A couple of oddities with Hell terrain:
1- When retreating or being sanctified, hell terrain will change all resources to a default one:
- corn, wheat rice > rice
- any plantation resource > cotton
- any pasture resource > cow
- marble correctly comes back since it's the only quarry resource
If hell terrain spreads to these tiles with resources again, and it is again cleansed, the resources will revert to their original status. So it takes two cleansing to bring them back right, but not always you will get that chance.
2- The turn after completing Genesis, in the lands of the player who completed it, Hell terrain will spread to some tiles it originally didn't spread to. When this happened to me, Hyborem had even been eliminated... and I was playing a Good Civ (originally Neutral - Amurites).