Methane
From Memory Alpha, the free Star Trek reference.
Methane (CH4) is an odorless simple hydrocarbon that can be combusted to produce energy. It exists on many planets, including Earth.
The atmosphere of a Class K planet, such as Theta 116 VIII, consists of methane, nitrogen and liquid neon. (TNG: "The Royale")
The Axanar breathe a nitrogen-methane atmosphere. (ENT: "Fight or Flight")
In 2151, the Klingon courier Klaang led two Suliban pursuers into a grain silo on Earth, owned by a farmer named Moore. The silo contained methane, which Klaang detonated behind him, killing the Suliban. (ENT: "Broken Bow")
In 2364, an accidental explosion from a methane-like gas occurred on the Sigma III colony on Quadra Sigma III. (TNG: "Hide and Q")
In an alternate timeline where the Borg assimilated Earth after stopping Zefram Cochrane's warp flight, the planet had an atmosphere of methane, carbon monoxide, and fluorine in 2373. (Star Trek: First Contact)
There were traces of potassium nitrate, ammonium, and methane in the back of a truck the USS Voyager found drifting in space in 2372. Kathryn Janeway identified it as horse manure. (VOY: "The 37's")
A class 6 gas giant encountered by the USS Voyager in 2375 had a layer of liquid hydrogen and methane ten thousand kilometers below the outer atmosphere. (VOY: "Extreme Risk")
