natural gas 
noun [mass noun]
Flammable gas, consisting largely of methane and other hydrocarbons, occurring naturally underground (often in association with petroleum) and used as fuel.This type of ceiling heater usually uses propane or natural gas as the fuel source....
- The country's main exports are crude oil, petroleum products, and natural gas.
- These cars are expected to use hydrogen taken from fossil sources such as natural gas or methanol.