Methane

Methane is a flamable gas; its molecular structure contains one carbon atom and four hydrogen atoms.

Borg drones are apparently prefer methane, as it is one of the compounds found in the preferred Borg atmosphere, along with fluorine and carbon monoxide. 

In an alternate timeline where the Borg were able to assimilate entire star systems in the Alpha Quadrant, increasing levels of methane were noted in the atmospheres of Alpha Centauri and Andor while the Borg adapted their worlds.