Technischer Bericht NTB 85-02

Sondierbohrung BöttsteinGeologie

Nagra, the National Cooperative for the Storage of Radioactive Waste, is carrying out a comprehensive geological research programme in an area of approximately 1200 km2 in northern Switzerland. This study, begun in 1980, will provide the scientific knowledge required to judge the suitability of the subsurface bedrock for disposal of nuclear waste.

The various investigations comprise a programme of deep boreholes, a regional geophysical reconnaissance of the petrographic and structural conditions, a hydrogeologic programme to classify ground-water chemistry and describe ground-water flow systems in the deep subsurface, and neotectonic observations to detect and measure active crustal movements in the area under study.

The Böttstein well was the first borehole in the drilling programme. Located in the community of Böttstein on coordinates 659'340.8/268'556.0, at an altitude of 347.46 m above sea-level, it lies approximately 10 km to the north of the town of Brugg (Ct. Aargau). Its total depth is 1501 m.

Drilling operations at Böttstein started in October 1982 and continued until June 1983. Except for a 250 m long crystalline interval drilled with wart-studded roller bits, both sediment and basement rocks were continuously cored. The orientation of cores with the multishot technique initially employed was not possible because of the strong turning movement of the inner core barrel. The cores therefore had to be oriented by means of the newly developed sonic televiewer tool (SA­BIS).

The 1250 m of cores from the Böttstein well were studied in detail both at the drill-site and in the laboratory. Apart from stratigraphic, sedimentologic, mineralogic and petrographic studies, the investigations included

  • Structural mapping of the cores (unwrapped core surface on transparent paper)
  • absolute and mercury injection porosimetry
  • permeability
  • fluorescence microscopy
  • specific surface
  • cation exchange capacity
  • thermal conductivity (quick thermal conductivity meter)
  • geomechanic properties
  • main and trace element analysis
  • stable and radioactive isotope analysis
  • fluid inclusion studies
  • maturation of organic material (in sediments only)

The present report contains a complete synopsis of the data obtained up to October 1984. They are interpreted with regard to a reconstruction of the geological evolution of the Böttstein area and with regard to a characterization of groundwater flow systems in sedimentary and crystalline rocks. The report also contains comments on the methodology applied.

After penetrating an approximately 300 m thick sedimentary sequence from Schilfsandstein (Middle Keuper) to Upper Buntsandstein (Lower Triassic), the Böttstein well entered the crystalline basement at a depth of 315.3 m. Down to the final depth at 1501 m, the cry­stalline rocks consist of biotite granite. Based on its mineralogic composition, texture and its tectonic position, this granite belongs to the type of late orogenic, postkinematic Variscan intrusions known from the southern part of the Black Forest Massif.

The stratigraphic sequence from top to bottom is as follows: A 17.5 m thick layer of Pleistocene fluviatile gravels (Niederterrassenschotter) lies directly on the partly eroded beds of the Schilfsandstein (20.5 m in the well). This member contains channel facies (fine-grained sands and silts) in the upper part, and quiet water facies (dolomitic marls) in the lower part. The Gipskeuper (74.8 m) consists of a heterogeneous sequence of clays, dolomitic marls and anhydrites. At their boundaries with the over- and underlying aquifers, the anhydrites have been altered to gypsum. The two members of the "Lettenkohle" (4.7 m) are the so-called "Grenzdolomit" and the Estherienschiefer interfinger. The Upper Muschelkalk can be subdivided from top to bottom into 3 members; the Trigonodus-Dollomit (28.9 m), the Plattenkalk (13.9 m) and the Trochitenkalk (32.4 m). Both Trigonodus-Dolomit and Trochitenkalk contain porous zones (porositity up to 30%) with vugs and pores caused by the solution of fossil shells and anhydrite/gypsum crystals. The Upper Muschelkalk, together with the underlying porous Dolomit der "Anhydritgruppe" (7.4 m), represents the well-known aquifer of the Muschelkalk. The lower part of the Middle Muschelkalk is composed of heterogeneous clayey evaporites of the Obere Sulfatschichten (6.2 m), the Salzschichten (2.1 m halite) and the Untere Sulfatschichten (6.2 m). The predominantly clayey Lower Muschelkalk, with a few intercalations of thin limestone beds, can be subdivided into Orbicularis-Mergel (9.4 m), Wellenmergel (26.1 m) and Wellendolomit (10.6 m). The Orbicularis-Mergel contain a 3 m thick layer of anhydrite; the Wellendolomit has a dolomitic facies only at its base. Below the Muschelkalk the quartzitic sandstones of the Upper Buntsandstein (8.2 m) have strongly varying porosities, and contain carneol in the lower part. The transition to the granites of the basement is abrupt. An early Triassic to pre-Triassic weathering (crumbling) of the granitic subcrop can be observed in the uppermost 3 m only.

The Böttstein granite is a coarse-grained porphyritic biotite granite with large crystals of potassium feld­spar. Its average mineral composition is 37.5% potas­sium feldspar, 27.5% plagioclase, 27% quartz, and 8% biotite. Radiometric dating of the biotites using the potassium/argon method indicates a Lower to Upper Carboniferous age. Aplitic to aplite-granitic, pegmatitic and rhyolitic dykes are frequent but generally rather thin. The tectonic movements caused by the subsiding Permo-Carboniferous trough to the South of Böttstein led to an intensive brittle deformation of the crystalline rocks, demonstrated by the strong fracturing of the granite and the occurrence of kakiritic fault zones. Early hydrothermal alteration accompanying the cooling of the intruded magma, and a late (Permian) hydrothermal "clayification" predominantly associated with kakiritic zones and fractures, account for the mineralogically, as well as petrophysically, very heterogeneous character of the granite. The composition of the replacement minerals both in the granite and fracture fillings is very similar. Clay minerals (inter­stratified illite/smectite, chlorite, and above 600 m depth, kaolinite), calcite, quartz and, above 1000 m depth, hematite have been identified. Absolute porosities vary between 0.2% in fresh granites, to 6.7 % in strongly altered kakiritic granites. The least altered and freshest granite was encountered in the lower part of the borehole between 1075 -1330 m and 1475 -1501 m.

Most fractures in the Böttstein granite are completely filled with clay minerals, calcite, and quartz. The frequency of occurrence of these fractures seems not to be related to the permeability of the surrounding rocks. The water flow observed in very narrow zones of the crystalline rocks is generally restricted to open fractures in pegmatitic to aplitic dykes and only partially healed quartz fractures and quartz veins that occur preferentially in kakiritic fault zones.

The relatively uniform spatial distribution of kakiritic fault zones in the Böttstein granite contrasts with the different and more varying position of the observed dyke rocks. Together, these features produce a complex pattern of intersections which is responsible for an equally complex distribution of flow paths.

Using the fluorescence microscope, it was possible to show the potential pathways of fluid migration between the open fractures and the surrounding porous and hydrothermally altered granites and dyke rocks. For the problems connected with the disposal of nuclear waste in geological formations, this result shows that diffusion, as an important means of nuclide transport from open water-bearing rock spaces into the surrounding rock matrix, is possible.