Ow... liquids mixing or even just flowing over each other... that will be FPS fun. So on second thought - we will probably not see that happen anytime soon.
It could be a contaminant though...
Hmmm...
Shale => Appropriate [Magma]Furnace-like workshop => oil/petroleum/etc?
Tars and pitches, also[1], maybe created, maybe natural seeps.
Brine. Dumped into specifically designated ponds (would also work with sea-water, of course), on warm enough biome with enough consecutive rainless days produces haulable piles of salt.[2] Consider also natural evaporite zones (and/or evaporite sedimentation layers being a worldgen creation... salt-mines that would react to water intrusion in an interesting manner roughly analogous to lignites/etc to fire... FPS being the first victim of such an 'improvement', however.)
A number of other connate fluids of various usefulnesses or otherwise.
Overpressured gaseous pockets of non-hydrocarbon gasses (very rare, would need to have been flash-formed in some way)
Treacle mines (of either the [sickly] sweet or unpleasant type, your choice).
Your various fantasy stuff. In keeping with DF's world of course, but I'm minded to mention "Fat deposits with BCGs" equivalents. Also Jam Butty mines (well, perhaps stone layers weeping preserved fruit product, much as per the former type of Treacle mine, though if it's between two layers of semi-fossilised grain, it can't be much worse than Pratchettian dwarf-bread! Or cylindrically folded to make close approximations to fossilised Swiss Roll... Whoops, is that my imagination running away with me...)
[1] Maybe needs to be applied to wooden constructions to make them waterproof. But if they ever burn, makes them even more !!FNU!! when exposed to heat.
[2] For trade, at least. Not sure if dwarves like seasoning in their prepared meals, but (along with some fantasy-equivalents to pepper, mustard, various herbs) could be integrated as a minor component (might need to revamp food quantities) to 'zhoozh-up' in value/satisfaction any particular meal. With the downside that certain dwarves might not like certain types of salty/spicy food and refuse to eat anything so treated. And others (perhaps after acquiring a taste for something spicy) complain of blandness when there's nothing of that kind available. Could be done similarly to encrusting with gems. Don't know whether to suggest it affects speed of rottenness (or effective rottenness, I suppose, if it covers up 'on the turn' food, and might even allow early-stage rottenness in foodstuffs to be overlooked if also available in cooking).