That's another problem that has reared its ugly head in Saudi. The quality of the oil from the older, larger fields is becoming more "sour" or "heavier" as it's called, and less light and sweet as they extract from deeper down in the reserves, making it more difficult to refine the higher grades of petroleum later during the refining process. That's in addition to deceasing internal pressures in the oil caverns themselves, making extraction more difficult and costly.

That's why Nigeria, which is even more unstable politicly, is such a influence on the oil markets. Their "bonnie light" sweet crude is among the highest quality oil on the planet.