Buntsandstein. We applied the stress limitation concept by Jaeger et al. (2007) and Moos & Zoback (1990) to constrain the upper and lower bound for S Hmax. In-situ measurements of the local stress field at the geothermal site of Riehen do not exist. Hence, comparative values of stress field investigations in the vicinity of Riehen are used. The 5000 m deep geothermal well Basel1 and the 2755 m deep observation well Otterbach2 (both 5 km SW of Riehen) provide information about the local stress field orientation and the magnitudes of the stress components (e.g. Häring et al., 2008; Sikaneta & Evans, 2012; Valley & Evans, 2009). We applied the stress field model derived for Basel for the slip- and dilation tendency analysis of the Riehen fault model, assuming a homogeneous stress state in this area. An overview of the determined stress field models for Bruchsal and Riehen can be seen in Tab. 1. Tab. 1: Applied stress field models for the slip- and dilation tendency analysis for the Bruchsal and Riehen geothermal sites. Bruchsal Model 1 Bruchsal Model 2 Riehen Model name Bruchsal (NF) Bruchsal (NF/SS) Riehen (SS) stress regime Normal faulting Transitional Normal faulting / Strike-slip Strike Slip S Hmax orientation N142 E ± 20 N142 E ± 20 N143 E ± 14 S Hmax magnitude S Hmax = 0.76 S v S Hmax = 1.0 S v S Hmax = 33 z [z in km] S hmin magnitude S hmin = 13.2 z [z in km] S hmin = 13.2 z [z in km] S hmin = 17 z [z in km] S V magnitude S V = 23.8 z [z in km] S V = 23.8 z [z in km] S V = 25 z [z in km] P P magnitude P P = 10.5 (z-0.06) [z in km] P P = 10.5 (z-0.06) [z in km] P P = 9,91 (z+0.2) [z in km]

3 Results and Discussion The Bruchsal and Riehen geothermal sites show comparable tectonic settings but different stress regimes. The calculations indicate that, regardless the stress state and faulting regime, the slip tendencies and dilation tendencies show strong anisotropic distributions within the fault models of Bruchsal and Riehen. T S and T D as functions of the fault patch orientations can be seen in Fig. 1. Here, each dot corresponds to an existing fault patch of the meshed fault model. The line elements display T S and T D values for generic fault models of normal faults (dip 70 ) and strike slip faults (dip 90 ). Grabenparallel and non graben-parallel fault trends at both sites are characterized by different probabilities to undergo shear failure or tensile failure and can be classified in three major groups. Class 1 (high T D and low T S ): Faults that strike almost parallel to S Hmax (NW-SE) show maximum T D values and very small T S values if they are subvertical with dip angles between 80 and 90. These structures are not prone to shear failure (reactivation) but are most likely for tensile opening. Class 2 (high T D and high T S ): Faults striking 30 around S Hmax but with fault dips between 60 and 70 will show again very high T D values but are also characterized by maximum T S values. In a normal faulting regime these structures are prone to shear failure and most likely for reactivation as normal faults. In a transitional or strike slip stress state also subvertical faults aligned in conjugated directions of about 30 around the S Hmax orientation show this combination. Here, these structures are optimally oriented for reactivation as oblique normal faults or strike-slip faults. Class 3 (low T D and low T S ): N-NW striking graben-parallel fault zones are characterized by relatively low T S and T D values. Vertical structures striking exactly parallel to S hmin show always minimum T S and T D. These structures are less prone to undergo shear failure (reactivation) and are not favorably oriented for tensile opening. On the one hand faults and fractures oriented favourably for frictional failure often dominate fluid flow (e.g. Barton et al., 1995). On the other hand extensional faults and fractures optimally oriented to undergo tensile failure act as main fluid-conducting pathways in reservoirs (e.g. Gudmundsson, 2000; Gudmundsson et al., 2002). The first group is characterized by high T S values; the second group is characterized by high T D values. Thus, critical T S and T D values of faults probably can be interpreted as indications of increased conductivities along them. High angle transfer faults (Bruchsal) or reactivated strike-slip faults (Riehen), both class 1 structures, show these indications and thus probably cause anisotropic permeability patterns in both reservoirs. At both sites the doublet is arranged in a graben-parallel NE-SW to NNE-SSW direction. Due to several hydraulic tests at both sites a graben-parallel alignment of the hydrothermal reservoirs are assumed but conducted tracer tests didn t show any evidence for a direct fluid flow paths between the wells. A zerorecovery of the tracer as well as no temperature variations during long term production at both locations possibly can be explained by a highly conductive fault zone located between both doublet arms striking more or less parallel to S Hmax. This can significantly affect the efficiency and the life-cycle of both geothermal projects. Furthermore the slip- and dilation tendency analysis can also provide useful indications for developing existing and future deep geothermal projects, since comparable tectonic settings and fault trends as in Bruchsal and Riehen can be found in the whole Upper Rhine Graben.

4 Fig. 1: Distribution of the slip tendency (T S, red) and dilation tendency (T D, blue) as a function of the fault plane azimuth, displayed for the Bruchsal fault model under normal faulting regime (A) and under transitional regime (NF/SS) (B), and for the Riehen fault model under strike slip regime (C). T S and T D for the generic fault sets were calculated for a depth of 2000 m in Bruchsal and 2500 m in Riehen respectively.

5 Barton, C., Zoback, M. and Moos, D., Fluid-flow along potentially active faults in crystalline rock. Geology, 23(8): Ferrill, D.A., Winterle, J., Wittmeyer, G., Sims, D., Colton, S. and Armstrong, A., Stressed rock strains groundwater at Yucca Mountain, Nevada., GSA Today, pp Fischer, H., Hauber, L. and Wittmann, O., Geologischen Atlas der Schweiz, 1:25000, Blatt Basel mit Erläuterungen, Geologische Kommission der Schweiz. Naturf. Ges. Gudmundsson, A., Active fault zones and groundwater flow. Geophysical Research Letters, 27(18): Gudmundsson, A., Fjeldskaar, I. and Brenner, S., Propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields. Journal of Volcanology and Geothermal Research, 116(3-4): Gürler, B., Hauber, L. and Schwander, M., Die Geologie der Umgebung von Basel mit Hinweisen über die Nutzungsmöglichkeiten von Erdwärme. Hauber, L., Der südliche Rheingraben und seine geothermische Situation. Bull. Ver. schweiz. Petroleum-Geol. u. -Ing., 60(137): Häring, M., Schanz, U., Ladner, F. and Dyer, B., Characterisation of the Basel 1 enhanced geothermal system. Geothermics, 37(5): Ito, T. and Zoback, M., Fracture permeability and in situ stress to 7 km depth in the KTB Scientific Drillhole. Geophysical Research Letters, 27(7): Jaeger, J.C., Cook, N.G.W. and Zimmermann, R.W., Fundamentals of Rock Mechanics. Blackwell Publishing, Malden MA, USA, 475 pp. Klingler, P., charakterisierung des geothermischen Reservoirs Riehen: 3D Struktur und Tracertest. Master Thesis Thesis. Larroque, J. and Laurent, P., Evolution of the stress field pattern in the south of the Rhine Graben from the Eocene to the present. Tectonophysics, 148(1-2): Moos, D. and Zoback, M., Utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses: Application to continental, Deep-Sea Drilling Project,and Ocean Drilling Program boreholes. Journal of Geophysical Research-Solid Earth and Planets, 95(B6): Morris, A., Ferrill, D. and Henderson, D., Slip-tendency analysis and fault reactivation. Geology, 24(3): Schumacher, M.E., Upper Rhine Graben: Role of preexisting structures during rift evolution. Tectonics, 21(1). Sikaneta, S. and Evans, K., Stress heterogeneity and natural fractures in the Basel EGS granite reservoir inferred from an acoustic televiewer log of the Basel-1 well, Thirty-seventh Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford University, Jan 30th - Feb. 1st. Townend, J. and Zoback, M., How faulting keeps the crust strong. Geology, 28(5): Ustaszewski, K., Reactivation of pre-existing crustal discontinuities: the Southern Upper Rhine Graben and the Northern Jura Mountains - a natural laboratory, University of Basel, Basel. Valley, B. and Evans, K., Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis. Swiss Journal of Geosciences, 102(3): Wiprut, D. and Zoback, M., Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea. Geology, 28(7): Ziegler, P., Collision related intra-plate compression deformations in Western and Central-Europe. Journal of Geodynamics, 11(4):