Natural gas

(materials) A combustible, gaseous mixture of low-molecular-weight paraffin hydrocarbons, generated below the surface of the earth; contains mostly methane and ethane with small amounts of propane, butane, and higher hydrocarbons, and sometimes nitrogen, carbon dioxide, hydrogen sulfide, and helium.

A combustible gas that occurs in porous rock of the Earth's crust and is found with or near accumulations of crude oil. Being in gaseous form, it may occur alone in separate reservoirs. More commonly it forms a gas cap, or mass of gas, entrapped between liquid petroleum and impervious capping rock layer in a petroleum reservoir. Under conditions of greater pressure it is intimately mixed with, or dissolved in, crude oil. See also Oil and gas storage.

Typical natural gas consists of hydrocarbons having a very low boiling point. Methane (CH₄) makes up approximately 85% of the typical gas. Ethane (C₂H₆) may be present in amounts up to 10%; and propane (C₃H₈) up to 3%. Butane (C₄H₁₀); pentane (C₅H₁₂); hexane; heptane; and octane may also be present.

Whereas normal hydrocarbons having 5–10 carbon atoms are liquids at ordinary temperatures, they have a definite vapor pressure and therefore may be present in the vapor form in natural gas. Carbon dioxide, nitrogen, helium, and hydrogen sulfide may also be present.

Types of natural gas vary according to composition and can be dry or lean (mostly methane) gas, wet gas (considerable amounts of so-called higher hydrocarbons), sour gas (much hydrogen sulfide), sweet gas (little hydrogen sulfide), residue gas (higher paraffins having been extracted), and casinghead gas (derived from an oil well by extraction at the surface). Natural gas has no distinct odor. Its main use is for fuel, but it is also used to make carbon black, natural gasoline, certain chemicals, and liquefied petroleum gas. Propane and butane are obtained in processing natural gas. See also Petroleum products.

Gas occurs on every continent. Wherever oil has been found, a certain amount of natural gas is also present. Successful exploitation of these resources involves drilling, producing, gathering, processing, transporting, and metering the use of the gas. Long before supplies of natural gas run out or become expensively scarce, it is expected that some process of coal gasification will produce a gas which is completely interchangeable with natural gas and at a competitive price. This is important because coal makes up a majority of the world's known fossil fuel reserves. But when energy consumers indicated in the marketplace their preference for fluid and gaseous fuels over the solid forms, coal gasification research, already well under way, was given additional impetus. See also Coal gasification; Oil and gas well drilling.