Technischer Bericht NTB 00-01

Sondierbohrung Benken Untersuchungsbericht

In 1980, Nagra initiated a regional geological investigation programme in Northern Switzerland, with the aim of exploring the underground geological structure of Switzerland in general and the crystalline basement beneath the Mesozoic sedimentary cover in particular. The results from a series of deep boreholes drilled as part of this programme provided key input for Nagra's Project Gewähr 1985. The evaluation of Project Gewähr by the authorities called for sedimentary options to be investigated, in addition to crystalline rock, as potential host formations for disposal of high-level waste.

Based on this requirement, Nagra began to evaluate sedimentary formations in terms of their suitability as host rocks for geological disposal; these studies were initially based on existing information. Efforts were concentrated on two formations, namely the Opalinus Clay and the Lower Freshwater Molasse. As part of a status review, it was agreed, in 1994, with the supervisory authority (HSK) and with experts from KNE and KSA to focus exploration effort on the Opalinus Clay. Important data for this exploration have been provided by the seismic campaign in the Zürcher Weinland (3D seismics), the deep borehole at Benken and the ongoing experimental programme in the Mont Terri Rock Laboratory. This database is supplemented by results from earlier 2D seismic surveys in Northern Switzerland, as well as from other deep boreholes drilled in the region.

The Benken borehole (coordinates: 690'988.80/277'842.90, 404.30 m a.s.l.) is located in the community of Benken in Canton Zürich, approximately 0.7 km SSW of the village boundary and to the west of the main road between Winterthur and Schaffhausen. The borehole reached a final depth of 1007 m, passed through the entire Mesozoic sediment sequence and finally encountered the crystalline basement.

Drilling commenced on 3rd September 1998 and continued until 12th May 1999, with several interruptions to allow scientific investigations to be carried out. The final activities in the drilling phase involved cementing the borehole down to a depth of 827.7 m and perforation of the casing in the intervals planned for long-term monitoring. During the period from 28th June 1999 to 17th July 1999, a multipacker system was installed for long-term monitoring of hydraulic potentials. The entire investigation programme was carried out successfully and as planned.

The drilling and test programmes were directed and managed by Nagra staff. More than 30 technical institutes, consultants and service companies from a range of countries were involved in the investigations.

This report documents only the results of the geological investigations carried out in the Benken borehole and, in this sense, does not draw any direct conclusions regarding the suitability of the Opalinus Clay as a host rock for a radioactive waste repository. Conclusions of this type can be found in a synthesis report, which includes the results of all other relevant investigations carried out (particularly from the 3D seismic campaign and from Mont Terri).

Geological overview

The Zürcher Weinland is located in the tectonically quiet region between the northern boundary of the Molasse Basin and the Tabular Jura. The Mesozoic sedimentary cover outcrops in the Tabular Jura of Schaffhausen and dips beneath the Tertiary deposits at an angle of approximately 3-5° towards the SE; between the crystalline basement and the Molasse, it comprises a sequence of Triassic and Jurassic sediments.

Objectives of the Benken borehole

The overall objective of the investigations in the Benken borehole was to obtain baseline data for the Opalinus Clay and neighbouring rock formations and to calibrate the seismic data with these results. Important individual aims were:

  • Determining the thickness, bedding and lithological and mineralogical/petrographical composition of the sediment sequence.
  • Characterising the fracture and fault systems and determining their spatial orientation.
  • Investigating the rock mechanical properties of the Opalinus Clay and determining the stress field and temperature conditions in the formation.
  • Investigating transmissivities and hydraulic potentials in aquifers and aquitards.
  • Investigating the hydrochemistry and isotope hydrogeology of deep groundwaters in aquifers and of porewaters in aquitards.
  • Acquiring geophysical reference data for calibration of seismic images.
Stratigraphy / lithology

In accordance with the boundary conditions specified in the work programme, the Benken borehole was drilled to a depth of 395 m with a roller bit and was then cored from this depth. This means that, for the first 395 m, drill cuttings are the only material available for geological examination. The core material from the borehole (395 m to final depth) was documented and selected cores were subjected to detailed field and laboratory experiments. Besides stratigraphic, mineralogical and petrographic studies, these experiments also covered structural geology, petrophysical characteristics and geochemical and isotopic properties.

After passing through unconsolidated Quaternary deposits to a depth of 68 m, followed by 131 m of Tertiary deposits, the borehole penetrated through the 784.3 m thick Mesozoic sequence. The potential host rock for a geological repository is made up of the Opalinus Clay (Dogger) and the argillaceous Murchisonae Beds, which are very similar in terms of facies. Together, these formations have a total thickness of 112.34 m (between 539.70 and 652.04 m depth). Below a depth of 983.30 m, the borehole penetrated the crystalline basement, composed of a biotite-plagioclase gneiss.

The Quaternary consists of a vertical and horizontal alternation of clay, silt, sand, gravel and lacustrine chalk. The upper boundary of the sequence is the overlying humus and the lower boundary the start of the consolidated rock. The Tertiary underlying the Quaternary is composed of a 124 m thick layer of Lower Freshwater Molasse (coloured marls and poorly cemented sands and sandstones) and a 7 m thick bean iron ore formation (mostly ochre-coloured clays with inclusions of bean ore).

The Malm, which is approximately 252 m thick, comprises a sequence of limestones with thin marl intercalations. Considered overall, the Malm at Benken is similar in composition to that of the Randen area and the western Swabian Alb where the Malm is characterised by well stratified, massive limestones. Compared with the neighbouring regions, the Malm at Benken is relatively thin, which can be explained mainly by the limited extend of the Effingen Beds.

The Dogger is made up of an approximately 200 m thick sequence of fine sandy claystones and marls with intercalated limestones, calcareous sandstones and iron oolites. In terms of lithological composition and facies, the Dogger is similar to the Brown Jura of the Wutach region and the Schaffhausen Jura. The Opalinus Clay (93.52 m thick) generally consists of dark grey, silty, calcareous and micaceous claystones. To a varying extent, these bear thin silt and sandstone inclusions, lenses and laminae, as well as brownish siderite concretions. At Benken, the Murchisonae Beds (19.68 m thick) above the Opalinus Clay consist of black silty to fine sandy, calcareous claystones and are thus similar, in terms of lithology, to the uppermost Opalinus Clay. They are thus different in mineralogical composition from the Murchisonae Beds in the Weiach borehole.

The Liassic comprises a heterogeneous marine sequence of marls, limestones, silts and claystones around 40 m thick. Despite being generally very thin, these formations can be followed, with the same or very similar lithology, as far as the Swabian Jura and Weiach.

A sequence of claystones and marls around 119 m thick, with intercalations of dolomites, sandstones and anhydrite, make up the Keuper. The sandy formations of the Stubensandstein and Schilfsandstein can be found in the upper third of the sequence, while the lower two-thirds consist of Gipskeuper which is rich in anhydrite. Occurrences of Keuper of similar thickness and composition can be found from the Aargau Jura to the Wutach area. In the Benken borehole, the Keuper can be subdivided into formations known from elsewhere, with only the Stubensandstein deviating from the usual picture due to its greater thickness and porous carbonate breccia.

The Upper Muschelkalk comprises massive porous dolomites, platy, partly oolitic dolomites and micritic limestones, as well as trochite and shell banks. Together, these are around 63 m thick and show the usual division into Trigonodus Dolomite and Main Muschelkalk. Besides laminated dolomites, marls and claystones, the Middle Muschelkalk (around 68 m thick) has an evaporite series with thick anhydrite banks and a rock salt deposit (13 m thick). The Lower Muschelkalk consists mainly of an approximately 34 m thick sequence of dark grey to black claystones with thin sandstone beds and small isolated carbonate banks. Marl is also present and, in the lowest section, dolomitic sandstone banks. Compared with the areas to the north, the Lower Muschelkalk in the Benken borehole is relatively thin; it is subdivided into Orbicularis Marl, Wellenmergel and Wellendolomit.

The Buntsandstein (approximately 8 m thick) consists of greenish and white sandstones and thin cornelian-cemented, clayey sandstones and a reworked horizon. Missing are the coloured, violet-red clayey and sandy palaeosols which are characteristic of the Buntsandstein in the Wutach area. The Buntsandstein in the Benken borehole can be subdivided into Plattensandstein, a cornelian horizon and a reworked horizon.

The crystalline basement (983.30-1007 m) consists of highly metamorphosed, partly migmatic gneisses and an aplite dyke.


A comprehensive geophysical programme was carried out in the Benken borehole; included were petrophysical and structural investigations and seismic measurements.

As expected, the petrophysical measurements in the sediments proved to be very suitable for determining the key formation parameters in situ. Even in the section drilled with the roller bit (down to 395 m), it was possible to obtain a detailed picture of the lithological conditions. Low-permeability, clayey areas can be clearly distinguished from calcareous layers.

Detailed logging of the structures on the borehole wall using a range of probes allowed the drillcores to be oriented to their true position; the spatial orientation of strata, fractures and inhomogeneities could also be determined.

Borehole seismics provides the link between the 3D seismic campaign and the Benken borehole. It was used to image the seismic reflectors close to the borehole and to calibrate them in terms of depth. Comparison of the results from borehole seismics with the results of the 3D campaign allowed the latter to be calibrated and the borehole results to be extrapolated laterally over the Mesozoic sediments in the investigation area.

Rock mechanics

The rock mechanics investigations were focused on characterising the Opalinus Clay and investigating the primary stress field.

The mechanical properties of the Opalinus Clay were found to be strongly anisotropic (transversally isotropic). The dependence of the elastic properties and strength parameters on water content confirm the mechanical interactions between rock deformation and pore pressure (hydro-mechanical coupling). In addition, a transition was observed from brittle to ductile behaviour dependant on water contents. The physical properties of the rock depend to a lesser extent on mineral composition. Although no direction-dependence was observed for Poisson's ratio, the moduli of elasticity show a clear anisotropy in relation to stratification, with Eparallel > E45° > Eperpendicular. The E-modulus (Young's Modulus) varies between 11 and 6 GPa as a function of orientation to stratification; the uniaxial compressive strength is approximately 30 MPa both parallel and perpendicular to the stratification, but is only around 6 MPa at an angle of 45° to the stratification.

To determine the size and direction of the in situ primary stress field, hydraulic fracturing (hydrofrac) tests were carried out and the orientation of borehole wall breakouts and induced fractures was studied. The results of the hydrofrac stress measurements gave amounts for the minimum and maximum horizontal principal stress (Sh and SH); these show clear variations and increase with depth in a non-linear manner. The results for the Opalinus Clay are consistent for the experiments performed at depths of 629.5 m and 632.5 m, with Sh = 14.6 MPa and SH = 19.7 MPa ± 2.6 MPa. The results were consistent in showing a NNW-SSE to N-S orientation of the maximum horizontal principal stress SH.


Particularly for the Opalinus Clay and other borehole sections passing through argillaceous formations, the concept and methodology planned for the hydraulic investigations outlined in the Benken work programme had to be checked first to ensure the success of the subsequent work in the borehole. The low hydraulic permeability of the rock (approximately 1·10-13 m/s) caused the test intervals to react strongly to changes in the pressure field (e.g. due to drilling). The tests were thus designed specifically to keep the influence of these perturbations on the test procedure to a minimum. Single packers were used to isolate a test interval of varying length from the bottom of the hole, while double packers were used to isolate test intervals in any section of the hole.

In general, the best fit with the test data was achieved using a radially symmetric model with two zones (= radial composite). The results for the inner zone can usually be traced back to perturbing effects (e.g. excavation disturbed zone surrounding the borehole) and are thus not representative of actual formation properties. Test results which are relevant for hydraulic characterisation of the rock formations should therefore be derived from the outer zone of the model.

The hydraulic conductivities measured in the Benken borehole vary between 1·10-14 m/s in the Opalinus Clay and 6·10-6 m/s in the Buntsandstein. The figures for the Malm were between 6·10-14 m/s and 1·10-8 m/s. Consistently low values were measured for the Upper Dogger (K = 2·10-13 m/s to 2·10-12 m/s). In the Opalinus Clay (including the Murchisonae Beds), very low conductivities (1·10-14 m/s to 6·10-14 m/s) were measured in the packer tests. The Liassic was also found to have a low conductivity in the order of 3·10-14 m/s. With the exception of the Stubensandstein (K = 1·10-7 m/s), the Keuper also has a low conductivity (Gipskeuper = 1·10-13 m/s). Two tests were performed in the Muschelkalk, giving conductivities of 1 - 5·10-7 m/s. The highest hydraulic conductivity in the borehole was measured in the Buntsandstein (6·10-6 m/s).

The gas entry, or threshold, pressure in the fully saturated Opalinus Clay was measured in test O5. The value of 1.27 MPa, determined as a function of the measured hydraulic conductivity of 1·10-12 m/s (inner zone), is consistent with values from the literature.

All values for the freshwater potential from the Malm show slightly sub-artesian conditions (approximately 11 - 20 m below terrain). The potentials generally increase massively from the Upper Dogger, in the Opalinus Clay to the Liassic. In this section, the measured potentials are 50 - 150 m above terrain. In the Upper Keuper (Stubensandstein), the potential is 464 m a.s.l. (approximately 60 m above terrain). In the lower section of the borehole (Muschelkalk and Buntsandstein), the potential is close to ground surface (404 m a.s.l.).

Following completion of the drilling and test activities, a multipacker system with nine observation intervals was installed in the borehole. This system is used for long-term monitoring of deep groundwaters.


The hydrochemical investigations in the Benken borehole had two main objectives. The first was to study the physico-chemical and isotopic composition of the deep groundwaters in the regional aquifers and in the water-bearing strata above and below the Opalinus Clay, while the second was to characterise the porewater in the Opalinus Clay and surrounding low-permeability formations. The results provide a better understanding of regional groundwater flow conditions and are an important factor in explaining material transport in the host rock and surrounding formations. As was expected, conventional groundwater sampling proved impossible for the very low permeability Opalinus Clay and the over- and underlying clay-rich formations. Special laboratory experiments on drillcores were therefore carried out to characterise the porewaters of these clayey lithologies.

The programme for characterising the porewater in the argillaceous formations comprised the following:

  • measurement of isotope ratios (δ18O and δ2H) using vacuum extraction and diffusive isotope exchange
  • measurement of noble gases
  • determining cation-exchange capacity and concentrations of soluble salts in the rock
  • investigation of the chemical composition and stable isotopes of expressed porewater.

For the groundwater in the well stratified limestones (Wohlgeschichtete Kalke) of the Malm, the high mineralisation, isotopic compositions and high He concentrations point consistently to a very old formation water of marine origin (Na-Cl-(SO4)-type), which has been modified by rock/water interactions (and possibly by mixing). The highly mineralised Na-SO4-(Cl)-type water in the Keuper is of meteoric origin and evolved mainly in the lithologies of the Keuper. The composition of stable and radiogenic isotopes, dissolved noble gases and high He contents are consistent in indicating a long residence time and infiltration during an interglacial period for the Keuper water. The Ca-Mg-SO4-(HCO3)-type water in the Muschelkalk and the Na-HCO3-Cl-(SO4)-type water in the Buntsandstein show a much lower degree of mineralisation. Based on its 14C activity, the Muschelkalk water is believed to have a residence time underground of approximately 13,700 years. This is in agreement with the composition of the stable isotopes, the relatively low He concentrations and the dissolved noble gases, which indicate that this water infiltrated during the last ice age. The Buntsandstein water contains no 14C and a minimum residence time of > 26,000 years can be calculated; this is consistent with the high He contents and the stable isotopes and dissolved noble gases, which indicate that infiltration took placed under colder climatic conditions than at present. The significant concentrations of radiogenic 4He and 40Ar are indicative of a long circulation time in a crystalline environment for this water. The deep groundwaters from the Malm, Keuper and Muschelkalk have very high 3He/4He ratios and the Malm and Keuper waters also have high 40Ar/36Ar ratios.

For the porewaters in the central part of the Dogger, chloride concentrations are in the order of 5,700 mg/l (approximately 0.16 M); for the distal zone at the base of the Opalinus Clay the values are around 3,000 mg/l (approximately 0.085 M).

From the base of the Malm to the top of the Keuper, the chloride concentrations, chlorine isotopes, stable water isotopes and dissolved noble gases in the porewaters show individual but typical concentration profiles, which can all be explained as diffusion profiles. The chloride concentrations and chlorine isotopes indicate that the boundary conditions for the build-up of their diffusion profiles must have changed many times during the course of geological evolution. These changes are reflected only partly in the stable water isotopes and noble gases.