Isn't coal the result of a massive accumulation of plant matter that couldn't be digested by any organism? But then, once the current mushrooms and bacteria ma

Isn't coal the result of a massive accumulation of plant matter that couldn't be digested by any organism? But then, once the current mushrooms and bacteria manage to digest it, there is no way for it to accumulate again like that. Petrol might be a different story, coming from the plankton falling at the bottom of the sea instead.