Chapters :

Environment UNCCD – 05

Lakebeds and seabeds are examples of favorable sedimentary environments. 5 Environmental Sciences Energy and Environment Fossil fuel classification and composition Subsequent sedimentation buries the organic debris. The rise and fall of sea level, as well as other geologic processes, continue the process of burying the organic debris. As burial continues, the organic material is subjected to increasing temperature and pressure, and is transformed by bacterial action into oil and gas.

Classification of fossil fuel – The organic materials are heated and compressed over time to form oil, gas and coal. The fossil fuel can be classified as solid, liquid and gaseous fuel.

Types of fossil fuels in utilization


Coal is a combustible rock that is composed primarily of carbon rich organic material. Coal is a heterogeneous sedimentary rock that reflects both the different sediment logical regimes within which peat forms and the varied vegetation types from which it is derived.  The formation of coal from organic debris is by a process of coalification. When some types of organic materials are heated and compressed over time, they can form water, gas, and coal. Peat formed by the decay of vegetation under anaerobic conditions is a coal precursor. Organisms that form coal when subjected to coalification include algae, phytoplankton, and zooplankton. Coal can also be formed by the bacterial decay of plants and, to a lesser extent, animals. Organic debris is composed primarily of carbon, hydrogen, and oxygen. It may also contain minor amounts of other elements such as nitrogen and sulfur. On the basis of the degree of coalification or maturation of carbonaceous material, coal is classified into:

  • Lignite, 
  • Sub-Bituminous Coal
  • Bituminous Coal
  • Anthracite 
  • Graphite 

The lowest rank coal is lignite, followed in order by sub-bituminous coal, bituminous coal, anthracite and graphite. High moisture content is associated with low heating value, while low moisture content is associated with high heating value. Coal rank is correlated to the maturity, or age, of the coal. As a Coal matures the ratio of hydrogen to carbon atoms and the ratio of oxygen to carbon atoms decrease. The composition of the highest rank coal, graphite, approaches 100% carbon. Coals burn better if they are relatively rich in hydrogen; this includes lower rank coals with higher hydrogen to carbon ratios. The percentage of volatile materials in the coal decreases as coal matures. Volatile materials include water, carbon dioxide and methane.

Coal gas is primarily methane with lesser amounts of carbon dioxide. It is absorbed in the coal structure or coal matrix, and adsorbed on the surface of the coal. Absorption is the penetration of one substance into another substance. By contrast, adsorption is the accumulation of one substance on the surface of another. 

A coal seam is the stratum or bed of coal. It is a collection of coal matrix blocks bounded by natural fractures. The fracture network in coal beds consists of micro fractures called “cleats.” An inter-connected network of cleats allows coal gas to flow from the coal matrix blocks when the pressure in the fracture declines. This is an important mechanism for producing coal gas.

Crude oil (liquid fuel)

Oil and gas are terms that refer to mixture of hydrocarbon molecules in the liquid phase and gas phase, respectively. Hydrocarbon molecules are compounds of carbon and hydrogen. 

  • Unconventional oil refers to hydrocarbon production from low permeability shale (shale oil) and tar sands. 
  • Unconventional gas refers to gas production from coal (coal gas), low permeability sands (tight gas), and low permeability shale (shale gas). 

The primary difference between conventional and unconventional oil and gas is the ability of the fluid to flow through rock. Most crude oil is refined for use in transportation, while gas is used to generate electric power and as a fuel in industrial, commercial, and residential sectors of the economy. 

The components of mineral elements in crude oil 

ElementComposition ( %by mass)
Carbon84 – 87
Hydrogen11 – 14
Sulphur0.6 – 8
Nitrogen 0.02 – 1.7
Oxygen0.08 – 1.8
Metals0 – 0.14

The actual elemental composition depends on factors such as molecular composition of the source, reservoir temperature and reservoir pressure. Hydrocarbon molecules in oil are compounds of carbon and hydrogen. Inorganic molecules such as nitrogen, carbon dioxide, and hydrogen sulfide can also be found in oil and natural gas. Since these fuels majorly contain all these elements and its burning for energy purpose is adding to the heat in the atmosphere to the levels now is causing global warming.

Petroleum is another term for a hydrocarbon mixture found in nature. Petroleum liquid typically refers to crude oil and natural gas plant liquids. 

Natural gas is typically methane with lesser amounts of heavier hydrocarbon molecules like ethane and propane. The liquids obtained from the gas are referred to as condensates or natural gas plant liquids. The total energy utilization in the world relies around 30- 35 % on petroleum liquid and 20-25% on natural gas. 

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