Depends what the constituent of the natural gas is. It will more than likely be methane, which LFL is 5 and UFL is 15. Here is a list of some flammability limits in the link below.
methane, odour is added to make it natural gas
What is the chief constituent of gober gas? What is the chief constituent of gober gas?
Propane is the major constituent of Liquid PropaneGas.
The major constituent of air is Nitrogen which is 78%
crude oil and natural gas