By Yevgeni KOZLOVSKY, Dr. Sc. (Tech.), Minister of Geology of the USSR, Chairman of the Joint Learned Council for the Studies of Mineral Resources and Deep Drilling
A special program on studies of the Earth through deep drilling was adopted. The first step towards its implementation was the borehole on the Kola Peninsula, with the explicit purpose of studying the structure of the Baltic crystalline shield. The USSR Academy of Sciences and more than 20 ministries are involved in the experiment. The actual drilling is being carried out by a special expedition of the USSR Ministry of Geology.
Drilling to a depth of even 6,000 m is a major event. And when the drill goes down to 9,000 m, it is regarded as a major scientific and technical achievement. Only two countries in the world can boast of having reached such depths. In the United States the drill has gone down to depths of 9,159 and 9,583 m. The Soviet Union is the only country where a depth of more than 10,500 m has been reached, revealing crystalline rocks from 1.5 to 3 bin years old.
What is the significance of the Kola drilling experiment? First, it has allowed people for the first time ever to observe and study a ten-kilometer cross-section of the Earth's crust. A great deal of new information was obtained on the physico-mechanical and chemical properties of rocks at great depths; traditional views were checked, so were the geophysical models of the structure of the Earth's crust, the range of application of the theory of elasticity in geological media, and hypotheses about ore-formation. Secondly, theoretical foundations were evolved, corresponding drilling equipment and measuring instruments were developed and tested. A number of alternatives were proposed: from using a winch 80 m in diameter to the building of shafts 300-400 m deep to install drilling pipes. There were many arguments about the design of the borehole, the ways of overcoming high temperature and pressure. But in the final count, an original project was drawn up for drilling the Kola borehole, and the project has been put into practice. Drilling proceeded without wall casing with steel pipes. This was the first time such a method had been tried in the world.
It had been believed that crystalline rocks breakdown at a drilling speed of 500 rotations per minute. This proved to be true only down to a depth of 7,500-8,000 m. After that special turbine engines had to be developed with a low rotation speed so as to continue drilling. Raising and lowering operations at the borehole have been automated, which helped to increase the drilling rate several times over.
Soviet specialists developed entirely new methods and means for continuous sampling, for studying the composition and physical properties of rocks in the process of drilling. They also developed facilities for controlling the technological parameters of drilling and monitoring the state of the area around the shaft.
Geophysicists believe that the Earth's crust consists of three layers. The top layer is composed of sedimentary volcanogenic rocks and is several km thick, the medium is made up of granite and is anything from 35 to 40 km thick, while the bottom layer is composed of basalts about 30 km thick. In some places the granite layer outcrops on the surface. It is in a place like that, in the crystalline rocks of the Baltic shield, that the Kola borehole was drilled. For hundreds of millions of years, the Baltic shield was subject to erosion. It has been estimated that in some places the
water and glaciers have shifted to other areas as much as 5-15 km of the upper part of the granite layer. Consequently, the drilling actually permits studies to a depth of 15-25 km, not 10. Another factor that influenced the choice of site for drilling was that the Pechenega area is basic to understanding the geology and ore formation of the entire Baltic shield, as well as the Earth's crust of other continents.
It was expected that when the drill reached the 4,700 m mark, it would have passed the sedimentary volcanogenic rocks (about 2,000 min years old) and at 7,000 m the granite would have been cut through and the basalt reached. In reality things were different. The sole of the sedimentary layer was reached only at a depth of 6,800 m, after which the drill penetrated a granite horizon estimated to be 2,500-2,700 min years old.
For the first time, the horizontal border was crossed where the dissemination of seismic waves changes sharply. According to the model of the Earth's crust, this border (the so-called Conrad line) was believed to coincide the world over with the divisional plane between the granite and basalt layers of the lithosphere. In reality, as the drill went deeper into the crust, no sharp increase was observed in the density of the rock, nor in the rate of dissemination of elastic vibrations, or in other physical parameters. What is more, in the central part of the section, in the interval between 4,500- 8,000 m, something of a waveguide was observed-a zone in which elastic vibrations disseminated slowly. Thus, the Kola drilling experiment introduced corrections into the existing view that it is possible to directly correlate geophysical data and the geological media composition in the depths of the Earth's crust. The results that were obtained will serve as the basis for new structure models of the Earth's crust and its mantle.
Up to a depth of 3,000 m, the rise in temperature followed the anticipated pattern. It was only deeper down that the expectations were upset: at a depth of 10,000 m, instead of the expected 100C, a temperature of 180C was registered. Such a temperature jump was due to strong heat flows from great depths. As a result hundreds of m 3 of drilling mud were heated up to 45C. Physicists proved that the role of the radioactive decay in rocks as a source of heat is insignificant. The main heat comes from the mantle. As a result of the Kola experiment, the modem and ancient geothermal zones of the Baltic shield were defined, and this will help to update the heat model of the Earth. Will it help with practical problems? Without doubt. Especially with those linked with the use of the Earth's own heat.
Before the Kola experiment the theory of the origin of ores rested on studies of existing open cut mines. It turned out that deep down in the Baltic shield highly-saline solutions are constantly on the move. They circulate along the extensive zones of tectonic breaks. These solutions contain bromine, iodine and heavy metals. Among the gases found are helium, hydrogen, nitrogen and methane. Consequently, the continental plates are an area of active ore- formation. At a depth of 1,600-1,800 m, sulphide ores with a commercial content of copper and nickel were discovered. At 4,500-4,600 and 6,000-6,500 m-a large zone of crushed rock was found. Their fragments were cemented together by quartz, copper sulphides, iron, lead, zinc, nickel and cobalt. The horizons, that had always been regarded as dead, had at one time been actively involved in biological processes. Remnants of live organisms, micro-fossils, whose age is considered to run into hundreds of millions of years were found here. And, finally, at a depth of more than 9,500 m, minerals with a high content of iron and various types of mica were discovered.
An entirely new page has been inscribed in the scientific and technical progress in studies of the Earth's crust. A unique borehole 1 m in diameter at the top and more than 20 cm at a depth of 10 km has been drilled. The vertical zone of a search for hydrothermal ore resources has been widened. The Kola experiment has proved that it will in future be possible to exploit the wealth secreted at great depths in continental plates.
After the completion of the Kola experiment, the borehole will become a laboratory. It will be used to test and improve existing instruments and methods of geophysical, geochemical and hydrogeological research. It will also permit observations of temperature fluctuations and physical and chemical processes in the Earth's crust. That is important for learning more about the Earth, and about the other planets of the Solar system.
The results obtained at the Kola borehole cover only a small part of the immense variety of geological formations. We still have to study the structure of young platforms, that is, those formed comparatively recently (500-800 min years ago). The program of research will include studies of volcanic and seismic regions. Deep drilling will begin in the main oil- and gas-bearing areas and principal ore-bearing regions of the country, first and foremost, in Western Siberia and the Urals.
Such drilling is expected to reveal new deposits at depths of 5-7 km. The young platforms will be used to investigate the changes taking place in the deep-lying oil and gas fields. In the ore areas, where ore has already been mined for many years, deep drilling will help to estimate more accurately the prospects for extending the mineral-resource base there. In the fold systems, deep drilling will advance the studies of the "mountains' roots". In modern volcanic areas, the practical application of such drilling will be to assess the possibilities for greater use of the deep-lying heat of the Earth.
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