Technischer Bericht NTB 86-03

The Schafisheim borehole was drilled in the borough of Schafisheim, situated in the Molasse Basin of the Swiss Mittelland, 5 km to the S of the Argovian Folded Jura (Coordinates 653'620/245'760, altitude 421.2 m above sea level). The location chosen lies 13 km W along strike from the Born-Engelberg anticline situated at the southern foot of the E Jura Mountains. Drilling work commenced on 26^th November 1983 and continued until June 1984. Down to a depth of 960.90 m, drilling was mostly carried out using a roller bit; beneath this and to maximum weIl depth coring was almost continuous. Coring permitted the first recovery from the Triassic evaporites of the main Jura thrust décollement horizon, the existence of which had been suggested but not previously proved.

The 1489.84 m thick sedimentary succession is divided into a 244 m thick Quaternary including lake deposits sequence, 322 m of Lower Freshwater Molasse, 10 m of Eocene, a 264 m thick Malm, 239.95 m of Dogger, Lias (25.60 m), Keuper (122.72 m), Muschelkalk (249.93 m) and 11.64 m of Buntsandstein.

The Mesozoic stratigraphy of Schafisheim shows good correlation with that of the nearby Argovian Folded Jura. The youngest Malm strata penetrated were the Wangener Schichten; i.e. the early Tertiary erosion cut down as far as the Upper Oxfordian. The Dogger contains a 73 m thick Hauptrogenstein (oolite), thus providing additional confirmation of the south-eastward extent of the Burgundy Platform. The Opalinus Clay (79.3 m) may be divided into four subunits according to the presence of sandy horizons and carbonate beds. The Lias shows a strongly reduced upper part, an absence of Posidonienschiefer and a thick sandy Arietenkalk (sensu lato), all characteristics of the Solothurn Jura facies. The muddy-marly Middle Keuper contains a relatively thick Schilfsandstein (22.22 m). The Gispkeuper can be divided into the same five subunits as at Böttstein and Weiach. The Upper Muschelkalk (59.44 m) includes a porous Trigonodus Dolomit with common anhydrite pore-fillings. A reverse fault was recognised within the Middle Muschelkalk. Lithological and litho-density-Iog correlation indicated tectonic repetition of the beds. The Anhydrit Gruppe is characterised by a thick layer of rock salt (total 37.78 m). The Lower Muschelkalk (36.41 m) occurs in typical mud-marI facies. The Buntsandstein consists of nottled and white sandstones containing mud and jasper. The sand composition and fabric show evidence of strong diagenesis.

Laboratory determinations of absolute and open porosities showed values of 10 % for sampIes from the Lower Freshwater Molasse, the Schilfsandstein and the Trigonodus Dolomit.

Structural analyses revealed that ductile deformation affected both" the Salzschichten and the anhydritic horizons of the Upper Sulfatschichten and Gipskeuper as a result of shear during thrust emplacement. The Lower Muschelkalk and the Buntsandstein remain in stratigraphic sequence above the Palaeozoic basement.

The carbonate Malm, the Hauptrogenstein and the entire succession from the Opalinus Clay to the Upper Sulfatschichten are all strongly fractured. The old, steeply-inclined (pre-Triassic) fractures can be distinguished from the orthogonal set of younger faults (synkinematic Alpine shear and glide planes). Use of horizontal stylolites as stress indicators suggests a generally S-N oriented principal stress field.

The crystalline basement consists of Variscan intrusive rocks with a thin intercalated gneiss band. The upper part of the basement is formed of biotite granite. A complex alternation of syenites, monzonites and diorites was penetrated in the lower part of the borehole. These mafic rocks give radiometric ages of 315 Ma, and are cross-cut by steeply-dipping veins of aplite and aplite granite which can be geochemically correlated with the biotite granite. The mafic lithologies are comparable to those of the so-called "Durbachites" of the northern Black Forest.

The basement was subjected to intense brittle deformation, which led to cataclasis of 90 % of the granites and 60 % of the syenites encountered. The intensity of cataclasis, 10 to 20 times greater than at Böttstein and Leuggern, reflects the location at the edge of the Permo-Carboniferous trough. The lower intensity of syenite cataclasis resulted from their mica content, which is higher than in the granites.

Hydrothermal alteration and partial plagioclase sericitisation occurred during cooling of the intrusive rocks. Strong influx of water-rich fluids during pre-Triassic cataclasis led to albitisation of the plagioclase, and not to its replacement by clay minerals as observed in the Leuggern and Böttstein boreholes. Illite, mixed-layer illite-smectite, and trioctahedral smectite fracture fillings are assigned to this phase. Fluid inclusion data suggest that the younger, near-vertical, calcite-filled fractures were filled by precipitation from hypersaline, CaCl₂-rich solutions. Similar fractures also occur in the overlying sediments and may be linked with Alpine movements.

The cation exchange capacities of granite and syenitic sampIes reach 5 – 12 mVal/100 g. Carbon and oxygen isotope characteristics of investigated fracture minerals indicate disequilibrium with present-day groundwaters.

The most important petrophysical parameters are (mean values given):

Parameter Unit Biotite-Granit syenitische Gesteine

 fresh

 intensively cataclasic

 fresh

 cataclassic

rock density g/cm³ 2.59 2.49 2.77

 2.63

absolute porosity

 Vol. % 1.5 5.9 1.4 2.3

open porosity Vol. % 2.4 4.7 1.5 3.5
 

specific surface m²/g –  12  –  100
 

thermal conductivity WmºK 3.2 2.77 2.10 2.10
 

hydraulic conductivity ^-1

 m/sek ^-12

 – 12^-

 5.5 – 13·10^-12

 –^-12

 70·10^-15

The macroporosity (> 7 μ) results predominantly from dissolution of cataclastic matrix and of calcite fracture fillings; the microporosity occurs as grain boundary porosity in the granites and aplites and as sheet-silicate porosity in the syenitic rocks. Interconnection of open fractures with the rock porosity is certain in most cases.

The Schafisheim borehole forms the southernmost extension of Nagra's hydrogeological testing / sampling network in Northern Switzerland. Among the consolidated sedimentary lithologies, the carbonate formations of the Middle Muschelkalk are cut by open fractures and are considered to have potential as fracture aquifers. The sandy parts of the Lower Freshwater Molasse and particularly the Buntsandstein show notable porosity and form aquifers. Extensive groundwater flow is also inferred for the open fractures in the crystalline basement which chiefly occur either in cataclastically-deformed biotite granites or in aplite-syenite alternations in the upper and lower parts of the basement succession respectively.