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The origin of petroleum has long been the subject of various conjectures, and as far back as 200 years ago many scientists raised the question of the source of the black oil liquid occurring in rocks.

The second half of the 19^th century saw the birth of various theories concerning the origin of petroleum. L.Lequeret believed that it was the decomposition of algae that resulted in the formation of petroleum. K.Engler and H.Hofer considered fats of sea animals to be its source material.

Special studies of the origin of petroleum were started at the beginning of the 20^th century. It was suggested that the source material for the formation of petroleum was not definite species of flora and fats of sea oozes consisting of the remains of plants and animal organisms. This suggestion was proved by G.P. Mikhailovsky, H. Potonie and N.I. Andrusov.

According to G.P. Mikhailovsky, the source material of mixed plant and animal origin is scattered in sea zones among mineral particles. The initial decomposition of the plant and animal remains is the result of the activity of micro-organisms. Further with the gradual getting and burial of sedimentary rocks and under the action of increasing temperatures pressures the organic matter undergoes changes which lead to the formation of the scattered petroleum, the latter with the passage of time, accumulating in porous rocks. These ideas suggested by G.P. Mikhailovsky are essentially the basis for our modern principles of the formation of petroleum from the organic matter contained in sedimentary rocks.

As we know from ancient manuscripts man first began to apply petroleum already some centuries B.C.

Only since XIX century A.D. people have begun to use petroleum as one of the most important sources of energy.

Petroleum or oil is a combustible oily liquid which occurs in sedimentary rocks of the Earth crust. Petroleum usually forms and accumulates in geological traps at the depth of 1200 - 2000 m and deeper.

Petroleum is a liquid which consists of different hydrocarbons, i.e. the compounds of carbon and hydrogen. Besides, it often contains smaller amounts of sulphur, nitrogen and oxygen.

The odour of petroleum depends on nature, composition and quantity of hydrocarbons and different impurities. The colour of oil varies from light brown to dark brown, nearly black. Specific gravity of oil determines its colour. The heavier the oil the darker is the colour.

For oil doesn’t conduct electricity people use some of its products in the manufacturing of insulators.

All sorts of petroleum are combustible. At present, petroleum is the most important fuel and energy source because of its high calorific value.

The geological science has not given a clear explanation for the petroleum formation. Most scientists, however, admit the organic origin of petroleum. They believe that carbon and hydrogen, i.e. the chemical basis of any oil, came from the sea and land plants and animals as a result of their decomposition.

Natural gas either occurs together with crude oil or forms separate deposits of gas alone. Decomposition of both animal and vegetable remains over a period of many centuries without air is the source of natural gas. A great volume of gas accumulates and penetrates into porous beds of sand, sandstones and limestones. In these beds gas can form natural deposits under great pressure. When a borehole reaches such a deposit the gas rushes up. This gas has to be collected from several boreholes with the help of pipelines over long distances. She Soviet Union has a gigantic network of pipelines.

Natural gas is an inflammable gas and consists of hydrocarbons with a very low boiling point. In contrast to crude petroleum natural gas has no distinct odour.

Little of natural gas we may use chemically, most of it we use as a fuel for the production of both heat and energy. Like all gaseous fuels natural gas has great advantages over liquid and solid fuels as it gives a great amount of heat. Natural gas is valuable also as an important chemical raw material for industry, as chemical technologies are able to obtain hydrogen, acetylene, carbon black and various chlorine derivatives.

Natural gas occurs mainly in Russia and in the United States where most of the extraction and utilization takes place. Before the war gas was utilized at Baku and in the Carpathian Mountains. Since then numerous new sources were discovered. In Europe they found rich deposits in Rumania, Italy, Austria and France.

Man for one reason or another has since the earliest known times, been digging holes in the earth’s surface. More recently, modern man has drilled into the earth for petroleum products, for minerals such as sulfur and to tap sources of geothermal energy.

Man has always needed a source of water so most early civilisations dug wells. As man’s technology improved, so did the methods of digging. Crude shaped tools were first used and later digging implements of bronze and then iron appeared. At first debris was handed up out of the hole in a basket but later it was hauled out with the aid of crude ropes and a windlass.

Oil, from seepages, was known to, and used by ancient man. They used it to caulk boats and baskets, for medical purposes and in crude lamps. So evidence exists that wells were dug into the seepage areas to obtain a greater supply of petroleum. No one knows for sure who was the first to drill for water, brine, or oil. But the art of drilling did begin many centuries ago. By 600 B.C. the Chinese were using percussion tools, the forerunner of cable tools, to dig brine wells. By 1500 A. D. they were drilling to depths of 2,000 feet. Their rigs were constructed almost entirely of bamboo, with the only metal being the actual drilling tool or bit on the end of the line.

Cable or percussion rigs remain in use to this day. It is interesting to note that the Chinese drilling methods, once perfected, changed little over the years.

When men first began to seek petroleum, the easiest way to find it was to look for evidence of oil seeps on the earth’s surface. Generally, oil seeps are either up-dips or seepage along a fracture. Observation of seeps has led to the discovery of many of the world’s great oil fields in the U. S., the Middle East, Venezuela, and at other points on the globe.

Indeed, the search for oil begins with geologists and geophysicists using their knowledge of the earth to locate geographic areas that are likely to contain reservoir rock. Once such a “likely area” is found, then more specific tests and investigations are made and the information gained from these is used to construct “maps” of the earth’s substructure. By 1920 it was found that looking for domes, seeps and anticlines on the surface maps was not sufficient. Thus geophysical methods were devised that gave the searchers an idea of what lay beneath the surface.

The basic tool in any search for oil is the knowledge of the earth itself – how it was formed, its composition and its present configuration. It is not enough though, to merely become aware of the existence of an oil accumulation at a given location. Before investing what may be millions of dollars, the operator needs to know if the well will be commercially feasible, or simply stated, will he recover his investment and perhaps make a profit?

The rocks of the Earth's crust are divided into three main groups: sedimentary rocks, which consist of fragments or particles of pre-existing rocks; igneous rocks which have solidified from magma and metamorphic rocks. Metamorphic rocks have been derived from either igneous or sedimentary rocks.

Sedimentary rocks represent one of the three major groups of rocks that make up the crust of the Earth. Most sedimentary rocks have originated by sedimentation. They are layered or stratified. Thus, stratification is the most important characteristic of sediments and sedimentary rocks. It is necessary to note that the processes which lead to the formation of sedimentary rocks are going on around us.

Sediments are formed at or very near the surface of the Earth by the action of heat, water (rivers, glaciers, seas and lakes) and organisms.

Strictly speaking, sedimentary rocks form a very small proportion by volume of the rocks of the Earth's crust. On the contrary, about three quarters of the Earth's surface is occupied by sedimentary rocks. It means that most of sedimentary rocks are formed by sediments, accumulations of solid material on the Earth's surface.

The most principal kinds of sedimentary rocks are conglomerate, sandstone, siltstone, shale, limestone and dolomite. Many other kinds with large practical value include common salt, gypsum, phosphate, iron oxide and coal.

The thickness of the layers of sedimentary rocks can vary greatly from place to place. They can be formed by the mechanical action of water, wind, frost and organic decay. Such sediments as gravel, sand and clay can be transformed into conglomerates, sandstones and clay schists as a result of the accumulation of materials achieved by the destructive mechanical action of water and wind.

Mechanical sediments can be unconsolidated and consolidated. For example, gravel, sand and clay form the group of unconsolidated mechanical sediments, because they consist of loose uncemented particles (grains).

On the Earth's surface we also find consolidated rocks, which are very similar to the loose sediments whose particles are firmly cemented to one another by some substance. The usual cementing substances are sand, clay, calcium carbonate and others. Thus sandstones are consolidated rocks composed of round or angular sand grains, more or less firmly consolidated. Like sand, sandstones can be divided into fine-grained, medium-grained and coarse-grained.

On the other hand, chemical sediments are the result of deposits or accumulations of substances achieved by the destructive chemical action of water. The minerals such as rock salt, gypsum and others are formed through sedimentation of mineral substances that are dissolved in water.

All rocks which are exposed on the Earth's surface (high mountain peaks, deserts) are decomposed to a certain degree. The process of rock disintegration by the direct influence of local atmospheric conditions on the Earth's surface is called weathering. This phenomenon is often referred to in geology because weathering is an active process. It takes place in the upper layers of the Earth's crust.

The main cause of physical weathering is the change in temperature that takes place with the succession of day and night. This phenomenon can best be observed in the deserts and high mountains where the changes in temperature are common.

During the day under the influence of heat, rocks expand whereas at night they begin to contract. As rocks are generally corn-posed of different minerals, their expansion and contraction do not occur uniformly. As a result of this rocks crack. At the beginning these cracks or fissures are hardly noticeable but gradually they become wider and deeper until the whole surface of rock is finally transformed into gravel, sand or dust.

In the regions of a moderate or cold climate, where the temperature in winter goes down to below 0 (zero), the decomposition of rocks is greatly facilitated by the action of water. When water freezes it increases in volume and develops enormous lateral pressure. Under the action of water, rocks decompose to pieces of varied forms and sizes.

The decomposition of rocks under the direct influence of heat and cold is called physical weathering.

Rocks are subjected not only to physical decomposition but also to chemical weathering, i.e. to the action of chemical agents, such as water, carbon dioxide and oxygen. In a general way, chemical weathering is an acid attack on the rocks of the Earth's crust, in particular an attack on the most abundant minerals — quartz (sand) and aluminosilicates (clays). Only few minerals and rocks are resistant to the action of natural waters. The solvent action of water is stronger when it contains carbon dioxide. Water causes more complex and varied changes. With the participation of oxygen and carbon dioxide up to 90 per cent of rocks is transformed into soluble minerals, which are carried away by the waters.

Organisms and plants also take part in the disintegration of rocks. Certain marine organisms accelerate the destruction of rocks by making holes in them to live in. The action of plants can often be even more destructive. Their roots penetrate into the fissures of rocks and develop the lateral pressure which fractures and destroys rocks.

10.

Most mineral resources are derived from the Earth's crust. The crust is composed of minerals that are crystalline solids with specific and rather simple composition. Minerals in the Earth's crust are concentrated into specific groups which are called rocks. Two distinctly different types of crust are recognized: oceanic and continental.

Since it is difficult to investigate the floor of the ocean, the composition of the oceanic crust is not known completely. Scientists say that it is relatively constant in composition. The oceanic floor consists largely of minerals rich in calcium, magnesium, iron and silicon, and it is formed by the cooling of lavas extruded on the sea floor to form a type of rock called basalt. It is subjected to the same forces of erosion and weathering.

The continental crust contains less iron and magnesium than the oceanic crust, but relatively more silicon, aluminium, sodium and potassium. The continental crust is more complicated and has a more variable thickness and a less well defined structure.

A systematic examination of all known rock types shows that two principal types predominate: 1) Igneous rocks which are formed by the cooling and crystallization of liquids from deep in the crust called magma; 2) Sedimentary rocks which are formed by sedimentation and gradual cementation of sediments by the action of water, ice, wind and organisms. They are layered or stratified. Most of the sediments are deposited in the sea along the continents.