Composition of flatus gases

Nitrogen is the primary gas released. Methane and hydrogen, lesser components, are flammable, and so flatus is susceptible to catching fire. Not all humans produce flatus that contains methane. For example, in one study of the feces of nine adults, only five of the samples contained bacteria capable of producing methane.[3] Similar results are found in samples of gas obtained from within the rectum.

The gas released during a flatus event frequently has a foul odor which mainly results from low molecular weight fatty acids such as butyric acid (rancid butter smell) and reduced sulfur compounds such as hydrogen sulfide (rotten egg smell) and carbonyl sulfide that are the result of protein breakdown. The incidence of odoriferous compounds in flatus increases from herbivores, such as cattle, through omnivores to carnivorous species, such as cats or dogs. Flatulence odor can also be caused by the presence of large numbers of microflora bacteria and/or the presence of feces in the rectum.

From Wikipedia