Deutsch (27.2 MB) - Nagra

iotite-plagioclase-gneisses (partly hornblendebearing)
with acidic dyke rocks. The latter make up
around 18% of the drilled section in the Crystalline.
The original material for the gneisses is assumed to
be dolomitic claystones with a low aluminium content
up to argillaceous to dolomitic graywackes
which were subjected to overprinting by a regionalmetamorphism
(amphibolite facies) with slight migmatization
during the Caledonian orogeny.
During the course of the Late Variscan-Upper Carboniferous
movements and granite intrusions
(around 300-320 Mio. years), the rocks at Weiach
were partly subjected to intensive tectonohydrothermal
overprinting. A combination of tectonic movement
and convective circulation of a hot H20-NaCI
fluid (possibly subject to local overpressures) led to
marked cat aclasis and hydrothermal alterations. The
metamorphic minerals were altered by the supply of
H20 (± C02), involving mainly intergranular material
exchange processes with conservation of overall rock
chemistries. Ore-forming enrichment processes
therefore did not occur. Alterations took place
under retrograde conditions at temperatures below
400°C and pressures below 1 kb. There must have
been a strongly increased geothermal gradient which
was probably caused by shallow intrusions in the
environment.
The calcite fractures, CaCh-rich fluid inclusions and
Ca2 -+ - interlayer coatings of the layer silicates
occurring in the Weiach Crystalline could all be evidence
of Permian movements. Intensive Permian
brittle deformation and breaking down into clay such
as is found in the Crystalline of the Bottstein, Leuggem
and Kaisten boreholes is not present in Weiach.
Over 90% of the fractures are healed with new formations
and contain mainly chlorite and calcite.
Open fractures were observed only in the aplites.
Sub-division into different fracture systems is not
possible.
Use of petrophysicallogs allowed a clear distinction
to be drawn between gneisses and aplites, even in the
bitted sections. However, further sub-division of the
gneisses and clear identification of the structures
present was not feasible.
For both the Crystalline and the sediments, parameters
such as composition of the clay minerals, rock
chemistries (main- and trace elements), cation
exchange capacity, rock and grain densities, specific
surfaces and thermal conductivity were determined,
the aim being, inter alia, to obtain base data on
radionuclide retention in the rock.
Borehole geophysics
A comprehensive geophysical investigation programme
was carried out in the Weiach borehole. The
resulting data-set was compared with the results of
drill-core analyses and tests carried out in the borehole.
This allowed detailed information to be obtained
even in sections with core-loss and in bitted sections.
As expected, the petrophysical logs carried out by
the Schlumberger company proved very suitable in
the sediment section from the point of view of measuring
the most important formation parameters "in
situ".
In addition a quantitative computer analysis using
the @ GLOBAL programme was carried out in the
sediments. This allowed identification of the rock
formations drilled through, independent of the drillcores.
It was also possible to make a volumetric analysis
of the most important mineral components such
as quartz, feldspar, calcite, dolomite, ankerite/siderite,
anhydrite, iron ore and coal/tuffites, as
well as of the porosity. The well-logging in the Crystalline
is also sufficiently precise to allow recognition
of qualitative features of the formations drilled
through; quantitative interpretation is not possible
since the specific composition of simultaneously
occurring minerals and the effects of material migration
and trace elements can not be calculated.
Nevertheless, the lithology in the hitted sections
could be determined satisfactorily using the
@ LITHO-analysis.
The orientation of the foliation and cleavage in the
sediments was determined using log data obtained
with the dipmeter probe. In the Crystalline the interpretation
of the dipmeter log was possible only to a
very limited extent since it is not always possible to
distinguish the cleavage from cataclasites and schistosity
planes.
Borehole seismic and gravimetric measurements provided
the basis for the recalibration of the sonic and
density logs and allowed precise, reliable interpretation
of the seismic reflection and gravimetric surface
data.
Evaluation of the geothermal data showed a correlation
between thermal conductivity and seismic velocity,
with a marked rise in the temperature curve in
the vicinity of the "heat-accumulating" coal intercalations.
The average geothermal gradient in Weiach
can be taken as 41-46°C/km.