Ok let's first look at the production of hydrocarbons.
1) Small organic lifeforms die in the upper levels of deep water low energy conditions, fall to the anoxic sea floor and are covered by fine-grained sediment.
2) Continual deposition of sediments and organics builds up a layer of organic rich mud called sapropel.
3) The paleoenvironment changes (e.g. an outbuilding landmass) and deposition of coarser sediment (which will form a porous and permeable reservoir rock) begins burying the sapropel. (increasing temperature and pressure and beginning the transformation to a source rock)
4) An impermeable layer is added to the top, the cap rock.
5) Faulting, folding and other tectonics creates a structure that will intercept and concentrate oil into economically viable reserve as it migrates upwards.
Hydrocarbon production is reliant on the enviroments of deposition not really timescale, we get lots of oil formed in the Jurassic at 180Ma-140Ma (or rather the source rock is that age) in the North Sea because that was when we had the right conditions in the right order not because 'oil takes 180Ma to form'.
steps 1-4 aren't that hard to imagine happening at some point in Scadrial's history, but when Harmony rearranged the geography of the landscape it would have been one hell of a job to not only preserve and move Kms of strata but position near to what would become Elendel. You also have to keep in mind this was also Sazed's first use of the shards and we know from past experience that the first time is the not always easy for a Shard-holder.
Diamonds come from deep down brought up in mantle plumes, the difference is (mostly) pressure but the pressure is on a different order of magnitude. If scadrian coal seams generally formed under higher pressure than Earth's we would see a greater proportion of higher grade coal however.