This shows, as I've oft said, that any fossil fuel problem will soon solve itself, at least as far as petroleum.
There are an estimated 892 billion tonnes of proven coal reserves worldwide. This means that there is enough coal to last us around 110 years at current rates of production. In contrast, proven oil and gas reserves are equivalent to around 52 and 54 years at current production levels.
https://www.worldcoal.org/coal/where-coal-found
After centuries of mineral exploration, the location, size and characteristics of most countries' coal resources are quite well known. What tends to vary much more than the assessed level of the resource - i.e. the potentially accessible coal in the ground - is the level classified as proved recoverable reserves.