It's a naturally-occurring
mixture of gaseous hydrocarbon
s which is often found along with
. Natural gas formed millions of years ago, underground
, as a result of
the same processes that formed petroleum.
To obtain natural gas today, people drill
into the Earth's crust
into pockets of gas trapped there. In the past, humans exploited naturally occurring
seepages of this gas, burning it to produce heat and light.
Natural gas is used as a fuel for heating, cooking, production of electricity,
and even for powering certain vehicles.
Natural gas is usually at least 90% methane, but can also contain propane,
ethane, and other lightweight hydrocarbons, in small concentrations. It is lighter
than air, and this is a good thing -- it won't collect in low-lying areas if it
escapes from the ground or from a pipeline.
It has no color or odor, but gas companies add
mercaptan, which is really smelly, to it so leaks or open valves (such as on an
appliance that someone
forgot to turn off) will be noticed. Gas leaks are very dangerous, because if the
concentration gets high enough, a spark from flicking a switch -- or even static
electricity -- can cause an explosion.
Natural gas started to be used in the 1800s, and at first it was used almost exclusively
for lighting of city streets. Eventually pipelines were installed to homes, and
electricity became more popular for lighting, so the use of natural gas shifted to
heating and cooking.
Five of the 50 states (New Mexico, Texas, Oklahoma, Louisiana, and Alaska)
contain more than half of the natural gas reserves in the US.