What Is Natural Gas ?

Natural gas is a subcategory of petroleum that is a naturally occurring,
complex mixture of hydrocarbons, with a minor amount of inorganic
compounds. Geologists and chemists agree that petroleum originates from
plants and animal remains that accumulate on the sea/lake floor along
with the sediments that form sedimentary rocks. The processes by which
the parent organic material is converted into petroleum are not understood. The contributing factors are thought to be bacterial action; shearing
pressure during compaction, heat, and natural distillation at depth; possible addition of hydrogen from deep-seated sources; presence of catalysts; and time (Allison and Palmer 1980).
Table 1-1 shows composition of a typical natural gas. It indicates that
methane is a major component of the gas mixture. The inorganic compounds nitrogen, carbon dioxide, and hydrogen sulfide are not desirable
because they are not combustible and cause corrosion and other problems
in gas production and processing systems. Depending upon gas composition, especially the content of inorganic compounds, the heating value of
natural gas usually varies from 700 Btu/scf to 1,600 Btu/scf.
Natural gas accumulations in geological traps can be classified as reservoir, field, or pool. A reservoir is a porous and permeable underground
formation containing an individual bank of hydrocarbons confined by
impermeable rock or water barriers and is characterized by a single natural pressure system. A field is an area that consists of one or more reservoirs all related to the same structural feature. A pool contains one or
more reservoirs in isolated structures. Wells in the same field can be classified as gas wells, condensate wells, and oil wells. Gas wells are wells
with producing gas-oil-ration (GOR) being greater than 100,000 scf/stb;

Table 1-1 Composition of a Typical Natural Gas

condensate wells are those with producing GOR being less than
100,000 scf/stb but greater than 5,000 scf/stb; and wells with producing
GOR being less than 5,000 scf/stb are classified as oil wells.
Because natural gas is petroleum in a gaseous state, it is always accompanied by oil that is liquid petroleum. There are three types of natural gases:
nonassociated gas, associated gas, and gas condensate. Nonassociated gas
is from reservoirs with minimal oil. Associated gas is the gas dissolved in
oil under natural conditions in the oil reservoir. Gas condensate refers to
gas with high content of liquid hydrocarbon at reduced pressures and
temperatures.