I've been reading articles about renewable energy, and in comments people talk about fossil fuels and petroleum as examples of non-renewable types of fuel. Sometimes they are used interchangeably, or even mentioned as different things. I couldn't find explicit explanation for which is which, and if one is a subset of the other. Can someone clarify this for me?
Fossil fuels are those that originate from ‘fossilised’, anaerobically partially degraded prehistoric living species (mostly plants and animals) and that take a few million years to form hence are not considered renewable.
Depending on how old the deposit is and which conditions it encountered across Earth’s history, the deposit will either turn into something gaseous, liquid or solid — and these categories are typically labelled ‘natural gas’, ‘crude oil’/‘petroleum’ and ‘coal’. Thus, petroleum is only one type of fossil fuel: the one that presently is in the liquid aggregate state.
However, equating petroleum and fossil fuels isn’t entirely unwarranted, since petroleum is both the most important fuel and the scarcest today.