Natural Sour Gas Creation??

After hundreds of hours into the game, and countless restarts, I've started getting into more of "mid-game" builds and been expanding my knowledge of the oil biome and its by products. I had been digging a relatively small cavern in the oil biome, and was about to start pumping some oil to add to my metal refinery cooling loop. After ignoring the cavern for 5-10 cycles, I scrolled down to task up the pump and its required power/plumbing, and saw that the cavern had been completely filled with Sour Gas. As far as I knew according to wikis and other videos, you can only make Sour Gas if have a very high heat source to do the oil->petroleum->sour gas transformation?

Looking at the various map overlays, I don't see a high heat source, and there is only 2 tiles of petroleum left that would give evidence that a transformation even happened. I'm very confused how this could have happened. Any ideas what could have caused the transformation?? Bug? Glitch?

I'd just run with it if I knew what caused it, but without knowing, I'm wondering if its smart to just do a restart.