Sour gas is natural gas or any other gas containing significant amounts of hydrogen sulfide (H 2 S). According to this reference ng.asp" target="_blank">NaturalGas.org website page Processing Natural Gas , natural gas is usually considered sour if there are more than 5.7 milligrams of H 2 S per cubic meter of natural gas, which is equivalent to approximately 4 _ppm by volume.

Although the terms acid gas and sour gas are used interchangeably, strictly speaking, a sour gas is any gas that contains hydrogen sulfide in significant amounts, whereas an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO 2 ) or hydrogen sulfide. Thus, carbon dioxide by itself is an acid gas but it is not a sour gas.

Before a raw natural gas containing hydrogen sulfide and/or carbon dioxide can be used, the raw gas must be treated to remove those impurities to acceptable levels, commonly by an amine gas treating process. The removed H 2 S is most often subsequently converted to by-product elemental sulfur in a Claus process.

Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide.

Hydrogen sulfide is a toxic gas. It also places restrictions on the materials than can be used for piping and other equipment handling sour gas, as many metals are sensitive to sulfide stress cracking. The presence of hydrogen sulfide in gas causes lower quality burning and the production of sulfur dioxide, and so is regulated in commercially sold gas.