I did my research, and didnt find anything dedicated to methane, now this may be silly but its otherwise a good element. I made it in my mod, heres the code: 

-- MTHN

local MTHN = elements.allocate("MOD", "MTHN")

elements.element(MTHN, elements.element(elements.DEFAULT_PT_HYGN))

elements.property(MTHN, "Name", "MTHN")
elements.property(MTHN, "Description", "Methane gas.")
elements.property(MTHN, "Colour", 0xFFE67C)
elements.property(MTHN, "MenuSection", elem.SC_GAS)
elements.property(MTHN, "MenuVisible", 1)

elements.property(MTHN, "Flammable", 600)
elements.property(MTHN, "Explosive", 15)
elements.property(MTHN, "Weight", 1)
i would add a reaction to make it, like C02+HYGN but i couldnt get the reactions to work, also that would break saves, so if you, as in the dev or another modder are making this, make the creation reaction smth like BCOL/CO2+HYGN+BOYL to sort of simulate an acetoclastic reaction.