Natural gas is a fossil fuel. Like oil and coal, this means that it is, essentially, the remains of plants and animals and microorganisms that lived millions and millions of years ago. Natural gas is a fossil fuel that is composed mainly of methane- composed itself of carbon and hydrogen – and is thus known as a hydrocarbon.
There are many different theories as to the origins of fossil fuels. The most widely accepted theory says that fossil fuels are formed when organic matter (such as the remains of a plant or animal) is compressed under the earth, at very high pressure for a very long time. This is referred to as thermogenic methane. Similar to the formation of oil, thermogenic methane is formed from organic particles that are covered in mud and other sediments. Over time, more and more sediment and mud and other debris are piled on top of the organic matter. This sediment and debris put a great deal of pressure on the organic matter, which compresses it.
This compression, combined with high temperatures found deep underneath the earth, breaks down the carbon bonds in the organic matter. As one gets deeper and deeper under the earth’s crust, the temperature gets higher and higher. At low temperatures (shallower deposits), more oil is produced relative to natural gas. At higher temperatures, however, more natural gas is created, as opposed to oil. That is why natural gas is usually associated with oil in deposits that are 1 to 2 miles below the earth’s crust. Deeper deposits, very far underground, usually contain primarily natural gas, and in many cases, pure methane.
The process that creates natural gas is the same as the process that creates oil and is generally the same in most areas. However, there may be different types of plant and animal debris that falls to the ocean floor and slightly different conditions. To form natural gas, the following steps are followed,
1. Dead plankton – both phytoplankton (including algae) and zooplankton – as well as other soft and hard organic matter tissues (including microbes), sink to the bottom of an ancient ocean and mix with inorganic, clay-like materials that enter these oceans from streams and rivers. This creates an organic-rich mud. This mud can only form in still water environments.
2. This mud cannot be exposed to too much oxygen, or else the organic matter in the mud would be decomposed by bacteria and disappear quickly. Therefore environments, where natural gas can form, are known as anoxic environments. Before this organic matter is destroyed, it is buried by more sediment and lithifies (becomes sedimentary rock), creating organic shale.
3. If this shale is buried between 2 and 4 kilometers, its temperature increases due to its location in the Earths interior. This increasing pressure and temperature of the shale transforms it into a waxy material known as kerogen. Shale that contains this material is known as oil shale.
4. If temperatures of the kerogen are greater than 90°C but lower than 160°C, the kerogen is transformed into oil and natural gas. At temperatures greater than 90°C, the only product is natural gas or graphite.
5. Natural gas and oil are both lighter than water, so as they escape from the source oil shale the products rise through pores in rocks, displacing water. Rock bodies that contain significant amounts of oil or natural gas are known as reservoir rocks. For the gas to remain trapped in the reservoir, there must be some sort of thick, impermeable layer of rock to seal the reservoir. If this seal exists, then oil, gas, and water are trapped beneath and can be drilled into to obtain the oil.
So, in conclusion, we can say
Natural gas is primarily methane with smaller quantity of other hydrocarbon.
It was formed million of years ago when dead organism sunk to the bottom of the ocean & were buried under deposits of sedimentary rock subject to intense heat & pressure in absence of air, these organism underwent a transformation in which they converted to gas over millions of years.
Natural gas is formed underground rocks called reservoirs, the rock have tiny spaces called pores that allow them to hold water, natural gas & sometime oil.
The Natural gas is trapped underground by impermeable rock called a cap rock & stays there until it is extracted.
Natural gas can be categorized as dry or wet. Dry gas is essentially a gas that contain mostly methane, wet gas on the other end contain compounds such as ethane & butane in addition to methane.
These natural gas liquids ( ethane, propane, butane, pentane) can be separated & sold individually for various uses.
Conventional natural gas can be extracted for drilling wells. Unconventional form of natural gas like shale gas, coal bed methane has specific extraction technique.
Natural gas can also be found in reservoir with oil & is sometimes extracted alongside oil, this type of natural gas is called associated gas. In the past, associated gas was commonly burned as a waste product but in most places today, it is captured & used.
Once extracted, natural gas is sent through small pipeline called gathering line to processing plants which separate the various hydrocarbons & fluid from pure natural gas to produce what is known as pipeline quality dry natural gas, before it can be transported.