The word “oil”, composed starting from the two Latin words will petra and oleum, means “stone oil literally”: one indeed finds oil in the sedimentary rocks of the earth's crust in liquid form. There also exists in gas or solid form.

Chemically, oil is a complex mixture of hydrocarbons with minor amounts of nitrogen, oxygen and sulfur. According to the layers, it presents a large variety, between natural gas, made up mainly of methane, and the solid or semi-solid forms as asphalt and the bitumens, via all the liquid intermediate forms.

Oil and the natural gas are currently the mineral and energy raw materials most used, in particular in the form of fuels, of drugs, plastics or even of cosmetics. Also oil it has become, for one century, a strategic product which was the stake many conflicts.

Oil is formed by degradation of the organic matter resulting from plants and microscopic organizations hidden with the fine sediments which accumulate at the bottom of the oceans. Due to bacteria, in mediums low in oxygen, the organic matter is degraded in a brownish substance complexes, the kerogen, which is in fact the “ancestor” of oil.

Genesis of oil

The layers of sediments are inserted and are consolidated, being transformed little by little into sedimentary rocks: it is what is called the diagenesis. The kerogen is then brought up to temperatures and with increasingly intense pressures, which break the molecules and reorganize them, thus giving hydrocarbons of oil. Beyond a certain depth, these phenomena are so intense that the only molecules which remain are gases and which there does not remain any liquid hydrocarbon. So the presence of liquid oil deposits can indicate that the rocks of origin did not have to undergo higher temperature with 140 °C approximately. On the other hand, the presence of natural gas in abundance makes it possible to deduce that this temperature was exceeded. In a general way, the temperatures favorable to the formation of liquid oil range between 60 °C and 120 °C, in correlation with depths of going hiding of the rocks of 2000 to 4000 Mr.

Another factor will condition the appearance of oil: time. The rise in temperature must proceed for one sufficiently long period “to cook” the starting ingredients. Thus, deposits containing of the organic matter and exposed to low temperatures, but over one long period of time, can produce oil in a way as effective as of the more recent deposits subjected to higher temperatures. Time is then known as geological, and corresponds to periods which reach several tens of million years.

The sedimentary basin

The geographical concept of basin recovers a geological reality: the basins are formed by the accumulation of sediments, transported from the active deepening reachs of the continent in a depression of terrestrial surface. Basins are currently in formation in the Gulf of Mexico, with broad of the delta of the Mississippi, where sands and muds brought by the river accumulate on kilometers thickness.

The weight of such accumulations causes the progressive depression of the whole of the basin: it is the phenomenon of subsidence; the basin keeps thus its character, the depression compensating for the filling. The older layers are found gradually hidden, and the growing weight of the overlying strata, formed by the arrival of new sediments, causes their compaction. The water which circulates, driven by the pressure increasing and charged with rock salt, will contribute to cement the grains or on the contrary to dissolve them partly. Thus, the sediments are transformed little by little into true rocks: sands become sandstones, and muds of clays.

The porosity of the rocks

The compaction and the cementing of the rocks cause to decrease the size of the interstices separating the grains, therefore the volume of vacuums where can place fluids such as water or hydrocarbons.

The relative volume of the vacuums compared to the total volume of the rock is called porosity: this parameter has an enormous importance in oil prospection, since it determines the potential volume of oil in a layer.

The permeability, which measures the facility that have the fluids to cross the rock, is it also an important parameter. A porous rock is not inevitably permeable: clays are impermeable, and yet porous (a dry clay can absorb water).

Bed rock, container

The pores of the underground sedimentary rocks are very often occupied by water. In the hydrocarbon layers, oil and the gas expelled water by occupying interstitial spaces. The rocks which impregnate oil thus are typically sandstones or rocks carbonated like limestones, whereas the sediments, where the organic matter was trapped, are rather muds which evolved in finer rocks like clays. This pleasing difference to make the distinction between the rock where oil was slowly formed, or bed rock, and that which forms the underground tank, or container. The term “container” is not completely adapted, since the layers are contained not in vast cavities but in the microscopic pores of the rock, where oil is exploited.

The containers must be at the same time permeable and porous: in this respect, a sandstone made up of coarse particles rounded of more or less uniform size is an ideal container. If the sizes of the grains are very variable, the rock will be less porous, and its degree of permeability is likely to be relatively weak.

Oil born in the bed rock must thus migrate, during its history, towards the container. This migration also allows the regrouping of the hydrocarbons, which in the beginning are disseminated in the sediments.

Migration of oil

With the accumulation of the sedimentary layers and the hiding, the weight increases and compacts the sediments. The intense pressure which results from it is probably the first engine of the migration of oil, the hydrocarbons extirpating too fine pores of the bed rock to gain rocks of higher porosity if it is presented some in the immediate vicinity.

Various routes of oil

The migration of the oil mass depends on the rocks met. Permeable rocks like the sandstones or limestones, coming from old reefs, allow the passage of hydrocarbons. Any impermeable rock, for example a clay, will stop them or modify their advance.

The ascending migration of oil can theoretically bring it to surface. In fact, one observes sometimes on the surface of the hydrocarbon seepages, which go from the simple traces, like an oil film on a water surface, with enough important reserves to be exploited.

The layers of surface are however exceptional: the natural gas is lost almost instantaneously in the atmosphere; oil is exposed to the action of the bacteria and the evaporation of its light components. When there remains hydrocarbons, they are heavy residues, tar or asphalts. Nevertheless, the presence of active seepages in an area is the proof of the gas or oil migration, and thus an index of the possible in-depth presence of a layer.

Traps with oil

The existence of in-depth layers is explained by the presence of certain geological structures which block oil and gas and prevent them from going to disappear on the surface. These structures, known under the name of traps, can take varied forms. They play a double part, since on the one hand they stop the migration of hydrocarbons and on the other hand they cause their accumulation “upstream”, as a stopping would do it. Generally, the traps put in contact a rather permeable and porous rock (a sandstone or a limestone) and an impermeable rock.

One distinguishes two big families from traps: the structural traps, which are consisted the flexible deformations (folds) or breakable (faults) of the rocks, and the stratigraphic traps, which bring into play the nature of the rocks. Very particular geological structures, the salt plugs (diapirs), often combine the two types of traps, because they have an environment which presents important stratigraphic variations and cause notable deformations with the wall-rocks.

When the layer is localized with certainty, of possible complementary drillings then calculations (known as “of tank”) make it possible to determine its exact extension and volumes of hydrocarbons contained. It is necessary to take account of geometrical parameters (total volume of the “oil pocket”), but also lithological, like porosity. These calculations will help to determine the best way of exploiting the layer.

The recovery of oil

In general, the pressure inside the geological tanks is enough to push hydrocarbons towards surface. But, under the best conditions, 25 % of hydrocarbons to the maximum can be thus extracted, because the pressure decreases little by little until becoming insufficient. Methods known as of recovery secondary, or assisted, are then used: they consist in injecting water or others liquid in wells designed for this purpose, in order to push oil towards the casings of production. It is possible also to act on the viscosity of the oil, which makes difficult its separation of with the container, by means of substances whose action is comparable with that of the soaps; in the long term, it will be possible to recover 50 % of the oil contained in the layers.

The refining of oil

If crude oil is a mixture of very varied hydrocarbons, the delivered products have, them, a quite precise composition. For example, the gasoline is manufactured starting from light hydrocarbons, contrary to the road coatings, where very heavy compounds are used. The refining thus makes it possible to separate the families from hydrocarbons. Separation is done by distillation; the progressive heating of a crude load makes it possible the very light compounds to pass in vapor phase and to be collected by condensation; then come from the less and less light compounds, the not evaporated residues representing the heaviest fraction.

The transformation consists in producing molecules different from those constituting the crude; two principal techniques are employed: the cracking, which consists in “breaking” the molecules, generally by heating them sufficient; reforming, opposite operation, which consists in “sticking” elementary molecules to form some heavier. At the exit of the refinery, a whole series of end products are obtained, such as the various fuels, but also of the compounds known as “major intermediates”, like butadiene, and which will be used for petrochemistry to manufacture products such as the plastics or the chemical compounds used in beauty care.

The transport of oil

The natural gas, the crude oil and the refined products of oil are transported by various means: pipelines and gas pipelines, oil tankers, and more rarely trucks or tank cars. At the origin, transport was done in wood barrels, and the barrel remained the unit of reference for the crude production (1 barrel is worth 159 L, and 1 T of crude represents 7.3 barrels).

Pipelines and gas pipelines

The fluid hydrocarbons can be transported in drains, the pipelines, as well as gas hydrocarbons (one speaks in this last case about gas pipelines). These drains are made up of welded tubes the ones with the others, of a diameter of a few tens of centimetres to more than one meter. They are generally buried, which attenuates the environmental impact. The hydrocarbons are transported under pressure, maintained by pumping stations installed with regular intervals.

In certain countries (Russia, the United States), where the surface transports imply considerable distances, most of oil and natural gas is conveyed by pipelines or gas pipelines. Those cover considerable distances, to several thousands of kilometers. There exist also underwater pipelines and gas pipelines, as those which cross the Mediterranean between North Africa and Europe. Until the years 1960, the gas pipelines were the only means of transport of natural gas.

However, this method of transport costs three times more expensive than the routing of an equivalent energy quantity of crude oil. A solution consists in liquefying gas while bringing it up to low temperature (- 160 °C). Tankers were especially designed to transport this liquefied natural gas (GNL) to the atmospheric pressure. The natural gas consumption coming from remote areas thus became possible. Important natural gas layers, like those of Alaska and of the Middle East, become from now on exploitable.

Oil tankers

The first oil companies, born in the United States, found that it was more interesting to amalgamate their activities within enormous trusts. In 1912, the Standard Oil Company, rested by John D. Rockefeller, was divided into small companies by the Supreme court of the United States, under the terms of the antitrust law. In spite of this measurement, the American oil industry, supported by the enormous demand for oil caused by the two world wars, was dominated during the following decades by some multinational companies, five American and two European (Royal Dutch-Shell and British Petroleum). In 1966, these Majors controlled 76 % of the worldwide production of oil. Although in fierce competition for the control of the production - for example, that of the layers of the Middle East then recently discovered -, they got along secretly between them to control the production, and consequently the prices, conclusive with their advantage of the agreements of production with the inexperienced governments of the producing countries.

The creation of OPEC

Certain producing countries (Mexico, Iran), eager to limit the privileges of the large foreign companies, undertook the nationalization of the wells exploited on their territory, or claimed an equal share of the incomes (Venezuela). In 1960, Venezuela founded, with several countries of the Middle East, Organization of Petroleum Exporting Countries (OPEC), the producing countries wanting themselves to control the prices. Several other countries rejoined this organization, which currently counts thirteen members.

Oil crises

The first oil crisis

The behavior more and more militant of OPEC as from the years 1970 brought famous “the oil crises”: the most known first and took place in 1973, at the time of the war of Kippour, when the countries of the Middle East decided rises in the price of the crude going up to 100 % and the embargo of the deliveries towards certain countries which supported Israel then.

The second oil crisis

It occurred in 1979: in one year, the oil prices doubled (from 14 to 28 dollars the barrel). Between 1973 and 1980, the oil price was thus multiplied by ten. Moreover, the variations of the courses of the crude were amplified by those of the courses of the currencies: oil being paid in dollars (“petrodollars”), any rise of this currency immediately involved an increase in the oil bill of the importing countries.

Consequences

This situation has increased dependence of consumer country (the EEC and Japan, in particular) compared to producing countries (75 % of the production are indeed consumed in countries which has only 10 % of oil world reserves) and contributed to the impoverishment of the majority of the Third World countries. Confronted with the oil crises, the importing countries took measures instituting of energy saving and supporting energy independence.

Thus, in 1974, certain European countries imposed prohibition to circulate or the closing of the petrol pumps during the weekend. It is also at that time that most country speed limits instituted, and that the fight against the wasting was launched, in incentive, for example, with the thermo isolation of the dwellings or the development of economic automobile engines while carburizing. In addition, the governments supported other forms of energy, by accelerating the nuclear programs or while pushing with research on solar energies, wind and geothermic low and high energy.

In order to reduce the dependence with regard to the traditional producing countries, a policy of diversification of the sources of supply was followed and exploration, in search of new layers, was developed. Thus the North Sea became a very important source of oil and gas.

These measurements bore their fruits: the oil worldwide consumption, which had culminated in 1979, fell of 14 % between 1980 and 1984 in the Western countries. The low growth of the industrialized countries also contributed to a decrease in the demand. It followed an overproduction which caused a price-cutting: the French oil bill, which was of 145 billion francs in 1984, went down to 68.2 billion francs in 1992.

Oil, strategic raw material

Whereas the production was at the beginning of the century the prerogative of the United States, today the Middle East, with 75 % of the worldwide production, is the first exporting oil area in the world, and the first supplier of North America, Western Europe and Japan. Also any rumor of instability of the area of the Persian Gulf makes it waver the world stock exchange places, oil having become the dominating raw material, which controls international economic flows partly.

Oil remains the “nerve of the war”: thus, Iraq, left economically weakened its war with Iran, militated for a rise in the prices of the crude which would have enabled him to pay its debts. The adjoining countries, producers also of oil and anxious of the growing hegemony of Iraq, supported the overproduction in order to maintain courses very low. In August 1990, Iraq, by invading Kuwait, caused the war of the Gulf, generating an imbalance of the world economic order. Out of fear of a third oil crisis the Western countries, via UNO, wanted to oblige Iraq to negotiate, by establishing an economic embargo. In front of the intransigence of the Iraqi president, Saddam Hussein, the United Nations decide to intervene militarily (operation “Storm of the desert”, January-February 1991), which caused in a few weeks the defeat of the Iraqis.

Oil reserves and resources

The specialists distinguish the resources, which are the whole of volumes of identified hydrocarbons, including those whose geologists do nothing but suppose the existence, and the reserves in a strict sense, which are volumes of the layers known with certainty, and which one knows that the exploitation will be profitable. But this profitability depends on the price to which it will be possible to sell extracted oil. If the courses of oil crumble, the layers most difficult to exploit will not be profitable any more, the cost of a possible exploitation exceeding the selling price. Deprived by these volumes, the reserves thus will decrease even if nobody still touched with the layers.

According to optimistic studies estimating oil world reserves, in 1996, to approximately 160 billion tons, those could last about forty years to the rhythm of consumption current; it should be hoped that the rhythm of the discoveries will follow that of consumption, and that it will be possible to affirm, in 2030, the existence of reserves until 2080; nevertheless, of many geologists affirm that the essence of oil and gas was already discovered, and that a fall of the production is inevitable. In 1997.80 % of produced oil came indeed from layers discovered before 1973; the majority of them are declining today. In the years 1990, the oil companies discovered on average a billion tons of oil per annum, but in 1997, they annually extracted from it more than three times this value. In addition, according to certain observers, crude oil world reserves are over-estimated (in particular by the national companies, which hold the monopoly of the oil rights in the countries of OPEC), and they rose, in fact, with the beginning of the year 1997, not to 160 billion tons but to approximately 120 billion tons (quantity of oil having a probability of 50 % of chance to be extracted from known layers). These reserves, and the new layers remaining to be discovered (estimated at 20 billion tons) would provide then hardly more than the cumulated quantity of oil already consumed to date (a little more than 110 billion tons).

Nonconventional oil resources

Certain geologists estimate that the not conventional reserves known as can represent a hope for the future. The hydrocarbons are indeed present everywhere in nature: the gases released by the decomposition of the living matter (marsh gas, for example) or the methane dissolved in subterranean water represent an immense potential source of natural gas. In the same way, of great quantities of rocks, the oil shales, contain kerogen which it is enough to bring up to a certain temperature to extract some from useful hydrocarbons. It is also by heating them that one can extract from heavy hydrocarbons of the tar sands, whose layers are abundant.