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PETROLEUM Source: Library of Congress Burkburnett Oil Field, Texas, 1919
The formation of petroleum		Where is the petroleum?			How do we use the petroleum?				Coal to Liquids Technology

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The formation of petroleum

Step 1: Diagenesis forms Kerogen

Diagenesis is a process of compaction under mild conditions of temperature and pressure. When organic aquatic sediments (proteins, lipids, carbohydrates) are deposited, they are very saturated with water and rich in minerals. Through chemical reaction, compaction, and microbial action during burial, water is forced out and proteins and carbohydrates break down to form new structures that comprise a waxy material known as “kerogen” and a black tar like substance called “bitumen”. All of this occurs within the first several hundred meters of burial.

The bitumen comprises the heaviest components of petroleum, but the kerogen will undergo further change to make hydrocarbons and, yes, more bitumen…

Step 2: Catagenesis (or “cracking”) turns kerogen into petroleum and natural gas

As temperatures and pressures increase (deeper burial) the process of catagenesis begins, which is the thermal degradation of kerogen to form hydrocarbon chains. Importantly, the process of catagenesis is catalyzed by the minerals that are deposited and persist through marine diagenesis. The conditions of catagenesis determine the product, such that higher temperature and pressure lead to more complete “cracking” of the kerogen and progressively lighter and smaller hydrocarbons. Petroleum formation, then, requires a specific window of conditions; too hot and the product will favor natural gas (small hydrocarbons), but too cold and the plankton will remain trapped as kerogen.

This behavior is contrary to what is associated with coal formation. In the case of terrestrial burial, the organic sediment is dominated by cellulose and lignin and the fraction of minerals is much smaller. Here, decomposition of the organic matter is restricted in a different way. The organic matter is condensed to form peat and, if enough temperature (geothermal energy) and pressure is supplied, it will condense and undergo catagenesis to form coal. Higher temperatures and pressures, in general, lead to higher ranks of coal. See the COAL page for more information.

So, the plankton is buried and it turns into oil and gas…but where does it go?

Where is the petroleum?

Because the earth is filled entirely by layers of solid (or at significant depths) molten rock, the petroleum it contains cannot exist within a self-contained “lake”, but must decide to live within the small fraction of space (or pores) that exist in these rocks. Like the sponge in your kitchen sink (albeit, less spongy and a bit heavier) certain kinds of rock (mainly sandstone and limestone) contain pores large enough and with enough connections to serve as storage and migration sites for oil or water or any other fluid wishing to call them home. Because most hydrocarbons are lighter than water and rock, those that exist within the earth will tend to migrate upwards until they reach the surface, or are trapped by an impermeable layer of rock.

There is a particular window of temperature that the zooplankton must find to form oil. If it is too cold, the oil will remain trapped in the form of kerogen, but too hot and the oil will be changed (through “thermal cracking”) into natural gas. Therefore, the formation of an oil reservoir requires the unlikely gathering of three particular conditions: first, a source rock rich in organic material (formed during diagenesis) must be buried to the appropriate depth to find a desirable window; second, a porous and permeable (connected pores) reservoir rock is required for it to accumulate in; and last a cap rock (seal) or other mechanism must be present to prevent it from escaping to the surface. The geologic history of some places on earth makes them much more likely to contain the necessary combination of conditions.

How do we use the petroleum?

To be of use to us, the crude oil must be “fractionated” into its various hydrocarbons. This is done at the refinery.

Oil can be used in many different products, and this is because of its composition of many different hydrocarbons of different sizes, which are individually useful in different ways due to their different properties. The purpose of a refinery is to separate and purify these different components. Most refinery products can be grouped into three classes: Light distillates (liquefied petroleum gas, naphtha, and gasoline), middle distillates (kerosene and diesel), and heavy distillates (fuel oil, lubricating oil, waxes, and tar). While all of these products are familiar to consumers, some of them may have gained fame under their refined forms. For instance, naphtha is the primary feedstock for producing a high octane gasoline component and also is commonly used as cleaning solvent, and kerosene is the main ingredient in many jet fuels.

In a refinery, components are primarily separated using “fractional distillation”. After being sent through a furnace, the crude petroleum enters a fractionating column, where the products condense at different temperatures within the column, so that the lighter components separate out at the top of the column (they have lower boiling points than heavier ones) and the heavier ones fall towards the bottom. Because this process occurs at atmospheric pressure, it may be called atmospheric distillation. Some of the heavier components that are difficult to separate may then undergo vacuum distillation (fractional distillation in a vacuum) for further separation. The heaviest components are then commonly “cracked” (undergoing catagenesis) to form lighter hydrocarbons, which may be more useful. In the same manner that natural mineral catalysts help to transform kerogen to crude oil through the process of catagenesis, metal catalysts can help transform large hydrocarbons into smaller ones. The modern form of “catalytic cracking” utilizes hydrogen as catalyst, and is thus termed “hydrocracking”. This is a primary process used in modern petroleum refining to form more valuable lighter fuels from heavier ones. All of the products then undergo further refinement in different units that produce the desired products.

Alkanes are saturated hydrocarbons with between 5 and 40 carbon atoms per molecule which contain only hydrogen and carbon. The light distillates range in molecular composition from pentane (5 carbons: C₅H₁₂) to octane (8 carbons: C₈H₁₈). Middle distillates range from nonane (9 carbons: C₉H₂₀) to hexadecane (16 carbons: C₁₆H₃₄) while anything heavier is termed a heavy distillate. Hydrocarbons that are lighter than pentane are considered natural gas or natural gas liquids (liquefied petroleum gas).

A few further refinement processes are described below:

· Desalting removes salt from crude oil before entering fractional distillation.

· Desulfurization removes sulfur from compounds, and several methods are possible. Hydrodesulfurization is the typical method, and uses hydrogen to extract the sulfur. This occurs after distillation.

· Cracking breaks carbon-carbon bonds to turn heavier hydrocarbons into lighter ones. This can occur thermally (as occurs during the petroleum formation process beneath the earth) or through the action of a catalyst:

o Thermal Cracking

§ Steam, visbreaking, or coking

o Catalytic cracking

§ Fluid catalytic cracking (FCC) cracks heavy oils into diesel and gasoline. Uses a hot fluid catalyst.

§ Hydrocracking (similar to FCC but lower temperature and using hydrogen as catalyst) cracks heavy oils into gasoline and kerosene

· A catalytic reformer converts naphtha into a higher octane form, which has a higher content of aromatics, olefins, and cyclic hydrocarbons. Hydrogen is a byproduct, and may be recycled and used in the naphtha hydrotreater.

· Steam reforming is a method of producing hydrogen from hydrocarbons, which may then be used in other processes.

· Solvent dewaxing removes heavy wax constituents from the vacuum distillation products.

See this very nice animation of distillation and good tutorial of the refining process. http://science.howstuffworks.com/oil-refining2.htm

Coal to Liquids Technology

But what if we want the same fuels that we get from petroleum, without the petroleum? Is there another way? Actually, yes, we can use coal. The only commercial coal to liquids (CTL) industry in operation today is in South Africa, where coal-derived fuels have been in use since 1955, and currently account for about 30% of the country’s gasoline and diesel consumption.

There are two different methods for converting coal into liquid fuels, direct and indirect liquefaction.

The direct liquefaction method dissolves the coal in a solvent at high temperature and pressure. While highly efficient, the liquid products generated this way require further refining to achieve a high fuel grade.

The indirect liquefaction technique gasifies the coal to form a “syngas” (a mixture of, primarily, hydrogen and carbon monoxide produced by breaking down the coal into its components using high temperature and pressure with the injection of steam and oxygen). This gas is then condensed over a catalyst (in the “Fischer-Tropsch” process) to produce a higher quality, cleaner fuel. Syn-fuel processes (such as Fisher-Tropsch) actually build up larger hydrocarbons from smaller ones, which is the opposite of cracking.

Both of these methods result in the release of carbon dioxide in a proportion greater than that produced during the extraction and refinement of petroleum, but the fuels they produce may be cleaner than final petroleum fuels. Carbon dioxide sequestration has been proposed as a method to counteract this downside, thereby achieving cleaner fuels, without the drawback of carbon dioxide release.

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