Hermann Pferfferkorn of the School of Arts & Sciences says, “But, most of the time, there isn’t very much — the coal is usually only found in certain areas, and the amounts that do exist do not compare to the huge quantities formed in the Carboniferous and Cenozoic.”
https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/21/new-research-digs-into-the-mysterious-origins-of-coal/ The Washington Post