What would happen if magma flows into a very large oil deposit?
An oil deposit is gravel and rock mixed with oil, capped by rock - like this:



My guess is that if somehow, likely from below or sideways, magma would intrude into this deposit, oil (and posibly water) would evaporate. The pressure rises, slowing the magma intrusion. If enough magma and enough heat is provided, the vapor pressure of the oil may be enough to crack the rock layer at places, oil would escape and condense in the shale layer.

My gut feeling is that we are talking about processes that take years to decades to play out.