What is Oil?

What is Oil?

Etymological Roots and the Compound

The word petroleum which is a compound word with Latin roots of petro (rock) and oleum (oil) means rock oil. Petroleum is a composite consisting of hydrogen and carbon, which also includes a small amount of nitrogen, oxygen and sulphur; it does not have a simple formula.


The name given to unrefined liquid oil is crude oil, oil in gas form is natural gas, oil formed with semi solid and solid heavy hydrocarbon and tar is called asphalt, bitumen, tar etc. depending on its properties and local use. Main components of crude oil and natural gas are hydrogen and carbon therefore it is also called hydrocarbon.


Crude oil is comprised of dissolved gas, tar and additives in hydrocarbons in different amounts. Light (high gravity) oil is light brown, yellow or green in color whereas heavy (low gravity) oil is dark brown or black. Mostly light and white products are produced with the refinery of high gravity oil such as oil, gas oil, and diesel oil; while heavy and black products are produced with the refinery of low gravity oil.


Oil Formation

All natural hydrocarbons have originated from the dissolution of the organics. Millions of years ago rivers and wind carried sand, aggregates, mud and various ions to the basins of the time, and all these materials formed ever-thickening deposits at the bottom of the water. The residues of the dead or alive living things also mingled with this deposit. Down layers solidified under the pressure of new layers in the upside which dissolve over them and transformed into sedimentary rocks.


Compound hydrogen and carbon molecules in the fossils mingled with mud were disintegrated in time with the effect of heat and pressure and formed the hydrocarbons. Therefore crude oil and natural gas is known as fossil fuel along with coal.


Oil in Nature

Oil and sea water leaked through the condensed deposits in drops in the quest of some spaces in which they could accumulate and moved towards more porous rocks with such spaces. This movement of oil from the main rock rich in high organic materials towards porous rocks is known as the primary movement.

As the oil is lighter than water and other pressure towards the sides and upwards within the new porous rock in which it moved. This movement within the porous (reservoir – dam) rock is called the secondary movement. Some oil somehow leaks to the surface or the bottom of the sea in this way, while some other accumulates in some underground compositions called oil trap and it is entrapped in cracks, chops and pores of the dam rock for millions of years. There is a need for non-permeable layer (covering rock) so that the oil can be entrapped in the dam rock which prevents such leakages and oil loss. The traps in which oil accumulates are formed as a result of tectonic events or stratigraphic (stratification) events.


Oil Exploration

The aim of the experts in oil exploration is to identify the oil traps. Therefore oil exploration is first of all a geological problem which requires special knowledge and technique. There is no method to directly show the existence of oil underground.


The first phase in oil exploration is to find out geologically convenient spots where hydrocarbons may be present. For this aim, aerial or space photos are used. It is necessary for the site where oil will be explored to be comprised of sedimentary rocks in the first place. Also it is necessary to thoroughly explore the following;

  • Main rock which may possible have formed oil,
  • Dam rock in which oil may have accumulated,
  • Covering rock to entrap the rock in the dam and prevent its loss,
  • Distribution, location and geological ages of the types of rock in the region,
  • Structural forms created by the curves and cracks in the crust of the earth,
  • And the deposit spots of the rocks.

First of all, geological etudes are carried out for the identification of oil and natural gas sites. Geophysical etudes such as seismic, gravity, magnetic and resistivity are followed to measure particular physical features of the crust of the earth. Seismic, which is the method most frequently used to find the hydrocarbon accumulations underground, means the return of the sound waves sent underground from an artificial source back to the surface through reflecting back from various rocks and its record with instruments called geophone. These records are processed and interpreted in computer programs and locations of possible oil accumulations are found. However the presence of any oil or natural gas reservoir underground can only be confirmed by drilling a well and making production.


  • The first well drilled in an attempt to find a new site for oil is called exploration well;
  • The well in which oil or natural gas was found is called expedition well;
  • The well in which oil or natural gas was not found is the dry well;
  • The well which only included water is a water well;
  • The well which is drilled after the expedition well on the same reservoir to confirm the expedition and to identify the size of the site is called a fixation well;
  • The wells drilled after the identification of the size of the oil site and the amount of producible oil to surface this oil are called the development or production wells.


Oil Production

Oil reserve of any region should be dealt with separately from the particular oil resources there. Known oil and natural gas amount in the reservoirs in the region comprises the in situ reserve. However, it is not possible to produce a large part of this reserve. After the identification of the spread of the oil bed, in situ reserve in the dam rock and how much of this amount can be produced is measured, which is the producible reserve.


Producible reserve depends on the quality of oil and the porosity and permeability features of the dam rock. This rate is between 5-44% in Turkey according to the quality of the sites. Oil outside this rate is left underground and fails to be produced. Oil resources are always more than the reserves. Because in situ reserve comprises of other resources which can generate oil also including unexplored and undeveloped reserves.


Drilled oil is transferred to the large tanks in the aggregation stations through pipe lines and from there to refineries. Crude oil is transformed into various oil products in refineries (liquid gas, diesel oil, gas oil, fuel-oil, asphalt, mineral oil, etc.) and offered to the customers through fuel oil stations.


Basic mechanisms, which enhance the flow of hydrocarbon from the reservoir in the porous and permeable environment underground to the well, include the rock and fluid expansion associated with pressure decrease to take place with production, expansion of the melted gas inside the oil which is freed due to the pressure decrease, water pressure and the effect of gravity. Oil bursts along the production wells if there is sufficient pressure from the oil dam and it is drilled through pumps if the pressure is not sufficient. Primary techniques used in the production include the following:

  • Horsehead
  • Gas injection into the well
  • Hydraulic
  • Bore
  • Electricity fuel pick-up unit

The production carried out as the hydrocarbons in the reservoir flow into the well with the help of these mechanisms is called the primary production. Increasing the final production through injection of various fluids into the reservoir is called the secondary production.


Oil Features and Classification

Following factors are taken into account in the classification of the oil produced in the world:

  • Specific gravity of the oil
  • Viscosity
  • Sulphur amount in content

The definition of API Gravity suggested by American Petroleum Institute (API) based on the specific gravity is generally acknowledged throughout the world for oil classification. Gravity, which is an international unit, varies between 10 and 48. Gravity does not mean the density of the oil. The formula of the gravity which is inversely correlated with density is as the following:


Density of the oil = 141.5 / (131.5 + gravity of oil)


API gravity of the oil with low specific gravity is high according to this formula. The classification of oil according to gravity is as the following.

  • Light > 31
  • Medium 20 – 31
  • Heavy 10 – 20
  • Natural Bitumen < 10

Today, 90% of world demand of oil is met with light or medium oil due to the convenience of production, transmission and processing. Only 25% of the oil resources in the world are comprised of light and medium oil. Heavy oil reserves in the world are located in Brazil, Canada, USA, Russia and Venezuela. Yet the transmission of heavy oil and its utilization in current refineries as raw material requires improvisation, which bears additional cost.


Another factor in the production and processing of the crude oil is the viscosity which is defined as the resistance against flow. As the production, transmission and processing of oil with low viscosity is more convenient and economic, such oil is preferred in the world.


Oil is also classified in terms of its sulphur content. Although there are no precise criteria set for such classification, generally oil is considered sweet if the sulphur percentage is under 0,5%.


Source: General Directorate of Petroleum Affairs