In addition to the excellent answer by @LShaver, other things to consider are:
Delivery of gas to the premises
My experience is that there is only one reticulated gas main in residential areas and that such mains supply methane. Similarly, the use of propane requires the use of on site pressurized gas containers which either have to be replaced with full ...
According to Wikipedia:
Propane is produced as a by-product of two other processes, natural gas processing and petroleum refining.
The amount from petroleum refining is minimal. The U.S. Energy Information Agency provides detail about how much propane (a hydrocarbon gas liquid) is produced by each source:
Continuing from Wikipedia, if propane wasn't ...