The Formation and Composition of Coal Gas and Natural Gas

Coal gas is a by-product of the evolution of plants into coal. Coal begins as an accumulation of terrestrial organic debris derived from plant tissues which, subsequent to the influences of heating and pressure (from burial at depths of several thousands of feet) becomes coal. This metamorphic process breaks the chemical bonds of the carbon-based organic matter causing the formation of methane, carbon dioxide, water, and trace amounts of ethane and propane, with very few heavier volatile hydrocarbons. (Some coal beds at depths greater than 4500’ can yield commercially significant volumes of light oils when the produced gas is carbon dioxide-rich. This is not typical of SJB coal gas, but is characteristic of coal gas produced from the northern Piceance Basin of Colorado) (Nelson, 1999). The amount of gas stored within the micro-pore structure of the coal is related to the rank of the coal. The more mature (higher-rank) coals, having been subjected to greater periods of burial and higher temperatures, yield proportionately greater volumes of gas. Fruitland coals are generally considered low to medium rank volatile bituminous.

Conventional natural gas is derived through heat and pressure-induced alterations of marine organic matter. Like coal gas, natural gas is essentially composed of methane, but generally contains higher percentages of heavier hydrocarbon fractions such as butane, pentane, hexane and condensates, giving natural gas a higher heating value.