Content is typically very high percentage methane. First stage processing removes propane, butane etc because they tend to liquefy at the pressures used in the pipelines. Then they collect in the low spots when the line isn't running at capacity. When the flow increases, you get a slug of liquid propane being pushed by the gas. A pipeline pump designed for a gas gets really unhappy when it gets a slug of liquid.
Detail analysis: https://www.uniongas.com/about-us/about-natural-gas/Chemical-Composition-of-Natural-Gas
Note that these are in moles, and hence will be volumetrically correct. Percentage by mass will skew away from methane. (Remember that a mole of any gas is essentially the same volume at reasonable temperature and pressure)
The higher alkanes are green house gasses, but have short lifetimes. See this good answer on quora: https://www.quora.com/If-methane-is-a-significant-greenhouse-gas-then-do-all-alkanes-also-have-potential-to-be-a-significant-greenhouse-gas
Turns out that right now the liquids are more valuable than the methane. Natural gas for heating would be a lot more expensive, but right now the drilling and fracking is being driven by the price of butane and propane. (Don't ask me for a source for that. Somewhere in the last year.)
The other significant variable component is hydrogen sulphide (H2S ) Smells of rotten eggs at < 2 ppm. Anesthetizes the nose at slightly higher levels. Recommended safety levels no more than 10 ppm for 10 minutes. High levels are deadly. 1000 ppm is "1 breath unconscious, a few minutes dead" Since you can't smell it, and it's heavier than air this is a major threat in confined spaces. There are wells in Alberta that produce up to 30% H2S (300,000 ppm)