I was raised from 1951-1969 in Texas City. It becomes a part of your psyche if you live there. The Van Camp smoldered for some time before exploding. I suppose it could have been mixed with fuel oil if a bulkhead split, but as for my memory of the tale from my father, it just exploded when it got hot enough. Maybe generated gases, maybe ship fuel, who knows. I think the explosion was pretty efficient as the ship anchor was almost blown to the north end of Moses Lake about 20 miles from Monsanto. I'm inclined to believe it was just the ammonium nitrate reaching critical temp.
Now if you are reading the Anarchist cookbook, yeah, diesel fuel is recommended.