BOOKLET
1. Background: "The Oil Adventure – its history and importance”
1.1 Conditions for recovering oil ... 3
1.2 Geological conditions for finding oil ... 3
1.3 Predictions about the future of the oil adventure ... 3
1.4 Denmark's oil adventure ... 4
1.5 Geological conditions in the North Sea ... 5
1.6 The battle for North Sea oil ... 5
1.7 The state's entry into the oil adventure ... 6
2. Theme: "Sedimentology" 2.1 Organic sediments ... 7
2.2 Oil formation ... 7
2.3 Traps ... 7
2.4 Crude oil – density and sulphur content... 9
3. Theme: "Seismology" 3.1 Seismic measurements ... 10
3.2 Gravimetric measurements ... 11
3.3 2D, 3D and 4D seismic analyses ... 11
3.4 Terminology for seismic analyses ... 13
4. Theme: "Oil wells" 4.1 Exploratory wells... 14
4.2 Drilling workplaces ... 14
4.3 "Anatomy" of an oil well ... 15
5. Theme: "Oil production and exploration" 5.1 Oil production ... 17
5.2 Maintaining oil production ... 17
5.3 Production costs ... 18
Population and income growth are the two most powerful driving forces
behind the demand for energy. Since 1900, the world’s population has
more than quadrupled. More people with higher income means that the
production and consumption of energy will rise, making oil an
indispensa-ble resource all over the world for decades to come.
1.1 Conditions for recovering oil
In many ways, crude oil is a fantastic source of energy: it is easy and safe to transport, it is widely dispersed, and only a small area of the earth's surface has been surveyed for oil. The working conditions are often harsh: cold weather, storms, sea ice and icebergs are part of the challenge. Oil exploration is a costly and demanding business. It requires a keen focus on safety. It also requires teamwork and specialists.
1.2 Geological conditions for finding oil
It is a geologist's job to designate the areas where the right conditions are present. If just one condition is lacking, the drilling of an oil well is usually futile. The five basic condi-tions for finding oil:
1. There must have been a "mother rock" where it was
possible for dead animals and plants to be converted into hydrocarbons.
2. A kilometres-thick stratum must be deposited on top of
this mother rock to create sufficiently high temperature and pressure.
3. There must have been cracks above the mother rock
that would enable the hydrocarbons to migrate up to higher strata.
4. Here there must be a porous stratum with space to
absorb oil and gas, like a sponge absorbs water. This stratum, which can be made up of sand or chalk, is called a reservoir rock.
5. Next, the reservoir rock must be topped by an
imperme-able stratum, such as clay, through which the oil cannot penetrate and "escape" to the surface.
1.3 Predictions about the future of the oil adventure
There have been many predictions of an abrupt end to the oil adventure over the years. In the early 1930s, the world's oil reserves were believed to be able to last for only another fifteen years – a very short time in relation to the rate of oil consumption at the time. In 1975, the reserves
were assessed as being able to meet 29 years of consump-tion. The current assessment is that known and estimated reserves will meet another 35 years of consumption, in part because alternative forms of energy are also gaining ground. Perhaps it goes without saying that determining the vol-ume of the world's total oil reserves is not easy. There is so much uncertainty about the assessment that at one point it was believed that the OPEC countries possessed two-thirds of all of the world's known oil reserves, but at another time, this was assessed to be only one-third!
The fact that Canada is considered as having the biggest oil reserves in the world is something new. Canada is followed by Saudi Arabia (because tar sand is also included today, even though it requires a different recovery technology).
1.
Background: "The Oil Adventure – History and importance”
Theme booklet
Top
3
oil-producing
countries in Europe:
NO
1.4 Denmark's oil adventure
Denmark is a player in the global oil-production indus-try. There are five important oil-producing countries in Europe: Norway, the UK (Scotland), Denmark, Germany and the Netherlands. Denmark has Europe's third-highest oil exports and is ranked 45th at world level. Denmark's Maersk Oil is involved in oil exploration and oil production all over the world.
From a global perspective, Denmark's oil fields in the North Sea produce only 0.5% of the world's total oil, and around 7% of Europe's oil.
High oil prices have probably stimulated oil exploration and help to prolong it, just as technological oil-production methods are developing rapidly.
Originally it was believed that only 6% of the oil in chalk reservoirs in the Danish North Sea basin would be profit-able to produce. Today, improved technology, such as water flooding and horizontal drilling, has increased this potential rate of recovery to 25–30%.
Thanks to these improvements in technology, Maersk Oil has been able to develop oil fields once considered uneconomical to produce from. However, there remain several unexploited but discovered oil fields in the Danish sector of the North Sea, which new technology may make it possible to produce from in the future.
Danish sector North Sea oil fields
1.5 Geological conditions in the North Sea
The North Sea basin was formed more than 260 million years ago during the Early Permian Period when the sub-strata began to sink. Initially, the basin was more clearly divided into three sub-basins: The Danish Basin, the North German Basin and the Central Graben. The differences between these basins disappeared later on.
Throughout most of its existence, the North Sea basin has been covered by water, but sedimentation has also been extensive. The geological development of sea and sedimentation, particularly in the Central Graben, led to the formation large deposits of oil and gas. Some deposits are thick, impenetrable layers of clay, which have sealed oil and gas reservoirs since the Tertiary Period.
Most of Denmark's oil and natural gas is produced from porous limestone reservoirs from the Upper Cretaceous (96–65 million years ago) and Danian (65–61 million years ago) periods, whereas a lesser volume of oil is produced from sandstone reservoirs from the Jurassic (203–135 million years ago) and Paleocene (61–53 million years ago) periods. There are 19 fields currently producing oil, where the fields Dan, Gorm, Skjold and Halfdan have produced most of the oil, and the fields Dan, Tyra, Halfdan and Harald most of the natural gas.
1.6 The battle for North Sea oil
Norway, Denmark and other countries entered into an agreement on the North Sea oil fields in 1963. The countries agreed on a centre line in the sea between the countries as an economic border. Germany would not recognise the cen-tre line, however, because it left Germany with too small a section of the continental shelf in the North Sea.
Instead, Germany intended to observe the "coastline princi-ple" according to which territorial waters are apportioned. The International Court upheld Germany's views in part in 1969. Denmark and the Netherlands argued that the division along the centre line was natural and expedient, but the court ruled that there was no precedence for this perception. If Germany's position had been accepted from the outset, the Ekofisk field, the richest in the North Sea, would not have gone to Norway but to Denmark.
Initially, there was only one concession holder in Denmark's oil exploration sector: A.P. Møller - Mærsk. At the time, however, the company did not possess oil-production expertise. Therefore, DUC (Danish Underground Consortium) was formed. It comprised three companies: Maersk, Shell and Chevron.
1.7 The state's entry into the oil adventure
Oil crises took place in 1973 and 1979. The crises aroused great concern within the then Danish government and there was uncertainty as to whether Maersk could "keep pace" with the oil production. For this reason, political pressure compelled the company to relinquish more and more of its offshore areas to other operators (in '76, '79, '81 and '82).
Today, the Danish state is also a member of DUC (2013) through the company Nordsøfonden (20%).
Concessions (permits, licences) under this division are granted through calls for tenders, where interested com-panies submit tenders for continental shelf areas (seabed areas less than 200 metres deep, bordering on mainland).
Interest is generally great, because excellent, promising new oil finds have been made in the North Sea in recent years. Thirty-one parties have already registered their interest in what is now the seventh round of tendering that concluded in late 2013. This is twice as many participants as in the last round in 2005.
The facts that a lot of oil is still recoverable in the North Sea, and that new technologies are enabling the production of ever increasing volumes of oil from substrata is good news for Denmark. Since 1964, 2,700 exploratory wells have been drilled in the North Sea, which has resulted in around 800 finds of oil. The oil on which these exploratory efforts are based translates into roughly 100 billion barrels of oil.
Oil and oil geology are actually the result of chemical processes that took
place deep inside the earth over millions of years. Most of the oil and gas
in Denmark's sector of the North Sea is found in substrata that are 60-70
million years old.
2.1 Organic sediments
There are three types of sediment: inorganic, organic and chemical. Organic sediment is particularly important in this context, because it is the decomposition of organic material, primarily algae and plankton in the sea, that formed the oil.
Hundreds of millions of years ago, the sea floor was low in oxygen. As a result, dead animals and plants that ended up there did not rot but were deposited as organic sediments in substrata and slowly pushed downward over the ages. Pressure from new overlying strata increased the pressure and temperature around the sediments, which was ideal for transforming the organic residue into oil.
The remains of the original organisms that could not be included in the chemical transformation process fossilised into layers of limestone or siliceous sedimentary rock. Rocks that can form oil or gas are called source rocks. A porous or permeable rock where the oil or gas collects after migrating is called a reservoir rock.
Click here to watch an animated film about how oil is formed or type http://www.youtube.com/ watch?v=nrvIabfQuic in your browser.
2.2 Oil formation
At the right pressure and temperature, a chemical process will take place in which the various components (fats, fatty acids, proteins, etc.) are slowly converted into hydro-carbons (the chemical designation for oil and gas). The temperature must be between 100 and 150 degrees Celsius. The ideal pressure and temperature conditions are called the "oil window". The correct temperature is usually found at depths of 2–3 kilometres below the surface.
Oil cannot form close to the earth’s surface, as tempera-tures and pressure levels are usually not high enough.
According to conservative estimates, up to 70% of the crude oil ever formed will never be found because it evapo-rated into the atmosphere long ago.
2.3 Traps
Density and pressure conditions in the dense source rock where oil and gas were formed force them away from the source rock. Oil and gas migrate upwards because they are lighter than the water contained in the rocks.
Oil or gas that does not dissipate completely but which accumulates in a reservoir rock requires the existence of an impermeable layer covering the reservoir to create a trap. There are three types of trap all of which comprise a porous stratum covered by a dense impermeable stratum.
Traps can form in folded strata where the trap is created in rising folds or, for instance, by salt vertically intruding into overlying rock from below to create a salt diapir/dome. The third type of trap is created by faults, in which the strata are displaced in relation to one another. North Sea oil is often located at fault traps or in salt diapirs (domes).
2.
Theme: "Sedimentology"
Theme booklet
70%
of the crude oil formed
will never be found.
Core samples of North Sea Oil
2.4 Crude oil – density and sulphur content
The oil that exists in natural underground reservoirs is called crude oil. Crude oil is a mixture of hydrocarbons that remain fluid at atmospheric pressure after being brought up to the surface.
Crude oil is not a "pure" product. There are wide regional differences in the chemical composition of crude oil. For instance several different hydrocarbons in varying phases appear as gas or fluid in crude oil. Also, crude oil contains a number of non-hydrocarbons, like sulphur and metals, which can affect its quality. Two of the most important quality parameters of crude oil are therefore density and sulphur content.
Crude oil is called "light" if it has a low density content of sul-phur and heavy natural components such as metal oxides. Light crude oil has a higher market price, solely because it is cheaper to process at an oil refinery into petrol or diesel oil. The opposite is obviously true of heavy crude oil.
Crude oil is called "sweet" if it has a low content of sulphur. Crude oil high in sulphur is called "sour".
The figure below depicts typical crude oils in relation to density and sulphur content.
According to the figure, "North Sea – Brent" is a relatively sweet, light oil whose average market price is high.
Density and sulphur content of selected crude oils
Seismic measurements are conducted to get information about substrata.
These measurements are specifically used by the oil industry to determine
the location of oil reservoirs.
3.1 Seismic measurements
A seismic measurement is performed by sending seismic waves from a sound source into substrata. When a seismic wave meets different geological strata, some of the wave is reflected to the surface. Sound waves move faster through hard materials than soft materials.
After this, the reflected wave is captured by geophones (microphones designed to listen to substrata) located in the area. The results form an image of the subsurface geology. The image can be used to find geological structures which could contain oil or gas if the right conditions are present.
Seismic measurement
3.2 Gravimetric measurements
In addition to seismic measurements, gravimetric meas-urements are used to locate oil reservoirs. These are grav-ity measurements taken from an aeroplane or helicopter which can help to identify whether the rocks below are dense (heavy) or saturated with oil or gas.
The technology is based on the laws of physics stating that gravitational acceleration at the surface is affected by the density of the rocks in the substrata. The greater a rock's density, the greater the gravitational acceleration. By measuring variations in gravitational acceleration across an area, it is possible to map the dispersion of heavy and light materials below the surface.
Gravimetric measurements
A geological model indicating density in g/cm3 (kg/m3 x 10-3) is shown at the bottom of the figure below.
Nederst på ovenstående figur ses en geologisk model med angivelse af
massefylde i g/cm³ (kg/m³ x 10-3). Kilde: geologi.dk/oliegas
Source: http://www.geologi.dk/oliegas
3.3 2D, 3D and 4D seismic analyses
Two-dimensional (2D) seismic analyses provide a cross-section of substrata. Collecting 2D seismic lines into a finely meshed network forms a three-dimensional (3D) image of the substrata. This is called 3D seismic imaging. When 3D seismic data are retrieved from the same area at annual intervals and compared, a fourth dimension arises: time. 4D seismic imaging can be used to depict the changes that have occurred in a producing oil field over time. 4D seismic imaging also makes it possible to determine the direction in which the oil has flowed towards the production wells and to determine which areas of a field have not been sufficiently drained. Oil companies can use this knowledge to optimise the oil recovery process and set up supplemen-tary wells at optimal locations.
Offshore seismic analyses
In the course of a few seconds, seismic analyses can gener-ate millions of data that have to be processed by powerful computers.
When data is collected at sea, the seismic equipment is towed by a specially converted measurement vessel. Seismic waves emitted by an air gun towed by the ship are captured by sensitive hydrophones (microphones designed for underwater use). The hydrophones are positioned along cables 5–8-kilometres in length towed by the ship, and from here the hydrophones can capture the reflected seismic signals.
Horizontal well
Maersk Oil has become a leader in the industry in terms of long-range horizontal well drilling, thanks to the develop-ment of excellent expertise in the fields of geocontrol systems, drill-bit positioning and charting, as well as well drilling and stimulation. Maersk Oil was the first oil company to use horizontal drilling technology in the North Sea and has become an expert in developing wells with patterns of closely spaced horizontal well lines. Lessons learnt in this field enable Maersk Oil to recover gas and oil from densely positioned oil fields.
Horizontal wells were first used in the light limestone of the Dan field in Denmark's sector of the North Sea, in 1987. Since then, horizontal wells have become the preferred technology of the entire oil industry for developing imper-meable reservoirs. The Halfdan Oil Field in Denmark's North Sea sector and the Al Shaheen Oil Field in the Persian Gulf off of Qatar were both developed using horizontal oil wells, including a number of auxiliary side-track wells. This enables the wells to cover a larger section of the reservoir and saves platform capacity.
Maersk Oil developed pioneering
technology in the field of horizontal
drilling. The whole industry now
uses this same technology.
If the recovery process takes place in shallow waters, the method is the same, but requires smaller vessels and shorter drill strings.
For offshore seismic analyses, special consideration must always be shown to marine mammals, such as porpoises, which communicate using sound signals. For this reason, the Danish Energy Agency must approve the analysis programmes in advance.
3.4 Terminology for seismic analyses
The properties and layering of the various rocks are differ-entiated by analysing seismic data during oil exploration.
Source rocks and reservoir rocks
A source rock usually contains sufficient quantities of organic material that at the right temperature and pres-sure conditions can can be converted into hydrocarbons (oil and gas). However, oil/gas does not have to stay in the rock afterwards. A reservoir rock, on the other hand, can contain oil/gas. Reservoir rock is porous and contains water, oil or gas (fluids) in the void between the grains of mineral, i.e. in the pores. Porosity indicates the volume of pores and thus how much space is available for fluids in the rock. The ability of fluids to penetrate the pores is called permeability. It indicates the ease with which fluids can flow through the rock.
Migration and seal
When hydrocarbons are formed in a source rock, a natural flow, called migration, will begin if the pressure is suffi-ciently high. This is because oil and gas are lighter than the water in the pores. Therefore, oil and gas migrate upwards. The migration can take place through pores and cracks and along faults in the various substrata. If the hydrocarbons migrate to a reservoir rock with a seal, the oil/gas can collect here. A seal of a trap (see section 2.3) is a dense, overlying cap rock such as salt or shale through which the oil/gas cannot penetrate.
A seismic profile of Denmark's North Sea sector.
The profile is 40 kilometres long and extends from the surface to a depth of about 4 kilometres. A salt dome (salt diapir) is visible in the middle of the profile.
The very first oil well in modern times was established in 1859. Back then, a
pole equipped with a chisel at the end was repeatedly hammered down into
the ground. Today, a rotating drill bit is used to dig down into substrata.
4.1 Exploratory wells
An oil well serves two obvious purposes. First, to determine whether an oil reservoir is found at the drilling site. Secondly, to make it possible to recover and produce oil (or gas). Once geologists and geophysicists have completed their preliminary work, exploratory wells are made first to determine whether actual production wells can be com-missioned. Even the finest seismic measurements cannot unequivocally identify an oil field's actual potential for oil production. The exploratory wells' profiles and a meticu-lous technological survey of the drilling profile's rocks form the basis for constructing a geological "scenario" of sorts. This "scenario" tells something about the area's general geological structure and the rocks' properties that are relevant to oil.
Making an oil well is a costly procedure, usually in excess of USD 55 million. The price depends on the depth of the well, whether the well is drilled onshore or offshore and whether technical complications arise. But the price also depends on the scope of the measurements conducted during the drilling process.
4.2 Drilling workplaces
Drilling for oil on the sea floor is a complicated process. It takes place from drilling rigs or drilling platforms, which come in different varieties depending on requirements and needs.
DRILLSHIP
Operates at water depths up to 12,000 ft.
SEMI-SUBMERSIBLE
Operates at water depths up to 10,000 ft. JACK-UP RIG Operates at water depths up to 500 ft. DRILLING BARGE Operates in shallow waters LANDRIG
Jack-up rigs
A jack-up rig is usually used in shallow water and stands on steel legs affixed to the sea floor. Once it has been put into place, the actual drilling process is not very different from a oil derrick on land. The system which hoists the rig up/down to its three legs is like a jack screw. This enables the rig to be relocated with relative ease for new jobs once its current task is completed. Jack-up rigs are used particu-larly in the North Sea, Persian Gulf and the Red Sea.
At deeper depths, however, different types of anchoring are used for both drilling and production platforms. For obvi-ous technical reasons, it is crucial that the rig is firmly in place and does not move from the position chosen. In these situations, GPS is a crucial auxiliary aid.
Semi-submersible rigs
Semi-submersible rigs are widely used in deep-water operations. These platforms are based on a hull of sorts pri-marily comprising pontoons that can be filled with water as required. This makes them very stable and seaworthy and it is possible to keep the structure afloat and firmly upright at the same time. It is relatively easy to move this type of rig from one place to another.
These rigs are particularly suited for working in particu-larly stormy areas with rough seas. The rigs are usually held in place by anchors but GPS positioning is also used. Semi-submersible rigs can be used at water depths of 60–3,000 metres and are suited for the shallow continental shelf (at depths of 0 to 200 metres) found in the North Sea.
Drillship
A marine vessel modified to drill oil and gas wells equipped with a drilling derrick. Typically employed in deep and ultra-deep waters from 2,000 to more than 10,000 feet. 4.3 "Anatomy" of an oil well
On the drilling rig, a drill bit containing three sharp conical rollers is screwed onto a long steel pipe (drill string). The drill bit crushes rock into fine, gravel-like material. The drill string is suspended from a drilling tower where it is rotated by a motor. Now the drill string is lowered from the deck down through a thick steel pipe to the sea floor and rotated to make the drill grind its way down into the substrata. Once the drill bit has worked its way a few hundred metres into the substrata, it is replaced. If particularly interest-ing strata are found along the way, the drill bit is briefly replaced by a type that is capable of removing a well core, which is brought to the surface for further investigation.
Watch an animated film about drilling for oil. Click here or type http://www.youtube.com/watch?v=iVXyrjrDo7I in
your browser.
Ultra Harsh
Jack-up Rig
The drill bit
grinds into
For every thirty metres of drilling, the drilling manager must stop the motor and attach a new section of drilling pipe before the drilling can continue.
The interconnected pipe sections are called the drill string. They keep the wellbore open and free of incident materials. The wellbore's sizes varies depending on the depth. At the surface, the hole diameter is usually 50–90 cm. Further down, the diameter of the wellbore narrows to 7–15 cm. There are a few open holes in the steel pipes that allow the oil in the surrounding strata to seep into the well from where it can be pumped up to the surface.
Temperatures and pressure
It is easy to imagine how drilling into rocks like granite and dense shale could cause overheating. To counteract this, drilling fluid (drilling mud) is used during the drilling process to lubricate, cool and carrying out drill cuttings to the platform where the fluid is cleansed and recirculated. Another property of drilling fluid is that it forms a lining of the borehole so the wellbore does not collapse during the work,
When drilling at great depths, the drill bit will encounter highly pressurised water, gas or oil. In these instances, the long column of drilling fluid regulates the pressure as its weight and the pressure with which it is conveyed serves as a counterweight to prevent an uncontrolled blowout of substrata material to the surface.
Producing crude oil is a lengthy, complex process. After geological
sur-veys, comprising geological surface charting, seismic analyses and
explor-atory wells, followed by analyses of the penetrated rocks, the next step is
to determine whether it is profitable to start up a production process.
5.1 Oil production
The rate of recovery from new oil fields is often uncertain. Oil prices are an important factor in deciding whether it is profitable to invest in producing oil at a new oil field. In 1999, the price of one barrel of oil was USD 11. This price had risen to USD 100 by early 2008. Over the next 25 to 30 years, the price is expected to stabilise at USD 100–200. When the price per barrel is low, small oil fields are unprofitable, whereas high prices can send the industry into overdrive.
Many wells – sometimes more than 30 – need to be made before an oil field is put into production. Whether a proven deposit can be put into production depends on the porosity and permeability of the reservoir rock and the actual oil saturation. The term "oil in place" denotes the capacity calculated for the oil field. More than 90% of all Danish oil wells are drilled in the western part of Denmark's North Sea sector where Denmark's oil and gas fields are also located.
Watch an animated film about how oil is stored in substrata. Click here or type https://www.youtube.com/ watch?v=OpmjdIai-go in your browser.
5.2 Maintaining oil production
The recovery of oil from oil fields, cannot last forever. Oil wells frequently need to be stimulated in order for produc-tion to continue. This can be done by pumping water or gas into substrata through a nearby well. Seawater can also be pumped down into substrata. More than 300,000 barrels of oil and more than 500,000 barrels of water are pumped up from North Sea oil fields every day. At the same time, 800,000 barrels of water are pumped back down. If a company wants to extend an oil field's lifespan, the company submits an application for extension. The exten-sion applied for may extend the life of the field by up to 30 years, because an exploratory well-drilling programme usually ends near the expiry of a licensing period.
5.
Theme: "Oil production and exploration"
Theme booklet
300,000
barrels of oil
are pumped
up from the
North Sea
every day
5.3 Production costs
The total costs of crude oil exploration and production are called "upstream costs". This figure is arrived at by adding lifting costs and finding costs.
Lifting costs are defined as the cost of operating and maintaining the wells with associated equipment and for bringing up the oil/gas to the surface.
Finding costs are defined as the costs of exploration and exploratory wells, the costs for licences/rights and the cost of extended exploration for reserves in adjacent areas.
As the table shows, production costs in the Middle East are far lower than in the US and much lower than in Denmark's oil fields as well.
To calculate the costs of transporting crude oil to a refinery and further processing the oil into products, even more costs have to be added.
The most important single offshore cost factor is the price of leasing a drilling rig. Drilling rigs are usually leased from rig owners and the medium sized cost of a medium-sized drilling rig varries from 160,000 - 500,000 USD per day. Drilling down to a depth of 4,000 metres for instance could be carried out in about 100 days, if everything goes according to plan. This does not include the cost of crew, lining pipe, drilling fluid, drill bits, transport and a wide range of other services.
Costs for producing crude oil and natural gas, 2007–2009 Lifting
costs Finding costs Total upstream costs
USA
average USD 12.18 USD 21.58 USD 33.76
Onshore USD 12.73 USD 18.65 USD 31.38
Offshore USD 10.09 USD 44.51 USD 51.60
Middle East
average USD 9.89 USD 6.99 USD 16.88
5,618 cubic feet of gas = 1 barrel of olie.
Source: Eia.gov
More than half of all
oil produced is used
to produce more than
different products
which you are
familiar with in
everyday life, such as
spectacles
5.4 Refining crude oil
Crude oil is a raw material that is unusable as is. Crude oil has to be processed into oil products like petroleum, jet fuel, petrol, diesel oil, fuel oil and liquid gases, which are used for paint, plastic and thin fibres like nylon and terylene that are woven into fabric.
Crude oil is made of many different components (mol-ecules) each of which has a different boiling point. This makes it possible to separate oil into several fractions by heating it. This process is called distillation. By contrast, natural gas is made up of only one type of molecule, overwhelmingly methane, which is why it only needs to be cleansed before it can be used.
Once oil has been refined, it is loaded onto small product tank ships that are designed to transport several different kinds of oil products. The product tank ships convey the various oil products to ports.
It is the refinery's job to deliver the products we need. Crude oils vary greatly. Some produce lots of petrol and only a little fuel oil. The reverse is true of others. Therefore, a refinery will chose crude oils that produce most of the products demanded by local consumers.
Crude oil tank Desalter
Fired heater
Distillation
Cracking
Distillation residue
20% petrol and gas, up to 165o
15% petroleum, 165–240o
28% light gas oil, 240–300o