...is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium.
I saw the graphic below in the BBC News article Trump climate: Challenges loom after Obama policies scrapped. It lists natural gas as the source of about 33% of the US electricity generation in 2015, but also lists "Other gases" as the source of less than 1%. Presumably it is a meaningful fraction of that 1% or else the BBC would not have included it. While the source of the data is listed as the US Energy Information Agency, the graphic itself was prepared by the BBC if I understand correctly.
Are thee other gases available from the Earth that could account for this fraction of 1%? I don't mean gases that are produced during an industrial refinement process, but perhaps gases that were simply separated. Just for example could it be natural propane? The same Wikipedia article mentions heavier hydrocarbons, but I don't understand if these are already present in the Earth and just being separated, or if they are produced primarily as reaction products.
EDIT: Based on comments, a substantially different source of gas that was chemically similar to natural gas would still be of interest. So just for example, methane from a bog should count in this case, since it does not involve many of the geological processes involved in the production of fossil fuels (e.g. the same timescales or temperatures).