Natural gas has been created over thousands of years through thermogenic and biogenic mechanisms. It is formed from the decaying remains of prehistoric plant and animal life. Similar to petroleum, most natural gas formation is due to the breakdown of prehistoric marine zooplankton, which are tiny animals found near the surface of aquatic environments. They subsist on a diet of phytoplankton (plants found in the sea), which in turn rely on the sun's energy to produce organic matter and energy through photosynthesis.
Natural gas is a hydrocarbon gas consisting mainly of methane with varying amounts of alkanes with a small percentage of nitrogen, carbon dioxide, hydrogen sulfide and sometimes helium. Along with petroleum they are found in deep underground rock formations and form part of hydrocarbon reservoirs in coal beds and methane clathrates. (For more information, see The Unique Challenges of Coal Bed Methane Production.)
They meet a very significant percentage of the world’s energy needs along with coal and oil. They come from various sources and have evolved either through thermogenic or biogenic processes. Fossil fuels are formed when the organic remains of animals and plants are suppressed beneath the earth’s surface under extreme heat and pressure. Over a long period of time, they are covered with mud and sediments that break down the carbon bonds in the organic matter. At relatively lower temperatures in shallower deposits, more oil is produced as compared to natural gas. At deeper deposits where temperatures are higher, more natural gas is found than oil. The natural gas extracted from such sources is referred to as thermogenic natural gas. On the other hand, natural gas formed due to the transformation of organic matter by tiny microorganisms is referred to biogenic natural gas. We will examine them in a bit more detail in the following sections.
Thermogenic Natural Gas Substance
Thermogenic natural gas originates from fine grain rock formation typically 3000 feet (914 meters) underground. Common sources of thermogenic natural gas include shale, coalbed, tight limestone, etc. It is a result of chemical reactions that occur without the presence of microorganisms. These decomposition reactions are triggered by the application of extreme heat and pressure underground. This gas contains a large percentage of non-methane hydrocarbons and forms a significant portion of natural gas production and reserves worldwide. In conventional hydrocarbon formation, traditional and standard drilling procedures can be adopted to extract them. However when the source rocks have limited permeability and are inaccessible, sophisticated drilling procedures such as fracking must be adopted so as to bring them to the surface. Most of the natural gas extraction and production comes through this conventional gas formation.
How Biogenic Gases are Formed
Natural gas, which is produced by methanogenic organisms, is called biogenic gas. Methanogens are tiny methane producing mechanisms that chemically break down organic matter to produce methane. They are found in marshes, bog, landfills and shallow sediments. Formation of methane through biogenic process takes place closer to the earth’s surface with majority of it getting lost in the atmosphere; however, some of them can be trapped underground. Methanogenic archaea is responsible for all biological forms of methane. As these methanogens live and grow, methane accumulates in their underground habitat. This biogenic natural gas is composed almost purely of methane and can be found in bodies of fine-grained sediment at depths of 10-300 feet underground.
It is interesting to note that the delicate balancing of earth’s radiation and moisture in atmosphere is taken care by biogenic gases. The biogases include oxygen, nitrogen, carbon dioxide, carbon monoxide, methane, ozone, nitrogen dioxide, nitric acid, ammonia, hydrogen and carbonyl sulfide and other complex non-methane substances. Unfortunately, all of the above gases fall under the greenhouse category with the exception of oxygen and nitrogen. Hence, this poses a challenge for extractors.
The principal source of biogenic gas containing methane are landfill gas, biogas and methane hydrate.
Natural gas formed due to the anaerobic decay of non-fossil organic matter (biomass) through anaerobic digester or fermentation is referred to as biogas. It is a form of renewable energy and can be produced from a wide range of organic feedstock such as biomass, agricultural waste, certain industrial wastes, municipal waste, sewage plants, etc.
A classic example of biogenic methane is landfill gas. Waste-containing landfills usually have significant amounts of natural gas due to the decomposition of the waste materials. Excluding water vapor, about half of landfill gas is methane. These landfills also contain small amounts of carbon dioxide, nitrogen, oxygen, hydrogen and small traces of hydrogen sulfide and siloxanes. If the gas is not removed, the pressure may increase significantly and work its way to the surface. This can result in significant damage to the landfill structure, cause a pungent odor and also sometimes cause an explosion. The gas must be vented to the atmosphere, flared or burned to generate electricity. (For more about flaring, see Flare Management for Efficient Operations.)
Landfills are designated locations for the disposal of waste collected from residential, industrial and commercial entities and are the third largest source of human-related methane emissions in the United States. It is important to note that landfill gas cannot be transported in utility natural gas pipelines as the carbon dioxide levels first need to be reduced significantly.
Methane hydrates are ice-like structures in which molecules of methane are trapped. The chief constituent of methane hydrates is natural gas. They are found either in the ocean floor or in the Arctic permafrost. Methane that forms the hydrate can be of biogenic origin from the biological activity of sediments in the ocean floor or of thermogenic origin caused by geological activity deep within the earth. Some estimates put the reserves of energy found in methane hydrates exceeding that of all other fossil fuels put together. However their development and commercial exploitation is still in the nascent stage due to environmental considerations.
Supply Chain Scenario
Thermogenic gas supplies have an established supply chain global network while biogenic gases have localized supply chain distribution networks from a storage, transport and consumption perspective. Most of the biogenic gases are consumed at the domestic level with a confined infrastructure and hence unlike thermogenic gases they do not require extensive transportation and distribution efforts. Also, most of the biogenic facilities are intended for captive consumption and so they do not come under the regulated or organized market environment. However, the role played by biogenic gases is significant in meeting the energy needs of unorganized market segments.