Technischer Bericht NTB 84-34

On the occasion of a review of safety aspects related to possible earthquake occurences in the area of the two nuclear power reactors at Beznau, several shallow boreholes were drilled on behalf of Nordostschweizerische Kraftwerke AG on Beznau island in the river Aare during winter 1979/80. One of the borings (coord. 659491/267242) located. approx. 75 m NNE of reactor building II was cored over its total length and taken to a greater depth of 321,8 m.

Because this weIl offered a good opportunity to gather important results for the research programme of Nagra in northern Switzerland, it was investigated in a very comprehensive way with sedimentological, mineralogical and geochemical methods and with geophysical, rock-mechanical, hydrogeological and radio-isotope techniques. This is the first deeper borehole drilled in Switzerland that has been evaluated to such a great extent.

Below a 20,3 m thick layer of Quaternary deposits, 301,5 m of an almost horizontal sedimentary series of the Tabular Jura was penetrated. This series extends from the lower Dogger (middle part of the «Opalinus-Ton») into the Triassic middle Muschelkalk («Obere Sulfatschichten» of the «Anhydritgruppe»).

A 45 m thick sequence of the middle and lower «Opalinus-Ton» (upper Toarcian-Aalenian) underlies the Pleistocene erosion surface. It consists of a series of seemingly monotone, grey, finely laminated silty clays and clayey siltstones with a CaCO₃ content of approx. 10%. The whole interval can be subdivided into 6 distinct types of lithological facies. The clay content of the formation increases with depth. This is demonstrated, on the one hand, by decreases in rock density, quartz content and microporosity and, on the other hand, by increasing values for specific surface and Al₂O₃ content. The organic carbon content also shows a linear increase with depth.

On the evidence of ammonites of the species Pleydellia found in some of the cores, the lowermost beds (12,36 m) of the «Opalinus-Ton» must be placed in the upper Toarcian.

In addition to this Liassic part of the «Opalinus-Ton» the Upper Lias («Jurensis-Mergel», 3,43 m, and «Posidoninenschiefer», 5,93 m) is represented by marlstones and calcareous clays. Strongly bituminous layers with up to 9% C_org occur in the «Posidonienschiefer». The Middle Lias (2,19 m) consists of an incomplete sequence of glauconitic fossiliferous micrites. The Lower Lias is represented by 13,66 m thick clayey siltstones («Obtusus-Tone»), 1,14 m of clayey biomicrites, ferruginous oolites («Arietenkalk») and thin bedded clays («Angulatenschichten») and 6,38 m thick bituminous (0,58% C_org) marls («Insektenmergel»).

The 12,06 m thick «Obere» and «Untere bunte Mergel» of the uppermost Keuper are developed as crumbly dolomitic marls. Intercalated in these marls is the 4,16 m thick «Gansinger-Dolomit» with its characteristic dolomite breccias and numerous solution vugs.  

The clay mineral association in the sediments of the Dogger, Lias and uppermost Keuper consists very uniformly of illite, kaolinite, chlorite and interstratified illite/montmorillonite. The more clayey rocks are characterized by a higher proportion of kaolinite. In the samples of the «Obere bunte Mergel» there is no kaolinite, whereas the proportion of illite and interstratified illite/montmorillonite is high. In the «Untere bunte Mergel» only chlorite in addition to illite was found.

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Technischer Bericht NTB 84-34

The 16,5 m thick «Schilfsandstein» (Middle Keuper) can be subdivided into an upper microcyclic series, a middle interval of massive, porous and fine-grained sandstones and a clayey-silty series at its base. The clay minerals form an association of illite, kaolinite (both partly authigenic in pores), chlorite and interstratified illite/montmorillonite.

The 85,15 m thick interval of the «Gipskeuper» is composed of clays, dolomites and mainly anhydrites in different structural manifestations. Sedimentological analysis permits the subdivision of this complex formation into 7 sub-units. The evaporitic facies is dominant. Only at the contact to the porous, water-bearing rocks of the overlying and underlying formations, are the anhydrites replaced by gypsum. Corrensite is the characteristic clay mineral of the «Gipskeuper». 

Based on facies considerations the 4,0 m thick «Lettenkohle» forms part of the upper Muschelkalk. It consists of an interfingering of the so-called «Grenzdolomit» with the thin «Estherienschiefer». In this succession, illite and kaolinite are the only clay minerals.

Close to the upper boundary of the «Trigonodus-Dolomit» (34,5 m), a chert containing dolomite layer with accompanying oolite was found in the cores. This key horizon is widespread in the Swiss Jura mountains. Underneath follows a 9,6 m thick sequence of porous dolomites with bioclastic intercalations, the so-called «Kaistener Schichten». They are underlain by massive to thinly bedded, finely crystalline dolomites. The strikingly high macroporosity of the «Trigonodus-Dolomit» is caused by vugs, joints and dissolved fossil shells. The pores are partly lined with crystals of calcite and frequently filled with white kaolinite.

The «Plattenkalk» (17,35 m) comprises interbedded dolomite and micrite. A 3,45 m thick biomicrite containing lammelibranchs and crinoids forms its base.

The «Trochitenkalk» (19,2 m) is developed in trochitic facies only in its uppermost part. Grey micrites are predominant with an oolite bed at the base.

The «Anhydritgruppe» of the middle Muschelkalk was penetrated in part only. It is composed of 11,45 m of beige-coloured, frequently stromatolitic dolomites with layers of celluar dolomite («Rauhwacke») and breccias displaying very high secondary porosity. This porosity is caused by dissolution of crystals and small nodules of anhydrite respectively gypsum.

The borehole reached its final depth in the «Obere Sulfatschichten» of the middle Muschelkalk after drilling through an 18,95 m thick section of this formation. It is composed of dolomitic and partly evaporitic marls which are underlain by massive anhydrite.

The content of radioactive elements in the rocks is low throughout. There is very good agreement between integral gamma-activity (broken down into K-, U- and Th-radiation intensity) and gamma-ray-Iog. In the interval of the «Opalinus-Ton» the radioactivity continuously increases with depth. This is due to the increase of all radioactive elements, whereby uranium accounts for the smallest contribution to the total activity.

Measurements of heat conductivity parallel and vertical to the bedding planes gave values that are considered typical for the different kinds of rock encountered in the well. The mean values measured in W/m °K lie between 1,14 («Insektenmergel») and 4,07 («Obere Sulfatschichten»). The pronounced variations within certain formations are caused by their lithological inhomogeneity. The heat conductivity of the clayey formations is clearly dependent on depth, as demonstrated by the values for the «Opalinus-Ton».

The cation-exchange capacity of all the analysed samples varies between 15 and 28 mval/100 g. It is primarily determined by illite and interstratified illite/montmorillonite but also by corrensite («Gipskeuper»). The porosity of clays and marls is dependent on the clay content of the sample and lies between 5% and 10%. The proportion of interconnected pores varies between 50% and 99%, with 70% as the most frequent value. The pore radii have their frequency maximum around 100 Å. High macroporosities were observed in the «Gansinger-Dolomit» and in the «Dolomit der Anhydritgruppe»

Strain measurements on samples from the «Opalinus-Ton» gave linear dilatation coefficients around 10^-5, both parallel and perpendicular to the bedding planes.

From 15 Jurassic and Triassic core samples polished sections of Kerogen concentrates were prepared to determine by reflected light microscopy the maturity and the maceral composition. Most samples analysed contain hydrogen lean Kerogen with a low H/C-ratio. Only Kerogens of the «Posidonienschiefer» and of some very thin interbeds in dolomites of the «Anhydritgruppe» consist predominantly of algae and hence reveal enhanced H/C-ratios. Although there is considerable scatter in the depth-plot of the vitrinite reflectance values (partly measured directly, partly converted from measured spectral fluorescence parameters), a maturity gradient of 0,10% R_m/100 m was estimated for the total depth interval investigated. Based on its maceral composition, especially its enhanced liptinite content, the «Posidonienschiefer» was recognized as the only potential source bed in the sequence penetrated by this well. The organic matter of this rock has reached a maturity level equivalent to the onset of hydrocarbon generation.

These petrographical results were confirmed by the measurement of yield and composition of the low and high molecular weight hydrocarbons (C₂-C₇ and C₁₅₊, respectively) extracted from clays of Lias and Dogger intervals. From the composition of the low molecular hydrocarbon fraction it could also be concluded that the uppermost 18,16 m of the «Opalinus-Ton» are influenced by weathering processes, a fact that was not evident from the mineralogical examination of this zone.

The borehole was logged over its total length with different geophysical tools. To this end electric, radiometric and acoustic methods as well as techniques for measuring formation temperature, borehole geometry, and water inflow were employed.

The resistivity logs as well as the radiometric logs fully confirmed the lithological boundaries established by sedimentological core analysis. The gamma-ray log shows relatively high radiation values in glauconitic beds of the Upper Lias and in clays and clayey marls of the Upper Keuper. The various values for rock densities, calculated from the gamma-gamma log, correspond generally well with values obtained from core samples in the laboratory. The curve characteristics of the neutron-gamma log are remarkably analogous to those of the resistivity log, both for the cased as well as the uncased borehole interval.

From the acoustic log exact velocities of longitudinal waves (p-waves) typical for different rocks could be determined. Down to a depth of 200 m only the transit time in the hard interval of the «Gansinger-Dolomit» gives values greater than 4000 m/sec. Below 200 m all acoustic velocities exceed 4000 m/sec except for the strongly porous «Trigonodus-Dolomit» and some fractured and brecciated layers in the upper part of the «Anhydritgruppe».

On the temperature logs a slow rise of the borehole temperature gradient from 0,04°C/m to 0,07 °C/m was registered between 30 m and 130 m depth. In the probably undisturbed interval between 314 m and final depth at 321,8 m the gradient is 0,038 °C/m with a maximum temperature of 28,16°C.

A combined interpretation of temperature, salinity, caliper and flowmeter logs made it possible to locate very accurately the points of groundwater entry in the well and to calculate the individual influx volumes. The enlarged hole diameters in the Muschelkalk at 230 m and 300 m depth correlate with observed water inflows from highly pourous and partly brecciated dolomites.

The rock mechanical characteristics of the «Opalinus-Ton» were investigated both in situ and on core samples in the laboratory. On the strength of their mechanical properties, these argillaceous sediments are defined as rocks. The reference values determined by the experiments show a considerable spread but no sudden fluctuations and only an insignificant tendency to change with increasing depth. This confirms the relatively homogeneous lithological and mineralogical structure of the penetrated 45,4 m of the formation.

With 9 in situ-measurements the moduli of deformation for the penetrated rocks were determined. From a depth of 38 m down to the bottom of the hole they gave rather uniform mean values. The considerably lower moduli at 26,5 m and 32,5 m are probably due in the first case to weathering and in the second case to a fractured zone of disturbance.

Characteristic of the investigated rocks are their low compressive strength and low shear strength, particularly parallel to the bedding planes, as well as their pronounced swelling tendency.

Below the productive groundwater stream in the Quaternary sands and gravels of the Aare valley the only other aquifers encountered in the borehole occur in the Triassic sequence. These aquifers are

• the slightly confined waters of the «Gansinger-Dolomit» (production 3 l/min with a small pump),
• the uppermost portion of the «Trigonodus-Dolomit» (outflow approx. 16,1 l/min),
• the «Trochitenkalk» (outflow 6,8 l/min) and
• the «Dolomit der Anhydritgruppe» (outflow 10,4 l/min).

All the waters in the Muschelkalk are confined, with overpressures reaching to approx. 15 m above ground-level. At the surface the total outflow from the Muschelkalk amounted to approx. 33,3 l/min, with an average temperature of 22-23°C.

The water from the «Gansinger-Dolomit» has a total mineralization of 14683 mg/l. It is characterized as a subthermal water (approx. 17,6°C) containing Na, Ca, Mg, S0₄, CI and, as minor constituents, Li, F, I, Fe and Br. In contrast, the thermal waters from the Muschelkalk show a considerably lower mineralization (6124 to 6951 mg/l). They are chemically very similar among themselves, containing Na, Ca, Mg and SO₄ with Li, Fe, F and Br as minor constituents and compare thus rather well with the thermal waters of Baden. The gases dissolved in the waters from the Muschelkalk show a relatively high percentage of helium. This inert gas presumably originates from the crystalline rocks of the basement.

On 6 groundwater samples several isotope analyses have been carried out. In order to arrive at a significant and meaningful interpretation of these obtained data, these measurements need to be placed in a wider regional context. This will be possible once the isotope determinations from the deep boreholes of the Nagra drilling programme in northern Switzerland and from outflows of mineral and thermal waters analysed within the framework of Nagra's regional hydrogeological investigations become available. As a preliminary, and in many respects still tentative result, residence times of several thousand years are indicated for the water of the «Gansinger-Dolomit». For the waters of the Muschelkalk the residence time, according to the ³⁹Ar-result, is more than 1000 years, according to the ¹⁴C-result probably more than 20'000 years. No significant admixture of recent fresh water could be deduced from the ³H- and ⁸⁵Kr-determinations.