Obviously the risk of fire or even an explosion.
I recently cut up an old fuel oil tank, that had not been in service for years, in order to remove it from a storage room. There was a very strong fuel odor. First, I borrowed a gas tester from work and verified the atmosphere would not support fire or explosion.
If you don't have access to a tester, and have someone able to explain how it functions, I think it's wise to avoid the job.
Charles Hall