So lets state that the oil in these barrels is Whale Oil.

Whale Oil has a flash point of 230 °C / 446 °F so to get a barrel to explode, you have to vaporize all the oil, mix it with an oxidizer and have it under pressure. Just how much energy are in those Fire Bolts?

If oil was as explosive as Larian is making out, no one would ever cook with it. Yes, oil fires can happen in a kitchen but they are the exception.