Sorry, Snorlax cannot learn any identify move at all.
Odor Sleuth is not a true 'identify' move, it reduces the opponent's evasiveness to normal, i.e. if they used Double Team to become more evasive. So Fissure will still only have 30% accuracy.
But if you definitely want Fissure you should be able to get it like this:
1. Get a Dugtrio, Camerupt or Hippowdon that knows it (by level up).
2. Breed it onto a Walrein.
3. Breed it onto Lapras.
4. Breed it onto Snorlax.