¿ìè¶ÌÊÓƵ

ON 8 SEPTEMBER, several hundred German dignitaries, scientists and engineers,
together with guests from 10 countries, gathered in the small Bavarian town
of Windischeschenbach to inaugurate the main borehole of the German Continental
Drilling Project (KTB).

At 83 metres high, the drilling rig is the biggest in the world. And
when it reaches its target depth of 10 kilometres in 1994, the hole will
be the second deepest ever; only the USSR borehole on the Kola Peninsula,
currently 12 kilometres deep, surpasses it.

The borehole will cost more than 500 million marks to complete; original
estimates were 450 million marks. The money comes from the German Ministry
of Research and Technology, in addition to the usual allocation for earth
science. The high level of funding, huge compared to most geological research,
arises because the KTB is seen as a prestige project. The coordinator, Rolf
Emmermann of Geissen University, says that the KTB, together with deep seismic
profiles, are ‘major German contributions to the understanding of the central
European crust’. This emphasis on pure research for such a major project
makes it unusual. But the technical benefits in terms of better drilling
techniques and the exploitation of geothermal power in the future also look
promising.

The pilot borehole, finished at a depth of 4 kilometres, has already
sprung some surprises for geologists. This part of central Europe has a
different structure from that inferred from seismic profiles of the area.
There is far more fluid circulating through the Earth’s crust here, and,
below about 500 metres, the rocks are hotter than researchers had thought.

Windischeschenbach was chosen because of its position in the geological
structure of Europe. The town is in the Oberpfalz, between three blocks
of Precambrian rocks – Eurasia, North America and Greenland, and Africa
– that were once part of the supercontinent Pangea. The drill will have
to cut through tough crystalline rocks, known as basement because they lie
beneath the surface sediments. The Bohemian Massif, stretching east from
the Oberpfalz, is the largest area of basement rocks at the surface in central
Europe, giving geologists plenty of basic data to work with.

In addition, the German Continental Seismic Reflection Programme (DEKORP)
has made a seismic profile reaching 30 kilometres deep below this area.
The DEKORP line picked up strong reflections from layers of rock and fault
zones that were, in the main, only gently tilted. In fact, one of the reasons
for drilling at Windischeschenbach was to find out what caused the reflections.
Strongly banded rocks, layers rich in fluids such as water and carbon dioxide,
and zones of cracking have all been suggested as possible causes.

The first borehole cut through metamorphic basement – gneisses, metabasics
and amphibolites – in steep, folded layers. The pilot project also indicates
that there have been relatively recent releases of methane, helium and oxygen
from the rocks, together with enrichment of graphite, open fractures several
centimetres across, severe fracturing and deformation produced by ancient
earthquakes and unusually high stresses below a depth of 3400 metres.

Although these surprises have challenged the techniques and theories
of the 190 scientists from 50 universities who are working on the project,
they were not unwelcome. One of the KTB’s main goals is to investigate and
interpret the data yielded by deep seismic reflection techiques and various
other methods of geophysical analysis of the crust.

According to Emmermann, the discovery of water rich in potassium and
twice as salty as the sea in open fractures 3400 metres below ground is
one of the bigger surprises for scientists working on the project. First
analysis of seismic data shows strong reflections from this depth: the fluid
seems to be the important factor. The main hole will cross reflectors below
8 kilometres.

Basement rocks of the type found beneath Windischeschenbach had been
considered essentially dry, with small amounts of fluid held in tiny pores.
But the pilot hole told a different story. ‘There are circulating brines
which are enriched in gases,’ says Emmermann. ‘At high temperatures they
precipitate secondary minerals. These secondary minerals fill veins and
cracks in the rock and seal the basement through hydrothermal mineralisation.’

The drill has penetrated three different layers of rock, whose fluids
are isolated from each other. And these layers seem distinct in other ways
too. ‘There’s basement at a depth of 500 metres, above which groundwater
is circulating,’ Emmermann said, ‘and a second region characterised by shear
zones and hydrothermal mineralisation at a depth of 2000 metres. Below,
in the third region, nearly vertical 70-degree fold structures and brine
filled fractures were encountered.’

The steep folds encountered in the core were not picked up on the seismic
profiles. This should not be a surprise: steeply tilted layers and faults
do not show up well on seismic sections. But the scale of the folds could
mean changes in the accepted models.

Since the test boring was completed in mid-April, Asaf Pekdeger of the
Free University of Berlin has been trying to figure out where water taken
from large open fractures encountered below 3400 metres came from, how old
it is and how it has interacted with the surrounding rocks.

The hole has yielded 38 000 litres of water with a temperature of 118
Degree C and a salinity of 60 grammes of salt per litre. According to Pekdeger,
its origin is something of a mystery. ‘It’s a riddle right now,’ he says.
‘It can’t come directly from the rocks. The crystalline rocks were at one
point magma. Now they have been transformed into metamorphic rocks.’ One
very broad theory is that the water was either forced directly into the
crystalline rocks at the time they were formed, or it worked its way in
over a period of a million years. ‘As a rule,’ Pekdeger says, ‘where there
are crystalline rocks, you find little salt water. It’s not like drilling
for oil, where prospectors find a great deal of salt in the rocks.’

The water is a calcium-sodium-chloride brine. Researchers suspect that
its composition results from chemical reactions between minerals and water,
as it circulated through the rocks. But where the water came from in the
first place is still an enigma. According to Pekdeger, ‘it’s not possible
to say if the water was originally fresh or salt.’

Nor can much be said about the age of the water, other than that it
is at least several million years old. ‘If the water was moving in terms
of centimetres per day or metres per day,’ Pekdeger says, ‘we could extrapolate
its age. But the water has been there for a long time. It’s trapped and
we’re not positive if it’s circulating. If it is, it’s not doing it very
fast, perhaps a millimetre per year. Right now we’re hoping that chemical
and isotopic analysis will give us some indication of age.’

He expects that more water will come to the surface during the current
state of the drilling project. ‘We now expect to find more salt water,’
he said, ‘but you never know. It becomes more interesting the deeper we
»å°ù¾±±ô±ô.’

Pekdeger predicts that the composition of the water will change at a
depth of 6000 metres. ‘There will be a continuous increase in carbon dioxide,’
he said. ‘The deeper we go, the more CO₂ there will be. The calcium
content will be reduced as a result of geochemical processes.’

Emmermann attributes the change in the chemical composition to higher
temperatures at greater depth. Temperature is one of the main factors that
control chemical reactions between rocks and circulating fluids. Reactions
with different minerals give fluid of different composition as the temperature
changes. ‘In the shallow parts of the borehole, the main reacting mineral
was plagioclase. At greater depths and temperatures, the plagioclase will
be stable. There, water composition will probably be determined by amphiboles
or pyroxenes.’

Relatively large amounts of methane and graphite were also recovered
during test drilling. According to Emmermann, the zones of graphite which
the drill bit into are products of a high-temperature chemical reaction
along fault zones, involving fluids consisting of carbon-rich gases. ‘We
think a reaction between methane and carbon dioxide produced graphite and
water, which probably happened during earthquakes 250 million to 300 million
years ago,’ said Emmermann.

But, he adds, there is also a possibility that the methane came from
the Earth’s mantle, a process called degassing. This could also explain
the helium from the test boring. Rock from below 3800 metres contained relatively
high proportions of the isotope helium-3.

Helium-4, the common isotope of helium, is produced constantly in the
Earth’s crust by the decay of uranium and thorium. ‘But helium-3 cannot
be produced by radioactive decay,’ says Emmermann. It could be that it came
from the degassing process as well, or perhaps the Tertiary Eger Graben
may be transporting it from the mantle.’ The Eger Graben is one of several
places in Europe where the crust has been stretched and thinned, making
it easier for fluids to rise.

When the pilot boring began on 22 September 1987, the scientists expected
that temperatures would rise by about 22 Degree C per kilometre. After going
below 500 metres, however, they met higher temperature gradients – something
that measurements from shallow boreholes and mines did not predict. Temperatures
climbed at a rate varying between 28 and 30 Degree C per kilometre. When
the drill stopped spinning 4000.1 metres down on 4 April 1989, the final
temperature hovered around 100 Degree C.

This pattern was unexpected, although other parts of Europe have even
greater gradients. Another possible site for the hole, in the Swartzwald,
was rejected because researchers predicted temperatures as high as 300 Degree
C at depths between 7 and 8 kilometres – adding to the technical difficulty
and cost of drilling. The unexpectedly high temperatures in the Oberpfalz
have meant that the target depth of the hole at Windischeschenbach has been
reduced from 12 to 10 kilometres.

But why was this high heat flow not apparent near the surface? One theory
is that the relatively low temperatures found above 500 metres arise from
systematic circulation of ground water, cooling the crust by convection.
But the Eger Graben, said Emmermann, may also be a factor in explaining
the higher temperatures.

Technological limitations, however, could hold back further discoveries.
‘Getting things out of the borehole is difficult because the density is
so high,’ says Pekdeger. ‘You need drilling fluid with a higher density
because the mud has to carry up rocks – it should be like quicksand.’ The
project has used a very special drilling fluid. ‘It can’t disturb the chemistry
of the coring and it must be functional up to 300 Degree C,’ he says.

The project’s drilling engineer, Axel Sperber, explained that consistency
in the composition of the drilling fluid is the project’s linchpin. ‘It’s
important that the mixture remains constant. The ingoing mud must always
have a constant composition. This characteristic enables us to detect even
minute traces of things in the mud that’s coming out of the borehole.’ Another
important trait of the mud is that its density can be changed to keep the
shape of the borehole constant. ‘The deeper you go’, says Sperber, ‘the
more lithostatic pressure you encounter. But if you adjust the density of
the mud its pressure increases and resists the forces pushing on the hole.’

But, according to Pekdeger, the acidity of the drilling fluid changed
after water was struck at 3400 metres. This changed the composition of the
mud and created problems. As the drilling continues, the researchers may
have to strike a compromise between the integrity of the drilling mud and
how much water they intend to draw from the hole, he said.

The technology, said Sperber, is not only suited to research drilling,
but also can be applied in digging geothermal wells. ‘Because we use special
technical devices – pipehandling in combination with a hook retractor –
it’s also well-suited for offshore drilling.’

* * *

Down, down, deeper and down

SOVIET scientists began to think about boring into the continental crust
in the 60s, at around the same time as American geologists were planning
to drill through the ocean crust – the Mohole project. In the past 30 years,
ocean drilling has continued worldwide, but not to such depths.

Plans for a hole on the Kola Peninsula, near Murmansk, arose partly
from scientific curiosity, but owed much to the desire to find new ore deposits.
In May 1970, the Soviet drills started to spin through the crystalline Precambrian
rocks of Kola, and by 1980 the well was 10.7 kilometres deep. As hoped,
the borehole penetrated valuable ores, copper and nickel about 1700 metres
down, and copper, lead and nickel between 6500 and 9500 metres below ground.

These and other early successes led to an expansion of the programme
in the 80s. Plans were laid for a network of deep seismic profiles, studded
by 10 boreholes up to 15 kilometres deep in key areas. But the future of
this ambitious project is no longer clear. Kola is just over 12 kilometres
deep, although there are plans to continue down to the target of 15 kilometres.

The US had problems even making plans for deep continental drilling.
The technology needed to reach 10 or 15 kilometres down into the Earth would
have been so expensive to develop that plans to drill a 10-kilometre hole
in the Appalachians were abandoned. The US drillers then turned to shallower
holes, in unusual areas. An especially exciting project was for a 5 kilometre
well at Cajon Pass, crossing a fault. But money was tight, drilling more
costly than anticipated, and the hole petered out at 3.5 kilometres.

France is also aiming for many shallow holes, using existing drilling
technology, rather than a few very deep and costly holes. Other European
countries take much the same approach. Sweden is the exception. At Siljan,
there is a well 6.8 kilometres deep, drilled to find oil and gas which may
come from the Earth’s mantle. So far, the hole has yielded only traces of
hydrocarbons, which many observers attribute to contamination.