>minecraft:overworld

/ Total rate split between the 2 cases to match the Water Lucky Block (overworld 1/600).
/ 1/750 + 1/3000 = 1/600 (same rate as Water). Firewell = ~1/5 of the chaos blocks.

/ block alone (neutral luck)
type=block,ID=lucky:chaosluckyblock@chance=750

/ inside a firewell: true jackpot (luck ~50, the rare case)
group(type=structure,ID=firewell;type=block,ID=lucky:chaosluckyblock,posY=#bPosY+3,tileEntity=(Luck=#rand(40,60)))@chance=3000