Inferring the in situ stress regime in deep sediments: an example from the Bruchsal geothermal site
© Meixner et al.; licensee Springer. 2014
Received: 27 January 2014
Accepted: 22 April 2014
Published: 6 June 2014
Knowledge of the ambient state of stress is of crucial importance for understanding tectonic processes and an important parameter in reservoir engineering. In the framework of the 2,500-m deep geothermal project of Bruchsal in the central part of the Upper Rhine Graben, new evidence is presented for the stress field in deep-seated sedimentary rocks.
With a sophisticated data analysis based on the concept of critical stress ratios, we evaluate the quality and uncertainty range of earlier stress field models in the Bruchsal area. New data from borehole logging and leak- off tests in deep sediments are used to propose an alternative stress profile for this part of the Upper Rhine Graben.
The revised stress field model for the Bruchsal area predicts a normal with transition to strike-slip faulting regime. Stress field perturbations and potential decoupling process within specific clay-, salt-, and anhydrite-bearing units of the Keuper can be observed.
By comparison with other models, we can show a regional consistency of our stress field model that is reliable throughout the central Upper Rhine Graben extending from Bruchsal in the East to the Soultz-sous-Forêts EGS site in the West.
In a regional context, the stress field is typically used for investigation of neotectonic and recent geodynamic processes. The world stress map (Heidbach et al. ) provides a sound database with respect to determination of fault reactivation, tectonic deformation, and related earthquake hazard (e.g., Hergert and Heidbach ). Moreover, stress is a key parameter in unconventional reservoir engineering. Faults and fractures that are favorably oriented and critically stressed for frictional failure often dominate fluid flow (Barton et al. ; Townend and Zoback ). In this respect, a higher resolution of the stress field is required and linear stress-depth profiles should be used with caution, as principal stress magnitudes can vary locally by topography, geological unconformities, stratifications, lithology, or geological structures like faults or fractures (Heidbach et al. ; Zang and Stephansson ). In sedimentary rocks, stress field orientation and principal stress magnitudes show significant variations depending on their rheological characteristics (Anderson et al. ; Cornet and Burlet ). Interstratification of stiff clastic sediments and clay-, salt-, and anhydrite-bearing formations causes significant deviations from linear stress-depth profiles in deep sedimentary basins such as the North German basin, the Paris Basin, and in continental rift systems such as the Upper Rhine Graben (URG) (Cornet and Röckel ; Wileveau et al. ). Thus, stress measurements in sediments (orientation and magnitude) need to be evaluated with respect to the lithological characteristics of the corresponding formation, and extrapolation of measured stress values to depth should be conducted with care, especially when only few measurements are available. With this in mind and although the world stress map provides a large amount of data, determination of local stress appears often to be insufficient.
In this study, we present a methodology for stress field estimation in areas where a detailed knowledge of the local stress conditions is unavailable. The approach is applied on the example of the Bruchsal geothermal site, where a number of earlier studies have been carried out with a similar aim. We will present a comparative review of the existing data and add new unpublished data from two leak-off tests (LOT) and our resulting approach applied to the Bruchsal area.
The 300-km-long URG represents the central part of the European Cenozoic rift system (Schumacher ), extending over a distance of more than 1,000 km across central Europe. It is subdivided into a NNE-striking southern, a NE-striking central, and a NNW to N-striking northern segment (Figure 1). The evolution of the Cenozoic URG was controlled by polyphase reactivations of a complex set of crustal discontinuities of Late Paleozoic structures (Ziegler ). The main extensional rifting and crustal NW-SE extension started in Late Eocene (Sissingh ) during which Late Variscan and Permo-Carboniferous crustal discontinuities were transtensionally reactivated (Schumacher ). The opening of the graben was controlled by a paleostress field with a SHmax orientation of NNE-SSW (Ahorner ; Illies ) resulting in the development of a NE-SW to NNE-SSW striking graben. A major reorientation of the regional stress field during early Miocene times established a NE-SW-extensional to transtensional stress field with reactivated fault segments showing sinistral and dextral oblique displacements but also local inversion and contraction (e.g., Illies and Greiner ). The change of the regional stress field initiated a new tectonosedimentary regime. The synrift deposits and older strata in the central and southern segments were uplifted and partly eroded due to transpressional reactivation of these graben segments (e.g., Rotstein et al. ; Rotstein and Schaming ). A number of thermal anomalies in the western part of the central and northern part of the URG are linked to different structural features such as zones of uplift (Baillieux et al. ). Subsidence and sedimentation were restricted to the northern graben segment with a maximum Cenozoic graben fill of up to 3.0 km (Bartz ; Pflug ).
Numerous local studies have been carried out to determine the tectonic stress field in the URG. Most of them are based on the analysis and interpretation of earthquakes, tectonic studies, overcoring data, hydraulic fracturing data, and borehole breakouts (Greiner ; Baumann ; Larroque and Laurent ; Plenefisch and Bonjer ; Valley and Evans ). Figure 1 presents an extensive data compilation for the present-day stress field in the URG and shows the abovementioned major structural units of the rift system. The seismological data are derived from fault plane solutions of earthquakes from 1971 to 1980 and incorporate 33 fault plane solutions selected by Larroque et al. (), based mostly on data from Bonjer et al. (). The fault traces from Illies and Greiner () are based on the interpretation of 2D seismic sections. The information on the stress field orientation and the faulting regime are taken from the world stress map (Heidbach et al. ).
The compilation of stress field indicators in Figure 1 highlights the generally uniform NW-SE orientation of SHmax demonstrated earlier by Müller et al. (). This general trend is confirmed by stress inversion of earthquake fault plane solutions (Delouis et al. ; Plenefisch and Bonjer ). The general trend of the stress field shows a local variation with SHmax orientation in the northern URG ranging N130°E to N135°E and in the southern URG/northern Switzerland ranging N145°E to N160°E. Interpretations of fault plane solutions also reveal a change in faulting regime in the URG (e.g., Plenefisch and Bonjer ). The northern part of the URG is characterized by an extensional stress state and active normal faulting (σ1 = Sv, σ2 = SHmax). In the seismically more active southern part, strike-slip faulting (σ1 = SHmax, σ2 = Sv) with secondary normal faulting is the predominant mechanism. The transition of the stress orientation and the change of the faulting regime by permutation of σ1 and σ2 (Larroque et al. ) occurs in the central segment of the URG, in the area of the site of investigation, probably causing a transitional stress state between normal and strike-slip faulting.
At the western central margin of the URG, an extensive set of in situ stress data is available for the geothermal site of Soultz-sous-Forêts (France). Measured and derived orientations of SHmax determined down to 5 km varies between N125°E and N185°E with a mean value of N175°E ± 10° (Cornet et al. ) and indicate a transitional stress state down to 5 km with a change from normal to strike-slip faulting at depths below 3 km (Cuenot et al. ).
Depth intervals of the analyzed borehole breakouts and determined S Hmax orientations in wells GB1 and GB2
Depth interval (MD)
1,650 to 1,700 m
N 104° E
Middle Muschelkalk and Upper Buntsandstein
1,700 to 1,775 m
N 137° E
1,775 to 1,850 m
N 142° E
1,850 to 1,900 m
N 145° E
Lower Buntsandstein and Upper Permian
2,026 to 2,070 m
N 090° E
2,070 to 2,130 m
N 163° E
2,130 to 2,230 m
N 125° E
Upper Muschelkalk and Upper Buntsandstein
2,250 to 2,328 m
N 125° E
2,330 to 2,385 m
N 145° E
Middle Buntsandstein and Upper Permian
2,385 to 2,475 m
N 131° E
2,475 to 2,535 m
N 128° E
The pore pressure, PP, was calculated similarly, assuming that it is close to hydrostatic. With an average depth of the free water table 60 m below ground level, the reservoir reveals tendency to slight under pressure condition. The fluid density of the geothermal brine is 1,070 kg m−3 (T. Kölbel 2013, pers. comm.). At mean reservoir depth of 2,000 m, a pore pressure of PP = 20.4 MPa and a gradient of 10.2 MPa km−1 was calculated. This leads to a ratio of pore pressure to vertical stress magnitude of PP = 0.43⋅SV.
Stress field profiles for Bruchsal and adjacent areas
With exception of SHmax of the Greiner profile, the two Bruchsal stress profiles differ significantly from the Soultz profiles (Figure 3). With a rather similar Sv gradient of 23.8 MPa km-1, the Rummel and Baumgärtner and the Greiner stress profiles would yield SHmax/SV ratios at depths over 500 m of 1.4 to 1.5 and 0.8 to 1.0, respectively (Figure 3A). Hence, the Rummel and Baumgärtner profile predicts a strike-slip regime, while the Greiner profile indicates normal faulting at depths below 500 m. In contrast, the ratios of Shmin/SV in both Bruchsal profiles are similar (0.8 to 0.9, see Figure 3B) but differ from the Soultz profiles with ratios of approximately 0.5. Finally, it must be stated that the two Bruchsal profiles are debatable since they are derived from measurements at shallow depth only.
Critical stress concept
Based on these equations, the critical ratio of the principal stress magnitudes depends on depth, pore pressure, and the coefficient of frictional sliding. A coefficient of friction of 0.85 has been shown to be applicable under shallow crustal conditions for normal stresses up to 200 MPa, i.e., depths of approximately 6 km (Byerlee ). However, it is generally accepted that hydrothermal alteration and clay content in shear planes contribute to a significant reduction of the friction angle during fault reactivation (Krantz ). But, since no data on the mineralogical composition of the faults and fractures in Bruchsal were acquired, we assume μ = 0.85, which results in an effective stress ratio of σ1/σ3 < 4.68. The possible mechanically stable stress conditions in different stress regimes have been comprised by Zoback et al. () to the so-called stress polygon. For a given friction coefficient, the area inside the polygon represents the allowed stress, whereas values lying outside the borders are instable and yield failure.
In the following, we will present the results of the determination of Shmin by two leak-off tests from the well GB2 as well as the application of the critical stress concept to further constrain Shmin and SHmax.
Leak-off tests and Shminmagnitude
Key parameters of the two leak-off tests conducted in the geothermal well Bruchsal GB2 in 1984
Drilling mud density
1.11 g cm−3
1.05 g cm−3
LOP = Shmin magnitude
15.3 MPa km−1
12.3 MPa km−1
The leak-off pressure (LOP) at the depth of the test is the sum of the well head pressure and the pressure in the wellbore due to the wellbore fluid. According to Zoback et al. (), we consider the clear LOP of each test (corresponding to a distinct break-in-slope of the linear pressure build-up) to be approximately equal to the Shmin magnitude. While LOT-2023 is conducted in the weak Keuper, LOT-2245 is located in stiff units of the Middle Buntsandstein. We would like to emphasize that LOT-2023 results in a higher LOP of 31.0 MPa compared to LOT-2245 with a LOP of 27.6 MPa. This finding is consistent with Shmin magnitudes obtained in comparable environments (Cornet and Röckel ). The determined Shmin/Sv ratios show contradictory results to those from Soultz.
Critical stresses of Shmin
There are no error estimates given on the Bruchsal LOT measurements. Based on the example from Soultz, we can assume a reasonable uncertainty range for Shmin magnitudes of ± 0.45∙z [km] + 1.82, as derived from several large and small volume injection tests (Valley ). If we assume similar error margins for Bruchsal, a bandwidth of the Shmin profile can be determined. An increase of the Shmin/Sv ratio from 0.52 to 0.55 is consistent with the critical stress concept and remains consistent with the error margins determined for the deep geothermal wells in Soultz. This represents a minimum Shmin gradient of LOT-2245, referred to as ‘LOT-2245 (SL)’ in the following (Figure 4, blue lines).
The hypothesis on the NF tectonic regime also influences the consistency with the critical stress concept. Following the same procedure, it is obvious that a strike-slip regime with S1 = SHmax > SV would result in an effective stress ratio of σ1/σ3 > 4.68. This would rather lead to a larger increase of the Shmin gradient for LOT-2245 to comply with the critical stress concept. Accordingly, this option is not further considered in this study.
Critical stresses of SHmax
The range of possible values from Shmin (i.e., measured LOT-2023 and corrected LOT-2245 (SL)) is constrained in Figure 6B. The lower bound of SHmax as defined from LOT-2245 (SL) would correspond to a radial extensional faulting regime. According to our assumptions on a NF regime, SHmax may take any value between Shmin and Sv (segments I to III in Figure 6B) and reaches the limit to a strike-slip regime. For LOT-2023 a stress ratio of SHmax = Shmin = 0.64⋅Sv marks the lower bound for SHmax. With a relation of SHmax = 1.42⋅Sv, a strike-slip regime seems possible, too.
The following discussion includes an assessment of the obtained stress profile in a regional context and with respect to the well-defined Soultz stress field. It should be mentioned that the sedimentary cover at Soultz reaches a maximum depth of about 1,400 m. Consequently, at the depth of the Bruchsal reservoir at Soultz, we are already in the upper granitic reservoir (Genter et al. ).
Stress field variations
Generally, SHmax orientation exhibits a high variability in the upper part of the investigated section of the Mesozoic sediments. Orientations stabilize at about 140° below the Muschelkalk in GB1. With the exception of the Middle Buntsandstein to Upper Permian in GB2, the orientations stabilize below the Keuper at about 130°. Thus, highest variation is observed for the sections with a high clay, gypsum, or anhydrite content. LOT-2023 is located in such lithology. Measured SHmax orientation is by about 30° to 40° off the general GB2 trend in this depth range. The determined Shmin magnitude of LOT-2023 is 3.4 MPa higher than that of LOT-2245. This observation is consistent with results summarized by Cornet and Röckel () and the increase in Shmin by about 4 MPa in the Callovo-Oxfordian argillite of the Paris basin (Wileveau et al. ). It suggests that the leak-off tests in rocks with a low friction coefficient behave generally in this manner, which is also indicated by laboratory experiments (e.g., Krantz ). The stronger rocks of the Middle Buntsandstein can support higher differential stresses without failing than the weaker rocks of the Middle Keuper. It is rather noteworthy that for the same stratigraphic unit, internal deformation processes have also been described (Wirth ). Litho-log interpretations of numerous oil wells of the former oil field Forst, 3 km North of Bruchsal, suggest massive internal shearing and sub-horizontal deformation within the Keuper and especially in the evaporitic layers of the Lower Keuper. With regard to our results, both orientation and magnitude of LOT-2023 show similar deviations and, thus, are not considered to be representative for the reservoir zone of Bruchsal. In contrast, LOT-2245 is located within the section of stable SHmax orientation.
Theoretically, our stress model has a SHmax-depth relation constrained by LOT-2245 (SL) with radial extension as a lower bound faulting regime and NF/SS transition as an upper bound (Figure 8A). Geologically, the validity of the radial extension under the tectonic settings of Bruchsal is, however, questionable. As discussed in Section 2, a transitional to normal faulting stress regime in the central part of the URG in general and in Bruchsal in particular is rather likely. This is consistent with the Soultz stress profiles or determined earthquake fault plane solutions (e.g., Ritter et al. ). Thus, we suggest our model to be valid for NF and NF/SS faulting regimes. It should be mentioned that the Greiner profile is within our range of SHmax for depths larger than 500 m.
The proposed Shmin-depth relation (Figure 8B) presents the steepest gradient (except the results for the uncorrected LOT-2245) and clearly deviates from the earlier Greiner and Rummel and Baumgärtner profiles.
Although located on the opposite side of the URG at the Eastern boundary fault and despite the different lithologies, the determined stress ratios SHmax/Sv of 1.0 and Shmin/Sv of 0.55 for Bruchsal are close to those obtained at the same depth at Soultz (Figure 8C,D).
According to these considerations, a stress range can at best be estimated. To better constrain this, we therefore strongly recommend detailed stress measurements, such as those proposed by Cornet and Burlet () in geothermal wells, in particular with respect to the option of hydraulic stimulation.
World-wide, geothermal projects are often located in active tectonic terrains with highly fractured subsurface reservoir rocks. The evaluation of the stress field in these projects is of main importance. It improves the understanding of the structural setting of the reservoir and supports the definition of an optimum exploration strategy by targeting zones of high transmissivity. The geothermal sites in the URG in central Europe are typically characterized in this context. The Rhine Graben tectonics yields large differential stress levels and favors therefore the existence of large fault and fracture systems. Due to a rather complex history, local evidence of the stress field needs to be evaluated. However, this importance of the stress field has only been pointed out gradually, and many earlier projects provide only little relevant data. In this perspective, new concepts and methodologies need to be considered. Our application of the critical stress concept to measured data may lead to better constrained results and this within their uncertainty of the measurements. The critical stress concept provides a number of solutions for the stress field model depending on possible stress regimes. Using indications such as the local geology and structural setting, the number of solutions has been reduced to normal faulting and transition of normal faulting to strike-slip regimes. Finally, based on new leak-off test data, we have constrained a new stress field model for the Bruchsal area.
Our results show that in accordance with earlier studies, sedimentary layers may have a large influence on both the orientation of the mean principal stress direction and the magnitudes. This applies in particular to clay- and anhydrite-rich layers that occur in this case in the Keuper formations and partly in the Muschelkalk. Stress decoupling and aseismic deformation within this formation cannot be excluded as also indicated by the results from the nearby Forst oil reservoir. We could show that stress magnitudes obtained in sedimentary formations vary significantly due to different rheological characteristics, and a linear stress-depth relation has to be treated only as a first approach. The result from a leak-off test in the Middle Buntsandstein in Bruchsal is, however, consistent with the Shmin/Sv ratio of Soultz. We may attribute this consistency across the entire E-W extension of the URG and different lithologies to small differences in the mechanical properties of the Soultz granitic basement and the sedimentary rocks of the Middle Buntsandstein in Bruchsal.
We would like to thank EnBW Energie Baden-Württemberg AG for providing the well logs and leak-off test data. Financial support has been kindly granted by the Federal Ministry for the Environment, Nature Conversation, Building and Nuclear Safety (BMU), and EnBW Energie Baden-Württemberg AG within the framework of the Logro project (FKZ 0325111 A). Further supports are from the Helmholtz Portfolio GEOENERGIE and the geothermal research center of Baden Württemberg (LFZG). We would like to thank I. Moeck, F. Cornet, B. Müller, and T. Röckel for fruitful discussions.
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