Natural gas like petroleum is generally believed to be derived from deposits of plant and animal remains from millions of years ago. Natural gas may be found along with oil or by itself as in many gas fields where little or no oil is found. The reasons for these differences, however, in origins for these two fossil fuels are not understood.

Natural gas as supplied is the cleanest fuel with sulfur removed (except for small amounts of odorants added), no ash and only molecular nitrogen, and a high hydrogen to carbon (H/C) ratio which minimizes the greenhouse gas CO2 emission. Along with methane which is by far the major combustible constituent of natural gas, other light hydrocarbons, namely ethane, propane, and butane are present in the natural gas. Raw natural gas may contain CO2 and sometimes N2 which have no heating value. CO2 is typically removed from the natural gas while ethane, propane, and butane are usually removed and marketed separately as special fuels or as feedstocks for the manufacture of petrochemicals. A number of other elements and compounds are also found in natural gas such as, H2, H2S and He. H2S is also removed from the natural gas before it is pipelined for sale.

Natural Gas Consumption and Reserves₁

The following table shows the natural gas consumption and the estimated reserves in 1996 in some countries (1 cubic Ft or 0.028317 cubic meter of natural gas contains roughly 1000 Btu or 1.0548 Mega Joules of energy):

.	China	Italy	Mexico	South Korea	U.S.
Consumption, 109 cubic Ft.	700	1,700	1,000	256	21,700
Reserves, 109 cubic Ft.	41,000	13,000	67,700	-	164,000

Reference:
http://www.eia.doe.gov/emeu/world/country