Technischer Bericht NTB 94-01

In connection with the geothermal borehole project at Bassersdorf, which was aimed at using the geothermal energy from the deep groundwater of the OMM (Upper Marine Molasse) to provide heating for a new nursing home, a wide range of geological investigations was carried out in the OMM and – in the extended borehole ­additionally in the USM (Lower Freshwater Molasse).

The main objective of the geothermal borehole – namely commercial exploitation of geothermal energy – was achieved. Pumping has to be at a rate of approx. 3 Is^-1 to realise the output of around 0.2 MW required for heating the building and this would cool the deep groundwater from approx. 23° C to 8° C. The maximum supply rate which is theoretically possible is around 7 Is^-1. There is no problem with allowing the deep groundwater pumped out of the borehole to seep away or directing it into the main drainage facility, because of its hydrochemical composition. Analysis of hydraulic tests has shown that there is no mutual influence or impairment of functioning between the two geothermal wells Bassersdorf and Kloten-1.

This report documents the operational investigations carried out as part of the Bassersdorf geothermal borehole project; the results of these investigations are also presented and interpreted in the report.

The section down to the bottom of the OMM was rotary drilled. The drill cuttings were characterised, the heavy minerals in the OSM (Upper Freshwater Molasse) and the OMM were analysed, geophysical borehole logging in the OMM, pump tests and fluid logging in the OMM and hydrochemical studies of the OMM waters were carried out. The aim of these investigations was to provide a geological, hydrogeological, petrophysical and hydrochemical characterisation of the OMM as a whole, particularly with reference to exploitation of geothermal energy from this formation.

The additional studies in the USM involved extending the borehole as a cored section by around 250 m, surveying the cored section in detail with a description of the lithofacies and division into architectural elements, division according to heavy minerals, mineralogical-petrographical and petrophysical laboratory investigations, geophysical borehole logging and interpretation of data, fluid logging and packer tests. The aim of this part of the programme was to provide a geological, petrophysical and hydrogeological characterisation of the drilled sections of the USM, particularly the “Upper Aquitanian Marl Zone" ("Oberaquitane Mergelzone", "OAMZ").

The composition of the OSM consists mainly of siltstones with differing clay, sand and carbonate contents; these were deposited primarily on a flood plain with marshes and shallow lakes in the axial part of the foreland basin. The OSM also contains fine sandstones with variable silt and clay contents which were deposited in crevasse channels and crevasse splays or as meander belt sands. These sediments can be assigned to the Alpine "Hörnli Deposition" (Hörnli-Schüttung); three interference zones of the "E-W Mica Sand Deposition" (E-W-Glimmersand-Schüttung) can also be identified.

In general terms, the OMM in the Bassersdorf borehole can be divided into a shallow upper section, which was deposited under nearshore conditions, and a thicker lower section laid down under a predominantly (deeper) marine deposition regime.

The petrophysical description of the OMM includes 6 lithofacies types which can be interpreted as carbonate deposits, conglomerates, coarse medium sandstones, fine to medium sandstones, siltstones and claystones. In addition to the predominantly fine sandy sediments, there are also "Muschelsandsteine", i.e. carbonate beds with echinoderm fragments; these occur mainly in the lower section. Conglomerate beds occur mainly in the upper section. The log facies in the lower section of the OMM show predominantly inversely graded coarsening upward sequences; in an upward direction towards the base of the upper section, these show a transition to normally graded fining upward sequences. Above this, the conglomerate deposits appear almost immediately. This situation suggests a change in the large-scale deposition regime, presumably caused by a drop in sea-level, which is associated with the advance of deltaic deposition systems from the regions of the large alluvial fans in the south. Heavy mineral investigations indicate, for the upper section, that sediments were introduced from the Hörnli and partly from the Bodensee alluvial fans; the lower section can be assigned predominantly to the "Bodensee Deposition" (Bodensee-­Schüttung). Based on log facies interpretation, parallels can be drawn between this boundary and the transition from fining upward to coarsening upward cycles. Comparison of the results from geological and geophysical investigations show that marked log facies types can be calibrated and characterised based on the description of the drill cuttings.

Hydraulic tests gave a bulk transmissivity (T) of 5.1 x 10^-5 m²s^-1. Fluid logging results show that around 90 % of the transmissivity can be accounted for by the permeable conglomerate zone at a depth of 315 – 340 m. Converted for this 25 m thick zone, this gives a transmissivity of approximately 4.6 x 10^-5 m²s^-1. This can be interpreted as a hydraulic conductivity (K) of 1.8 x 10^-6 ms^-1 (porous medium with unrestricted lateral extent). A hydraulic conductivity of 1.9 x 10^-8 ms^-1 can be derived for the residual component of the transmissivity of approximately 0.5 x 10^-5 m²s^-1 in the remaining OMM section; this is two orders of magnitude less than in the conglomerate zone.

Based on the results of hydrochemical and isotope hydrology investigations, the deep groundwater encountered in the OMM can be characterised as an Na-CI-SO₄-HCO₃ water, i.e. a mixed water of the Na-CI type with a certain component of Na-HCO₃ water which infiltrated under ice age conditions.

Based on geological and petrophysical investigations, the USM can be divided into an upper silt/clay-rich section (the so-called "Upper Aquitanian Marl Zone", "OAMZ") and a lower sand-rich section.

The profile shows a lithological change over a short vertical distance. The sedimentation cycles, which are mostly 2 – 3 m thick, show an increase in grain size from bottom to top (coarsening upward) and are divided from one another by proto­soil horizons. They can be interpreted as deltas which prograded into lakes, or as crevasse splays. In the Bassersdorf borehole, the USM is characterised by an absence of sandstone sequences of the meander belt type.

Based on information from core studies, the lower boundary of the "OAMZ" is formed by a thick palaeosol. The "OAMZ" can be distinguished in the geophysical logs and the heavy mineral profile by its generally higher carbonate content. This deposition sequence is dominated by sediments from the Hörnli alluvial fan; based on heavy mineral investigations, the lower section of the USM, including the palaeosol, can be assigned to the "Napf Deposition" (Napf-Schüttung).

In the absence of reliable test results, the hydrogeological characterisation of the USM is restricted to qualitative or, at best, semi-quantitative information. Laboratory values for absolute porosity mainly from sandstones of crevasse channels and splays as well as meander belts vary from 3 to 23 %, while laboratory measurements of permeability cover a range of values from < 0.01 – 500 md.

There are presently no suitable samples available for investigating the hydrochemistry of the deep groundwater from the USM.