Natural gas is a naturally occurring hydrocarbon fluid found in subsurface geological formations, typically in association with crude oil reservoirs or in standalone gas-bearing formations. Its composition is dominated by methane (\(CH_4\)), usually ranging from about 70% to over 90% by volume, with lesser amounts of other light hydrocarbon such as ethane, propane, and butane, and varying concentrations of non-hydrocarbon gases including carbon dioxide, nitrogen, hydrogen sulfide, and water vapor.
Natural gas is a fossil fuel energy source that forms naturally in underground geological formations through the transformation of ancient organic matter, primarily marine microorganisms, which becomes buried under layers of sand, silt, and rock over millions of years. Under conditions of intense heat and pressure in the absence of oxygen, this organic material undergoes thermal decomposition, resulting in the creation of natural gas alongside other fossil fuels such as coal and petroleum. The gas migrates through porous and permeable rock until it is trapped by impermeable cap rocks, forming accumulations that can be commercially produced.
In petroleum engineering, natural gas is classified based on occurrence and composition, including associated gas (produced with crude oil), non-associated gas (found in gas-only reservoirs), and unconventional gas such as shale gas, tight gas, and coalbed methane, each defined by reservoir characteristics that are well established. Once produced, natural gas is processed to remove impurities and separate valuable components, after which it is transported via pipelines or as liquefied natural gas (\(LNG\)) for use as a fuel and as a feedstock in chemical manufacturing.
Natural gas occurs in various reservoir types, including conventional deposits trapped in porous rock formations, unconventional sources within low-permeability shale or tight sandstone, and coalbed deposits where it is adsorbed onto coal seams. This resource serves as a versatile fuel for heating, electricity generation, and industrial processes, as well as a feedstock for manufacturing materials and chemicals, after it is processed into a dry, marketable form.
