This paper talks about combustion
It is the fact that combustion properties of large-scale crude oil pool fire have great significance for security design and firefighting of current crude oil reserves. Burning rate, the flame shape and radiation intensity are the most important parameters for fires properties.
And this is an image of a fire on an oil spill in the Arctic
To have an explosion you would need to have finely dispersed droplets of oil in the air to greatly increase the combustion efficiency. With a calm pool that doesn't happen.
Incidentally, this is why it is dangerous to try to extinguish a flame in a frying pan with water: the water will sink below the frying oil, turn into vapor and explode nebulizing the oil above it and producing the explosiona fireball/explosion.