According to generally accepted theory, petroleum is derived from ancient biomass. It is a fossil fuel derived from ancient fossilized organic materials. The theory was initially based on the isolation of molecules from petroleum that closely resemble known biomolecules.
Structure of vanadium porphyrin compound extracted from petroleum by Alfred Treibs, father of organic geochemistry. Treibs noted the close structural similarity of this molecule and chlorophyll.
More specifically, crude oil and natural gas are products of heating of ancient organic materials (i.e. kerogen) over geological time. Formation of petroleum occurs from hydrocarbon pyrolysis, in a variety of mostly endothermic reactions at high temperature and/or pressure. Today's oil formed from the preserved remains of prehistoric zooplankton and algae, which had settled to a sea or lake bottom in large quantities under anoxic conditions (the remains of prehistoric terrestrial plants, on the other hand, tended to form coal). Over geological time the organic matter mixed with mud, and was buried under heavy layers of sediment resulting in high levels of heat and pressure (diagenesis). This process caused the organic matter to change, first into a waxy material known as kerogen, which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis.
Geologists often refer to the temperature range in which oil forms as an "oil window" below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil which is formed at extreme depths may migrate and become trapped at much shallower depths than where it was formed. The Athabasca Oil Sands is one example of this.
Diagram from http://letslearngeology.com
A small number of geologists adhere to the abiogenic petroleum origin hypothesis and maintain that hydrocarbons of purely inorganic origin exist within Earth's interior. Chemists Marcellin Berthelot and Dmitri Mendeleev, as well as astronomer Thomas Gold championed the theory in the Western world by supporting the work done by Nikolai Kudryavtsev in the 1950s. It is currently supported primarily by Kenney and Krayushkin.
The abiogenic origin hypothesis has not yet been ruled out. Its advocates consider that it is "still an open question". Extensive research into the chemical structure of kerogen has identified algae as the primary source of oil. The abiogenic origin hypothesis fails to explain the presence of these markers in kerogen and oil, as well as failing to explain how inorganic origin could be achieved at temperatures and pressures sufficient to convert kerogen to graphite. It has not been successfully used in uncovering oil deposits by geologists, as the hypothesis lacks any mechanism for determining where the process may occur. More recently scientists at the Carnegie Institution for Science have found that ethane and heavier hydrocarbons can be synthesized under conditions of the upper mantle.
Crude oil reservoirs
Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.
The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where hydrocarbons are broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The latter set is regularly used in petrochemical plants and oil refineries.
Wells are drilled into oil reservoirs to extract the crude oil. "Natural lift" production methods that rely on the natural reservoir pressure to force the oil to the surface are usually sufficient for a while after reservoirs are first tapped. In some reservoirs, such as in the
Unconventional oil reservoirs
Oil-eating bacteria biodegrades oil that has escaped to the surface. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping and being biodegraded, but they contain so much migrating oil that, although most of it has escaped, vast amounts are still present, more than can be found in conventional oil reservoirs. The lighter fractions of the crude oil are destroyed first, resulting in reservoirs containing an extremely heavy form of crude oil, called crude bitumen in
On the other hand, oil shales are source rocks that have not been exposed to heat or pressure long enough to convert their trapped hydrocarbons into crude oil. Technically speaking, oil shales are not really shales and do not really contain oil, but are usually relatively hard rocks called marls containing a waxy substance called kerogen. The kerogen trapped in the rock can be converted into crude oil using heat and pressure to simulate natural processes. The method has been known for centuries and was patented in 1694 under British Crown Patent No. 330 covering, "A way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone." Although oil shales are found in many countries, the
The petroleum industry generally classifies crude oil by the geographic location it is produced in (e.g.
The American Petroleum Institute gravity, or API gravity
Is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is thus a measure of the relative density of a petroleum liquid and the density of water, but it is used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically API gravity has no units (see the formula below), it is nevertheless referred to as being in “degrees”. API gravity is graduated in degrees on a hydrometer instrument and was designed so that most values would fall between 10 and 70 API gravity degrees.
The formula used to obtain the API gravity of petroleum liquids is thus:
Conversely, the specific gravity of petroleum liquids can be derived from the API gravity value as
Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60°F) would have an API gravity of:
Here is a useful photo so you don't have to do the API calculation.
The geographic location is important because it affects transportation costs to the refinery. Light crude oil is more desirable than heavy oil since it produces a higher yield of gasoline, while sweet oil commands a higher price than sour oil because it has fewer environmental problems and requires less refining to meet sulfur standards imposed on fuels in consuming countries. Each crude oil has unique molecular characteristics which are understood by the use of crude oil assay analysis in petroleum laboratories.
Barrels from an area in which the crude oil's molecular characteristics have been determined and the oil has been classified are used as pricing references throughout the world. Some of the common reference crudes are:
There are declining amounts of these benchmark oils being produced each year, so other oils are more commonly what is actually delivered. While the reference price may be for West Texas Intermediate delivered at Cushing, the actual oil being traded may be a discounted Canadian heavy oil delivered at Hardisty, Alberta, and for a Brent Blend delivered at the Shetlands, it may be a Russian Export Blend delivered at the port of Primorsk.
The petroleum industry is involved in the global processes of exploration, extraction, refining, transporting (often with oil tankers and pipelines), and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline (petrol). Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics. The industry is usually divided into three major components: upstream, midstream and downstream. Midstream operations are usually included in the downstream category.
Petroleum is vital to many industries, and is of importance to the maintenance of industrialized civilization itself, and thus is critical concern to many nations. Oil accounts for a large percentage of the world's energy consumption, ranging from a low of 32% for
The pour point of a liquid is the lowest temperature at which it will pour or flow under prescribed conditions. It is a rough indication of the lowest temperature at which oil is readily pumpable.
Also, the pour point can be defined as the minimum temperature of a liquid, particularly a lubricant, after which, on decreasing the temperature, the liquid ceases to flow.
Measuring the pour point of petroleum products
The specimen is cooled inside a cooling bath to allow the formation of paraffin wax crystals. At about 9oC above the expected pour point, and for every subsequent 3oC, the test jar is removed and tilted to check for surface movement. When the specimen does not flow when tilted, the jar is held horizontally for 5 secs. If it does not flow, 3oC is added to the corresponding temperature and the result is the pour point temperature.
It is also useful to note that failure to flow at the pour point may also be due to the effect of viscosity or the previous thermal history of the specimen. Therefore, the pour point may give a misleading view of the handling properties of the oil. Additional fluidity or pumpability tests may also be undertaken. An approximate range of pour point can be observed from the specimen's upper and lower pour point.
Two pour points can be derived which can give an approximate temperature window depending on its thermal history. Within this temperature range, the sample may appear liquid or solid. This peculiarity happens because wax crystals form more readily when it has been heated within the past 24hrs and contributes to the lower pour point.
The upper pour point is measured by pouring the test sample directly into a test jar. The sample is then cooled and then inspected for pour point as per the usual pour point method.
The lower pour point is measured by first pouring the sample into a stainless steel pressure vessel. The vessel is then screwed tight and heated to above 100oC in an oil bath. After a specified time, the vessel is removed and cooled for a short while. The sample is then poured into a test jar and immediately closed with a cork carrying the thermometer. The sample is then cooled and then inspected for pour point as per the usual pour point method.
The sample is then cooled and then inspected for pour point as per the usual pour point method.
Group 2, Group 2,
Pour Point 0ºC Pour Point 30ºC
These two crude oil samples show the difference between pour points will need different responses. The Nigerian will loose approximately 35% to evaporation where as the Indoneasian will loose nothing. The Nigerian could be chemically disperse,where as the Indoneasian will need to be recovered completely.
Viscosity is the resistance to flow. The higher the viscosity the slower the liquid will flow and the lower the quality.
Resistance of a fluid to a change in shape, or movement of neighbouring portions relative to one another. Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules. Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating. The viscosity of liquids decreases rapidly with an increase in temperature, while that of gases increases with an increase in temperature.
The SI unit for viscosity is the newton-second per square metre (N-s/m2).
This is a simple explanation taken from oilspilltraining.com
You can tilt each beaker or all together and reset them at the top. This shows the effect of cold temperature on the same oil types by clicking on the thermometer.
In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force.
Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u /∂y, in the direction perpendicular to the layers.
Here, the constant μ is known as the coefficient of viscosity, the viscosity, the dynamic viscosity, or the Newtonian viscosity.
The relationship between the shear stress and the velocity gradient can also be obtained by considering two plates closely spaced apart at a distance y, and separated by a homogeneous substance. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. If this force causes the substance between the plates to undergo shear flow (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. Combining these three relations results in the equation F = μ (Au/y), where μ is the proportionality factor called the dynamic viscosity (also called absolute viscosity, or simply viscosity). The equation can be expressed in terms of shear stress; τ = F/A = μ (u / y). The rate of shear deformation is u / y and can be also written as a shear velocity, du/dy. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained.
Ratio of the density of a substance to that of a standard substance. For solids and liquids, the standard substance is usually water at 39.2°F (4.0°C), which has a density of 1.00 kg/liter. Gases are usually compared to dry air, which has a density of 1.29 g/liter at 32°F (0°C) and 1 atmosphere pressure. Because it is a ratio of two quantities that have the same dimensions (mass per unit volume), specific gravity has no dimension. For example, the specific gravity of liquid mercury is 13.6, because its actual density is 13.6 kg/liter, 13.6 times that of water.
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage.
If a substance's relative density is less than one then it is less dense than the reference; if greater than one then it is denser than the reference. If the relative density is exactly one then the densities are equal; that is, equal volumes of the two substances have the same mass. Simplified, as water is most often used as the reference, if a liquid has a density less than 1, then it will float in water. Hence methylated spirits, with a density less than 0.8, floats on the top of water. On the other hand, an ice cube with a density of about 0.91, will sink to the bottom if placed into methylated spirits.
Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm equal to 101.325 kPa. Where it is not it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice the specific gravity as specified above is multiplied by 1000.
Relative density (RD) or specific gravity (SG) is a dimensionless quantity, as it is the ratio of either densities or weights
where RD is relative density, ρ is the density of the substance being measured, and ρ is the density of the reference. (By convention ρ, the Greek letter rho, denotes density.)
The reference material can be indicated using subscripts: RD, which means "the relative density of substance with respect to reference". If the reference is not explicitly stated then it is normally assumed to be water at 4 °C (or, more precisely, 3.98 °C, which is the temperature at which water reaches its maximum density). In SI units, the density of water is (approximately) 1000 kg/m3 or 1 g/cm3, which makes relative density calculations particularly convenient: the density of the object only needs to be divided by 1000 or 1, depending on the units.
The relative density of gases is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3. Relative density with respect to air can be obtained by
Where M is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure i.e. they are both Ideal gasses. Ideal behaviour is usually only seen at very low pressure. For example, one mol of an ideal gas occupies 22.414 L at 0 °C and 1 atmosphere whereas carbon dioxide has a molar volume of 22.259 L under those same conditions.
The density of substances varies with temperature and pressure so that it is necessary to specify the temperatures and pressures at which the densities or weights were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (101.325 kPa the variations caused by changing weather patterns) but as specific gravity usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products) variations in density caused by pressure are usually neglected at least where apparent specific gravity is being measured. For true (in vacuo) specific gravity calculations air pressure must be considered (see below). Temperatures are specified by the notation Ts/Tr) with Ts representing the temperature at which the sample's density was determined and Tr the temperature at which the reference (water) density is specified. For example SG (20°C/4°C) would be understood to mean that the density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures we note that while SGH2O = 1.000000 (20°C/20°C) it is also the case that SGH2O = 0.998203/0.998840 = 0.998363 (20°C/4°C). Here temperature is being specified using the current ITS-90 scale and the densities used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982071 and 0.9999720 resulting in an SG (20°C/4°C) value for water of 0.9982343.
The temperatures of the two materials may be explicitly stated in the density symbols; for example:
relative density: or specific gravity:
where the superscript indicates the temperature at which the density of the material is measured, and the subscript indicates the temperature of the reference substance to which it is compared.
Property of a liquid surface that causes it to act like a stretched elastic membrane. Its strength depends on the forces of attraction among the particles of the liquid itself and with the particles of the gas, solid, or liquid with which it comes in contact. Surface tension allows certain insects to stand on the surface of water and can support a razor blade placed horizontally on the liquid's surface, even though the blade may be denser than the liquid and unable to float. Surface tension results in spherical drops of liquid, as the liquid tends to minimize its surface area.
Surface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid (as in connecting bits of water or as in a drop of mercury that forms a cohesive ball).
Surface tension is caused by cohesion (the attraction of molecules to like molecules). Since the molecules on the surface of the liquid are not surrounded by like molecules on all sides, they are more attracted to their neighbors on the surface.
Applying Newtonian physics to the forces that arise due to surface tension accurately predicts many liquid behaviors that are so commonplace that most people take them for granted. Applying thermodynamics to those same forces further predicts other more subtle liquid behaviors.
Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent — but when referring to energy per unit of area, people use the term surface energy — which is a more general term in the sense that it applies also to solids and not just liquids.
In materials science, surface tension is used for either surface stress or surface free energy.
Flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid's flashpoint requires an ignition source. This is not to be confused with the autoignition temperature, which requires no ignition source. At the flash point, the vapour may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is defined as the temperature at which the vapour continues to burn after being ignited. Neither of these parameters is related to the temperatures of the ignition source or of the burning liquid, which are much higher. The flash point is often used as one descriptive characteristic of liquid fuel, but it is also used to describe liquids that are not used intentionally as fuels. Flash point refers to both flammable liquids as well as combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43°C are flammable, and those above this temperature are combustible.
Surface tension is caused by the attraction between the liquid's molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which is balanced only by the liquid's resistance to compression, meaning there is no net inward force. However, there is a driving force to diminish the surface area. Therefore, the surface area of the liquid shrinks until it has the lowest surface area possible. That explains the spherical shapes of water droplets.
Another way to view it is that a molecule in contact with a neighbour is in a lower state of energy than if it weren't in contact with a neighbour. The interior molecules all have as many neighbours as they can possibly have. But the boundary molecules have fewer neighbours than interior molecules and are therefore in a higher state of energy. For the liquid to minimize its energy state, it must minimize its number of boundary molecules and must therefore minimize its surface area.
As a result of surface area minimization, a surface will assume the smoothest shape it can (mathematical proof that "smooth" shapes minimize surface area relies on use of the Euler–Lagrange equation). Since any curvature in the surface shape results in greater area, a higher energy will also result. Consequently the surface will push back against any curvature in much the same way as a ball pushed uphill will push back to minimize its gravitational potential energy.
Diagram shows, in cross-section, a needle floating on the surface of water. Its weight, Fw, depresses the surface, and is balanced by the surface tension forces on either side, Fs, which are each parallel to the water's surface at the points where it contacts the needle. Notice that the horizontal components of the two Fs arrows point in opposite directions, so they cancel each other, but the vertical components point in the same direction and therefore add up to balance Fw.
Surface tension, represented by the symbol γ is defined as the force along a line of unit length, where the force is parallel to the surface but perpendicular to the line. One way to picture this is to imagine a flat soap film bounded on one side by a taut thread of length, L. The thread will be pulled toward the interior of the film by a force equal to 2L (the factor of 2 is because the soap film has two sides, hence two surfaces). Surface tension is therefore measured in forces per unit length. Its SI unit is newton per metre but the cgs unit of dyne per cm is also used. One dyn/cm corresponds to 0.001 N/m.
An equivalent definition, one that is useful in thermodynamics, is work done per unit area. As such, in order to increase the surface area of a mass of liquid by an amount, δA, a quantity of work, δA, is needed. This work is stored as potential energy. Consequently surface tension can be also measured in SI system as joules per square metre and in the cgs system as ergs per cm2. Since mechanical systems try to find a state of minimum potential energy, a free droplet of liquid naturally assumes a spherical shape, which has the minimum surface area for a given volume.
The equivalence of measurement of energy per unit area to force per unit length can be proven by dimensional analysis.
Pond skaters use surface tension to walk on the surface of a pond—hydrophobic setae on the tarsi keep the insect afloat while an apical hydrophilic claw penetrates the surface, allowing it to "grip" the water. The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface, increasing its surface area. This represents an increase in potential energy through the surface tension of the water equal to the loss of potential energy of the insect's lowered center of mass.
Degree to which a substance dissolves in a solvent to make a solution (usually expressed as grams of solute per litre of solvent). Solubility of one fluid (liquid or gas) in another may be complete (totally miscible; e.g., methanol and water) or partial (oil and water dissolve only slightly). In general, "like dissolves like" (e.g., aromatic hydrocarbons dissolve in each other but not in water). Some separation methods (absorption, extraction) rely on differences in solubility, expressed as the distribution coefficient (ratio of a material's solubilities in two solvents). Generally, solubilities of solids in liquids increase with temperature and those of gases decrease with temperature and increase with pressure. A solution in which no more solute can be dissolved at a given temperature and pressure is said to be saturated.
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution. The solubility of a substance strongly depends on the used solvent as well as on temperature and pressure. The pressure also affects the solution whether it is gas or liquid, like temperature. So, in definition of solubility we always mention the pressure and temperature "fixed". The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration where adding more solute does not increase the concentration of the solution.
The solvent is generally a liquid, which can be a pure substance or a mixture. One also speaks of solid solution, but rarely of solution in a gas.
The extent of solubility ranges widely, from infinitely soluble (fully miscible ) such as ethanol in water, to poorly soluble, such as silver chloride in water. The term insoluble is often applied to poorly or very poorly soluble compounds.
Under certain conditions the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable.
Solubility occurs under dynamic equilibrium, which means that solubility results from the simultaneous and opposing processes of dissolution and phase separation (e.g. precipitation of solids). The solubility equilibrium occurs when the two processes proceed at a constant rate.
The term solubility is also used in some fields where the solute is altered by solvolysis. For example, many metals and their oxides are said to be "soluble in hydrochloric acid," whereas the aqueous acid degrades the solid to irreversibly give soluble products. It is also true that most ionic solids are degraded by polar solvents, but such processes are reversible. In those cases where the solute is not recovered upon evaporation of the solvent the process is referred to as solvolysis. The thermodynamic concept of solubility does not apply straightforwardly to solvolysis.
When a solute dissolves, it may form several species in the solution. For example, an aqueous suspension of ferrous hydroxide, Fe(OH)2, will contain the series [Fe(H2O)6 − x(OH)x](2 − x)+ as well as other oligomeric species. Furthermore, the solubility of ferrous hydroxide and the composition of its soluble components depends on pH. In general, solubility in the solvent phase can be given only for a specific solute which is thermodynamically stable, and the value of the solubility will include all the species in the solution (in the example above, all the iron-containing complexes).
Solubility is defined for specific phases. For example, the solubility of aragonite and calcite in water are expected to differ, even though they are both polymorphs of calcium carbonate and have the same chemical formula.
The solubility of one substance in another is determined by the balance of intermolecular forces between the solvent and solute, and the entropy change that accompanies the solvation. Factors such as temperature and pressure will alter this balance, thus changing the solubility.
Solubility may also strongly depend on the presence of other species dissolved in the solvent, for example, complex-forming anions (ligands) in liquids. Solubility will also depend on the excess or deficiency of a common ion in the solution, a phenomenon known as the common-ion effect. To a lesser extent, solubility will depend on the ionic strength of solutions. The last two effects can be quantified using the equation for solubility equilibrium.
For a solid that dissolves in a redox reaction, solubility is expected to depend on the potential (within the range of potentials under which the solid remains the thermodynamically stable phase). For example, solubility of gold in high-temperature water is observed to be almost an order of magnitude higher when the redox potential is controlled using a highly-oxidizing Fe3O4-Fe2O3 redox buffer than with a moderately-oxidizing Ni-NiO buffer.
Solubility (metastable) also depends on the physical size of the crystal or droplet of solute (or, strictly speaking, on the specific or molar surface area of the solute). For quantification, see the equation in the article on solubility equilibrium. For highly defective crystals, solubility may increase with the increasing degree of disorder. Both of these effects occur because of the dependence of solubility constant on the Gibbs energy of the crystal.
Petroleum ( petroleum, from Greek πετρέλαιον, lit. "rock oil") or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, that are found in geologic formations beneath the earth's surface.
The term "petroleum" was first used in the treatise De Natura Fossilium, published in 1546 by the German mineralogist Georg Bauer, also known as Georgius Agricola.
In its strictest sense, petroleum includes only crude oil, but in common usage it includes both crude oil and natural gas. Both crude oil and natural gas are predominantly a mixture of hydrocarbons. Under surface pressure and temperature conditions, the lighter hydrocarbons methane, ethane, propane and butane occur as gases, while the heavier ones from pentane and up are in the form of liquids or solids. However, in the underground oil reservoir the proportion which is gas or liquid varies depending on the subsurface conditions, and on the phase diagram of the petroleum mixture.
An oil well produces predominantly crude oil, with some natural gas dissolved in it. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas. A gas well produces predominately natural gas. However, because the underground temperature and pressure are higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, hexane, and heptane in the gaseous state. Under surface conditions these will condense out of the gas and form natural gas condensate, often shortened to condensate. Condensate resembles gasoline in appearance and is similar in composition to some volatile light crude oils.
The proportion of hydrocarbons in the petroleum mixture is highly variable between different oil fields and ranges from as much as 97% by weight in the lighter oils to as little as 50% in the heavier oils and bitumens.
The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The exact molecular composition varies widely from formation to formation but the proportion of chemical elements vary over fairly narrow limits as follows.
Composition by weight
83 to 87%
10 to 14%
0.1 to 2%
0.1 to 1.5%
0.5 to 6%
less than 1000 ppm
Four different types of hydrocarbon molecules appear in crude oil. The relative percentage of each varies from oil to oil, determining the properties of each oil.
Composition by weight
15 to 60%
30 to 60%
3 to 30%
Is the common name for the alkane hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to the solids with 20 ≤ n ≤ 40 .
The simplest paraffin molecule is that of methane, CH4, a gas at room temperature. Heavier members of the series, such as that of octane, C8H18, and mineral oil appear as liquids at room temperature. The solid forms of paraffin, called paraffin wax, are from the heaviest molecules from C20H42 to C40H82. Paraffin wax was identified by Carl Reichenbach in 1830.
Paraffin, or paraffin hydrocarbon, is also the technical name for an alkane in general, but in most cases it refers specifically to a linear, or normal alkane — whereas branched, or isoalkanes are also called isoparaffins. It is distinct from the fuel known in
The name is derived from the Latin parum (= barely) + affinis with the meaning here of "lacking affinity", or "lacking reactivity". This is because alkanes, being non-polar and lacking in functional groups, are very unreactive.
Also called, Cycloalkanes especially if from petroleum sources are types of alkanes which have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds which have only single chemical bonds in their chemical structure. Cycloalkanes consist of only carbon (C) and hydrogen (H) atoms and are saturated because there are no multiple C-C bonds to hydrogenate (add more hydrogen to). A general chemical formula for cycloalkanes would be CnH2(n+1-g) where n = number of C atoms and g = number of rings in the molecule. Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins.
Cycloalkanes are classified into small, common, medium, and large cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the common ones, cyclooctane through cyclotridecane are the medium ones, and the rest are the larger ones.
Aromatic compound: (Meanings related to odor)
In organic chemistry, the structures of some rings of atoms are unexpectedly stable. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. It can also be considered a manifestation of cyclic delocalization and of resonance.
This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. This commonly-seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.
As is standard for resonance diagrams, a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond.
A better representation is that of the circular π bond (Armstrong's inner cycle), in which the electron density is evenly distributed through a π-bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.
The single bonds are formed with electrons in line between the carbon nuclei—these are called σ-bonds. Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals:
Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.
Aromatic compounds are important in industry. Key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and para-xylene. About 35 million tonnes are produced worldwide every year. They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar, and are used to produce a range of important chemicals and polymers, including styrene, phenol, aniline, polyester and nylon.
Other aromatic compounds play key roles in the biochemistry of all living things. Three aromatic amino acids phenylalanine, tryptophan, and tyrosine, each serve as one of the 20 basic building blocks of proteins. Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines. As well as that, the molecule haem contains an aromatic system with 22 π electrons. Chlorophyll also has a similar aromatic system.
The overwhelming majority of aromatic compounds are compounds of carbon, but they need not be hydrocarbons.
In heterocyclic aromatics (heteroaromats), one or more of the atoms in the aromatic ring is of an element other than carbon. This can lessen the ring's aromaticity, and thus (as in the case of furan) increase its reactivity. Other examples include pyridine, pyrazine, imidazole, pyrazole, oxazole, thiophene, and their benzannulated analogs (benzimidazole, for example).
Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms. Examples are naphthalene, anthracene and phenanthrene.
Many chemical compounds are aromatic rings with other things attached. Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol, and the nucleotides of DNA.
Aromaticity is found in ions as well: the cyclopropenyl cation (2e system), the cyclopentadienyl anion (6e system), the tropylium ion (6e) and the cyclooctatetraene dianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such as tropone. Aromatic properties are tested to the limit in a class of compounds called cyclophanes.
A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp³ hybridized carbon atom.
When carbon in benzene is replaced by other elements in borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine or pyrylium salts the aromaticity is still retained. Aromaticity also occurs in compounds that are not carbon-based at all. Inorganic 6 membered ring compounds analogous to benzene have been synthesized. Silicazine (Si6H6) and borazine (B3N3H6) are structurally analogous to benzene, with the carbon atoms replaced by another element or elements. In borazine, the boron and nitrogen atoms alternate around the ring.
Metal aromaticity is believed to exist in certain metal clusters of aluminium. Möbius aromaticity occurs when a cyclic system of molecular orbitals, formed from pπ atomic orbitals and populated in a closed shell by 4n (n is an integer) electrons, is given a single half-twist to correspond to a Möbius strip. Because the twist can be left-handed or right-handed, the resulting Möbius aromatics are dissymmetric or chiral. Up to now there is no doubtless proof that a Möbius aromatic molecule was synthesized.
Aromatics with two half-twists corresponding to the paradromic topologies, first suggested by Johann Listing, have been proposed by Rzepa in 2005. In carbo-benzene the ring bonds are extended with alkyne and allene groups.
Molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and alkanes (i.e., saturated hydrocarbons). The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties. Asphaltenes in the form of distillation products from oil refineries are used as "tar-mats" on roads.
Asphaltenes consist primarily of carbon, hydrogen, nitrogen, oxygen, and sulfur, as well as trace amounts of vanadium and nickel. The C:H ratio is approximately 1:1.2, depending on the asphaltene source. Asphaltenes are defined operationally as the n-heptane (C7H16)-insoluble, toluene (C6H5CH3)-soluble component of a carbonaceous material such as crude oil, bitumen or coal. Asphaltenes have been shown to have a distribution of molecular masses in the range of 400 u to 1500 u with a maximum around 750 u.
difficult to ascertain, due to the complex nature of the asphaltenes, but has been studied by all available techniques including X-ray, elemental, and pyrolysis GC-FID-GC-MS. However, it is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e. polycondensed aromatic benzene units with oxygen, nitrogen, and sulfur, (NSO-compounds) combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures. Furthermore, asphaltene rotational diffusion measurements show that small PAH chromophores (blue fluorescing) are in small asphaltene molecules while big PAH chromophores (red fluorescing) are in big molecules. This implies that there is only one fused polycyclic aromatic hydrocarbon (PAH) ring system per molecule. Very recent fragmentation
studies by FT ICR-MS strongly support this 'island' molecular architecture refuting the 'archipelago' molecular architecture.
Most of the world's oils are non-conventional.
Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish or even greenish). In the reservoir it is usually found in association with natural gas, which being lighter forms a gas cap over the petroleum, and saline water which, being heavier than most forms of crude oil, generally sinks beneath it. Crude oil may also be found in semi-solid form mixed with sand and water, as in the
^9 m3) of bitumen and extra-heavy oil, about twice the volume of the world's reserves of conventional oil.
Petroleum is used mostly, by volume, for producing fuel oil and gasoline (petrol), both important "primary energy" sources. 84% by volume of the hydrocarbons present in petroleum is converted into energy-rich fuels (petroleum-based fuels), including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas. The lighter grades of crude oil produce the best yields of these products, but as the world's reserves of light and medium oil are depleted, oil refineries are increasingly having to process heavy oil and bitumen, and use more complex and expensive methods to produce the products required. Because heavier crude oils have too much carbon and not enough hydrogen, these processes generally involve removing carbon from or adding hydrogen to the molecules, and using fluid catalytic cracking to convert the longer, more complex molecules in the oil to the shorter, simpler ones in the fuels.
Due to its high energy density, easy transportability and relative abundance, oil has become the world's most important source of energy since the mid-1950s. Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics; the 16% not used for energy production is converted into these other materials. Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. There is also petroleum in oil sands (tar sands). Known reserves of petroleum are typically estimated at around 190 km3 (1.2 trillion (short scale) barrels) without oil sands, or 595 km3 (3.74 trillion barrels) with oil sands. Consumption is currently around 84 million barrels (13.4×10
^6 m3) per day, or 4.9 km3 per year.
Octane, a hydrocarbon found in petroleum, lines are single bonds, black spheres are carbon, white spheres are hydrogen.
Petroleum is a mixture of a very large number of different hydrocarbons; the most commonly found molecules are alkanes (linear or branched), cycloalkanes, aromatic hydrocarbons, or more complicated chemicals like asphaltenes. Each petroleum variety has a unique mix of molecules, which define its physical and chemical properties, like color and viscosity.
The alkanes, also known as paraffins, are saturated hydrocarbons with straight or branched chains which contain only carbon and hydrogen and have the general formula CnH2n+2 They generally have from 5 to 40 carbon atoms per molecule, although trace amounts of shorter or longer molecules may be present in the mixture.
The alkanes from pentane (C5H12) to octane (C8H18) are refined into gasoline (petrol), the ones from nonane (C9H20) to hexadecane (C16H34) into diesel fuel and kerosene (primary component of many types of jet fuel), and the ones from hexadecane upwards into fuel oil and lubricating oil. At the heavier end of the range, paraffin wax is an alkane with approximately 25 carbon atoms, while asphalt has 35 and up, although these are usually cracked by modern refineries into more valuable products. The shortest molecules, those with four or fewer carbon atoms, are in a gaseous state at room temperature. They are the petroleum gases. Depending on demand and the cost of recovery, these gases are either flared off, sold as liquified petroleum gas under pressure, or used to power the refinery's own burners. During the winter, Butane (C4H10), is blended into the gasoline pool at high rates, because butane's high vapor pressure assists with cold starts. Liquified under pressure slightly above atmospheric, it is best known for powering cigarette lighters, but it is also a main fuel source for many developing countries. Propane can be liquified under modest pressure, and is consumed for just about every application relying on petroleum for energy, from cooking to heating to transportation.
The cycloalkanes, also known as naphthenes, are saturated hydrocarbons which have one or more carbon rings to which hydrogen atoms are attached according to the formula CnH2n. Cycloalkanes have similar properties to alkanes but have higher boiling points.
The aromatic hydrocarbons are unsaturated hydrocarbons which have one or more planar six-carbon rings called benzene rings, to which hydrogen atoms are attached with the formula CnHn. They tend to burn with a sooty flame, and many have a sweet aroma. Some are carcinogenic.
These different molecules are separated by fractional distillation at an oil refinery to produce gasoline, jet fuel, kerosene, and other hydrocarbons. For example 2,2,4-trimethylpentane (isooctane), widely used in gasoline, has a chemical formula of C8H18 and it reacts with oxygen exothermically:
The amount of various molecules in an oil sample can be determined in laboratory. The molecules are typically extracted in a solvent, then separated in a gas chromatograph, and finally determined with a suitable detector, such as a flame ionization detector or a mass spectrometer.
Incomplete combustion of petroleum or gasoline results in production of toxic byproducts. Too little oxygen results in carbon monoxide. Due to the high temperatures and high pressures involved, exhaust gases from gasoline combustion in car engines usually include nitrogen oxides which are responsible for creation of photochemical smog.
Empirical equations for the thermal properties of petroleum products
Heat of combustion:
At a constant volume the heat of combustion of a petroleum product can be approximated as follows:
Qv = 12,400 − 2,100d2
where Qv is measured in cal/gram and d is the specific gravity at 60°F.
The thermal conductivity of petroleum based liquids can be modeled as follows:
where K is measured in BTU · hr-1ft-2 , t is measured in °F and d is the specific gravity at 60°F.
The specific heat of a petroleum oils can be modeled as follows:
where c is measured in BTU/lbm-°F, t is the temperature in Fahrenheit and d is the specific gravity at 60°F.
In units of kcal/kg°C, the formula is:
where the temperature t is in Celsius and d is the specific gravity at 15°C.
Latent heat of vaporization
The latent heat of vaporization can be modeled under atmospheric conditions as follows:
where L is measured in BTU/lbm, t is measured in °F and d is the specific gravity at 60°F.
In units of kcal/kg, the formula is:
where the temperature t is in Celsius and d is the specific gravity at 15°C.
These are the most important processes from a response point of view:
Oil that spreads and moves, when lighter than water, forming slicks that spread on the surface, on streams, rivers and ponds in percentages that are influenced by gravity, surface tension, viscosity, point of fluidity, winds and currents.
The temperature is another crucial variable to control spreading due to the dependency that viscosity has on temperature. One should note that crude oils vary widely in composition and their behaviour on the ocean also varies. Even viscous crude oils can spread quickly in thin layers. The action of the currents and wind spreads and breaks the slicks into mobile portions of oil that will have the largest amounts (thicker) near their leading edges.
Both wind and current affect the movement of the portions in the water. The effect of the currents is 100% in rivers, while that of the wind is around 3% of the wind speed. The effect of the wind is little felt in rivers, contrary to what happens in a pond where the wind is the predominant element in oil displacement.
Evaporation due to the high percentage of volatile components in most crude oils and the percentage for the loss of these oil volatiles in rivers and ponds is substantially important. Such evaporation occurs quickly and is physically related to the process of dissolution that is promoted by the spreading in high temperatures of water and fast-moving rivers (that generate water spray and bubbles that pop and eject the oil into the atmosphere). Studies have demonstrated that up to 50% of crude oil can be lost to evaporation, usually within 24 to 48 hours. This compares to only 10% of heavy or waste fuel oil, 75% of diesel and eventually 100% of kerosene or gasoline.
The formulation of water emulsions in oil (in contrast to oil dispersions in water, which is dealt with below) leads to many difficulties. The tendency to form emulsions, which are persistent and thick, stick as masses that are often called “chocolate mousses”, depends on the type of oil involved, but it is caused by turbulent conditions in rivers. Under adequate conditions emulsions containing up to 80% of water can form quickly. Their formation adds to the difficulty in cleaning, both on the river margins as in inaccessible areas, increasing the volume and the viscosity of the material to be removed and, because of that, the difficulty in treating and disposing of the oil.
It has been postulated by some that the processes of decomposition can convert emulsions into tar slicks, and particles that can persist in specific environments for long periods and travel long distances and be released into river mouths. Most of the tar balls are dispersed and decompose during the river’s journey to the sea, but those that reach deltas and estuaries can degrade more slowly. Studies on the biological effects of tar on life in temperate areas have shown that such effects probably do not represent an ecological threat, although the aesthetical and economic consequences from tar that runs aground into river margins can be serious, apart from hampering water collection.
Depending on the type of the crude oil involved, the spontaneous formation of small droplets of oil in the water can occur quickly, due to the action of winds. The temperature of the river and other factors contribute to this process. Natural dispersion can be useful to mitigate the effects of Spilled Oil, dissipating the oil and thus reducing its toxicity for aquatic life.
The gradual and spontaneous disappearance of crude oil that has been spilled is aided through dispersion processes. Due to the presence of oil on the water surface the small particles (globules) created by the oil are more easily biodegraded by micro-organisms, due to bigger areas of contact. These droplets lose their volatile and more toxic soluble components more quickly than continuous and larger oil slicks and are quickly dissipated by the action of currents.
The dissolution of dispersed oil many times prevents the oil from travelling away from the surface slicks, thus reducing the likelihood of it reaching farther areas. The dispersion also reduces bird contamination hazards.
The finest example of this process was the MV Braer, Shetland Islands, UK 1993 as metioned in the Major spill section.
In spite of the solubility of most of the hydrocarbons in water being substantially low, some crude oil components, notably low-boiling-point light aromatic hydrocarbons, are soluble enough to quickly penetrate the water following an oil spill. The dissolution rate depends on such factors such as water turbulence and temperature.
These more soluble fractions are also the more volatile oil components and, for this reason, they tend to evaporate instead of transferring themselves to the water phase. This has been confirmed by analytical measurements of concentrations of dissolved hydrocarbons that may remain below or near the location of the spill. Dissolution is not as important as other processes such as evaporation in the determination of weathering of spilled oil.
Sedimentation is the process where oil particles reach the bottom of rivers and ponds. For this to happen it is necessary that the oil particles, which are less dense than the water, are modified through evaporation of lighter components and, more important, through the incorporation of particled material present in the water column which causes them to be denser than the water. Due to the high levels of particled material in rivers that have eroding characteristics, sedimentation should occur in these environments.
This process becomes more important in areas near estuaries and mangroves, where suspended sediment can be found. Some types of oil have a density higher than 1 (Group V) and unavoidably, in the event of an oil spill, will sink to the bottom.
Oil, when subjected to sunlight on water and on land surfaces undergoes chemical changes, usually photochemical oxidation. These changes degrade some oil components and make it more soluble in water and are subject to dissipation through dissolution and dilution. The rates for photochemical oxidation are higher at the water’s surface or in exposed or physically stranded oil.
Water and sediments in the whole world contain micro-organisms (bacteria, yeast and fungi) that use and degrade oil components. A very high number of microorganism species that can degrade oil have been identified in rivers and on land.
Biodegradation is the most important process in determining the final destination of oil in the environment, in spite of not immediately reducing the volume of oil or its impact on the environment after the spill. Biodegradation is promoted by the dispersion of oil slicks into small particles over a large surface area. This is applied when the dispersion occurs naturally. It is interesting to note that biodegradation increases the rate of natural oil dispersion.
For biodegradation to happen at reasonable rates, nutrients such as nitrogen, phosphorous and potassium (NPK) should be present. Thus, biodegradation happens more quickly in eutrophized waters (which contain much more of these nutrients).
Most of the crude oil components can be degraded with the aid of micro-organisms, but lower-molecular-weight light components are degraded more quickly than the heavier ones. Higher temperatures speed up biodegradation but still occur at significant rates even in arctic regions.
“They came up very suddenly out of the seafloor: There were seven of them. The largest we called Il Duomo, and it is about the size of two football fields side by side and as tall as a six-story building,” said David Valentine, an earth scientist at the University of California, Santa Barbara.
“Nobody knew what the domes were made of,” said Chris Reddy, a marine chemist at Woods Hole Oceanographic Institution.
One of Valentine’s colleagues, Ed Keller, had spotted them, and in 2006 he suggested some possibilities. Large deposits of carbonate rock? Mud volcanoes created by mammoth burps of subsea natural gas? Or, most intriguing to Valentine and Reddy, perhaps they were remnants of oil that had erupted from the seafloor, hardened, and piled high to form something never seen before—volcanoes made naturally out of the same material that people use to pave roads: asphalt.
The WHOI undersea vehicle Sentry collected sonar data to create this map of the undersea asphalt mound called Il Duomo, the largest of seven similar domes in the Santa Barbara Channel. It covers twice the area of a football field and rises 30 meters, or six stories, above the seafloor. The scale at right is in meters below the sea surface.
(ABE/Sentry Group, Woods Hole Oceanographic Institution)
In 2007, off Santa Barbara aboard the research vessel Atlantis, Valentine and Reddy seized an opportunity to use the submersible Alvin to investigate the mysterious mounds. They reported what they found April 25, 2010, in the journal Nature Geoscience. We interviewed the two scientists on a bi-coastal conference call.In 2007, UCSB scientist Dave Valentine (right) and WHOI scientist Chris Reddy investigated the largest mound in the submersible Alvin. Using Alvin's manipulator they brought back a large sample of rock from the undersea dome called Il Duomo.They could heft it easily because it was made of asphalt, the solidified residue of oil.
The WHOI undersea vehicle Sentry collected sonar data to create this map of the undersea asphalt mound called Il Duomo, the largest of seven similar domes in the Santa Barbara Channel. It covers twice the area of a football field and rises 30 meters, or six stories, above the seafloor. The scale at right is in meters below the sea surface.
(ABE/Sentry Group, Woods Hole Oceanographic Institution)
The area around Santa Barbara is very geologically active, because of the movement of the San Andreas and other faults. Extensive faulting or rupturing in the Earth allows oil and gas from subterranean reservoirs to seep up to the seafloor and ultimately into the ocean and to the atmosphere. But some oil solidifies to create asphalt volcanoes.
(Jack Cook, Woods Hole Oceanographic Institution)