minecraft-assets/data/minecraft/worldgen/configured_feature/lake_lava.json

{
  "type": "minecraft:lake",
  "config": {
    "barrier": {
      "type": "minecraft:simple_state_provider",
      "state": {
        "Name": "minecraft:stone"
      }
    },
    "fluid": {
      "type": "minecraft:simple_state_provider",
      "state": {
        "Name": "minecraft:lava",
        "Properties": {
          "level": "0"
        }
      }
    }
  }
}