How is Petroleum formed?
Most of the oil and gas deposits on the Norwegian shelf originate from a thick layer of black clay that currently lies several thousand metres under the seabed.
The black clay is a source rock, which means a deposit containing significant quantities of organic residue. The clay was deposited around 150 million years ago at the bottom of a sea that covered much of present-day northwestern Europe. Much of the seabed here was dead and stagnant, while the upper water layers were teeming with life.
As the microscopic phytoplankton died, they sank to the bottom and accumulated in large quantities in the oxygen-free sediments. Over time, they were buried deeper and subjected to a long process of chemical conversion by bacterial decomposition and maturing under a thickening pile of sediment. This caused the formation of liquid and gaseous hydrocarbons in the source rock.
One of the products of anaerobic decomposition of organic matter is kerogen, which at high temperature and pressure slowly generates oil and gas. On the Norwegian continental shelf, the temperature rises by about 25 °C per kilometre of depth. After more than a hundred million years of erosion and sedimentation, the source rock may be buried under several kilometres of clay and sand deposits. Oil is generated when the kerogen temperature reaches 60-120 °C; at higher temperatures, it is mainly gas that is generated.
As oil and gas form, they seep out of the source rock. Because hydrocarbons are lighter than water, the oil and gas migrate upwards in porous water-bearing rock. Oil and gas migration takes thousands of years, and may extend over tens of kilometres until it is stopped by impermeable layers of rock, or the oil or gas leaks out into the sea.
Reservoir rocks are porous and always saturated with water, oil and gas in various combinations. Most of Norway’s petroleum resources are trapped in reservoir rocks deposited in large deltas formed by rivers that ran into the sea during the Jurassic Period.
The main reservoirs of the Gullfaks, Oseberg and Statfjord fields are in the large Brent delta that formed in the Jurassic. There are also large reservoirs in sand that was deposited on alluvial plains during the Triassic Period (the Snorre field), in shallow seas in the Late Jurassic (the Troll field) and as subsea fans during the Paleogene Period (the Balder field). In the southern part of the North Sea, thick layers of chalk composed of microscopic calcareous skeletons of plants and animals form an important reservoir rock, as in the Ekofisk Field.
Mudrocks and other impermeable deposits influence migration routes from the source rock to the reservoir. In addition, impermeable rock has to be present to stop petroleum escaping from reservoir rock. Impermeable rock that forms a seal over reservoir rocks is called cap rock. In addition, the configuration of the reservoir rocks must be such that the oil collects in a trap.
If there is a suitable combination of source rock, reservoir rock, cap rock and a trap in an area, recoverable oil and gas deposits may be discovered there.
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).
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
Pros and Cons of Petroleum
Petroleum provides transportation, heat, light, and plastics to global consumers. It is easy to extract but is a non-renewable, limited supply source of energy. Petroleum has a high power ratio and is easy to transport.
However, the extraction process and the byproducts of the use of petroleum are toxic to the environment. Underwater drilling may cause leaks and fracking can affect the water table. Carbon released into the atmosphere by using petroleum increases temperatures and is associated with global warming.
*Pros
Stable energy source
Easily extracted
Variety of uses
High power ratio
Easily transportable
Carbon emissions are toxic to the environment.
Transportation can damage the environment.
Extraction process is harmful to the environment.
Investing In Petroleum
The energy sector attracts investors who speculate on the demand for oil and fossil fuels and many oil and energy fund offerings consist of companies related to energy.
Mutual funds like Vanguard Energy Fund Investor Shares (VGENX) with holdings in ConocoPhillips, Shell, and Marathon Petroleum Corporation,3 and the Fidelity Select Natural Gas Fund (FSNGX), holding Enbridge and Hess, are two funds that invest in the energy sector and pay dividends.
Oil and gas exchange-traded funds (ETFs) offer investors more direct and easier access to the often-volatile energy market than many other alternatives. Three of the top-rated oil and gas for 2022 include the Invesco Dynamic Energy Exploration & Production ETF , First Trust Natural Gas ETF , and iShares U.S. Oil & Gas Exploration & Production ETF.
What Are Alternatives to Petroleum
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