I think it depends on the depositional environment, its still possible for trees to be buried before decomposition. Especially in forested areas with significan

I think it depends on the depositional environment, its still possible for trees to be buried before decomposition. Especially in forested areas with significant water flow. Once a log hits the muddy, anerobic bottom of a river/lake it essentially enters stasis, people fish centuries old logs out of rivers for profit. All of these things happen at massive scales, so even though the vast majority of wood does decompose before it can be preserved, even a few percent is probably enough coal to power some future post nuclear apocalypse society's industrial revolution in a few dozen million years time.