A gaseous fuel occurring in nature and consisting mostly of a mixture of organic compounds, normally methane, ethane, propane, and butane. The calorific value of natural gases varies between about 26. 1 MJ/m3 and 55. 9 MJ/m3 (700 Btu per ft3 and 1500 Btu per ft3), the majority averaging 37. 3 MJ/m3 (1000 Btu per ft3).