That depends on how the tree decays. There is a lot of coal near the surface of the western US because prairie grass burned yearly in every dry month (winter when it was too cold to grow - they may still be as much rain/snow!). This yearly burning didn't get warm enough to burn all the carbon and so the rest got deposited as charcoal.
Moral of the story: you should start regular forest/grass fires to turn the stored carbon into charcoal. A biologist can give you a number of other reasons why this is good for the forest as well: common knowledge about forest fires is almost completely false.
Imagining a forest where dead trees never decompose is quite a mindbender. Certainly a better explanation for the presence of fossil fuels though than just a handwavey mention of geological timeframes.
"Based on a genetic analysis of mushroom fungi, it was proposed that large quantities of wood were buried during [the Carboniferous] period because animals and decomposing bacteria had not yet evolved enzymes that could effectively digest the resistant phenolic lignin polymers and waxy suberin polymers. They suggest that fungi that could break those substances down effectively only became dominant towards the end of the period, making subsequent coal formation much rarer."
https://en.m.wikipedia.org/wiki/Carboniferous See Rocks and Coal