Having wood come from carbon and oxygen isn't unreasonable, those are the basic elements in most organic matter anyway (and hydrogen, but they want to keep the low level products 2 materials).
I guess what annoys me is that the game does so much right: silumin, duralumin, inconel are all real alloys, for example. Of course I'm not expecting modeling real world properties or anything, but the idea that "you refine ores to get rid of the mass you don't want and get the pure element inside" is so basic it breaks my immersion a little bit when it's so obviously violated.