Natural gas is a fossil fuel formed by layers of buried plants and animals exposed to intense heat and pressure over thousands of years. Thus it is a nonrenewable energy source.
Any hydrocarbon that remains gaseous under normal atmospheric conditions, directly extracted from oil and gas reservoirs, is considered natural gas. It is found in nature as a varied hydrocarbon mixture consisting primarily of methane but with small amounts of ethane, propane, CO2 and nitrogen.
The main characteristics of natural gas are: lower density than air, low point of vaporization and high burning power. The gas is odorless and invisible under normal conditions. It is odorized by the distribution companies for the society safety.
Natural gas is the third largest primary energy source in the world, surpassed only by petroleum and coal.
Source: Petroleum Law no 9.478/97and GasNet