The 96% natural gas that we use is essentially 1,000 BTU's per cubic foot. Some coal gas is only about 500 BTU's, so yes you have to put some higher BTU content gas (propane, etc) mix into it to get it up to 1,000. All natural gas appliances are designed for 1,000 BTU gas. Another problem is during extremely cold weather, the pressure drops on the distribution system because of the demand, so in some areas of the country, a mixture of propane-air is injected into the system to keep the pressures up. Propane has a higher BTU content, so it has to be diluted with air to bring it down to 1,000 BTUs, unless the appliances are designed for propane only. Some appliances are designed for both, but have to be adjusted for one or the other.
Natural gas is found naturally in oil wells, in wells by itself, and in coal seams. Propane is a very light gas formed as a by-product of oil refining. Essentially propane is a refined gas from oil. When oil is refined, it makes diesel, light oils, kerosene, gasoline. Also thick oils like asphalt, and very light like propane and ethane. Natural gas can also be made from algae, seaweed, cow manure, and synthetically from coal. It is still not cost effective to make it from these renewable sources yet.
If they would let us drill in Anwar in Alaska, we have a 67 year supply of natural gas up there for the lower 48 states. I know the environmentalists say there is only a 2 year supply, but they don't tell you about the natural gas. When the oil well is drilled, the natural gas is on the top, the oil on the bottom. They use the natural gas pressure to force the oil up and out first. When all the oil is out, then they release the gas, which is lighter than air and rises. So after the oil comes out, the gas will come out for many years. There is also enough natural gas in hydrates in the Bermuda triangle to last the whole world for 30,000 years. It is however very deep and harder to get to. Discovery channel had a special on this. The gas being occationally released in these hydrates, causes ships to sink, and airplanes to explode. It is not Atlantas or aliens, but just plain old natural gas.