I'm just grasping at straws here... but oil is revealed with Sci.Meth., isn't it? What if it's some weird bug involving an oil plot under a city, which then must update it's graphics, and that for some strangeness causes a crash? Or maybe oil in an ocean tile, which also has a ship sitting on it... any combination of oil and byzantine graphics. It's probably not that, but could you check the worldbuilder, to see if there is any oil under a Byzantine city?