Formation of Gas

There is a wide variety of natural gas occurrences whose composition and modes of formation may be rather different. However, in this chapter only natural petroleum gas will be considered. While methane (CH₄) is always a major constituent of the gas, other components may be present: hydrocarbons heavier than methane (mostly ethane C₂H₆, propane C₃H₈, butane C₄H₁₀),carbon dioxide CO₂; hydrogen sulfide H₂S,nitrogen, hydrogen, argon, helium,condensate (liquid hydrocarbons dissolved in the gas: they are separated when the gas reaches surface or shallow subsurface positions).