OK, OP... you might have heard about volcanoes containing gas, but it’s not the same kind of gas you are thinking of.
Volcanoes don’t really work the way it seems you might think.
Yes, what volcanoes contain is hot and burns, but not in the same way that petroleum does. Volcanoes are either made from silica-based minerals or basalt, depending on the type. However, because none of this was ever a living thing, these minerals have never contained energy that way, so energy can not be extracted while burning it.
The energy within a volcano is not in its minerals, but in the heat and pressure applied to the minerals. That is what causes the eruption.
Sorry if no one really cares about this, I just am totally fascinated by volcanoes and I take any chance to talk about them that I get.