Ceiii2K:

Your colorful comment

I remember reading Ghawar oil was made from the excrement of some long dead worm, or some such.

deserves a little clarification, from a geologist's perspective: What you probably read was that the actual ROCK from which the oil in Ghawar field is produced, i.e., the stratum that holds the crude oil in its pore spaces, is in part a peloidal limestone, and that these peloids are the fossilized excrement of ancient worms, or crustaceans, or some other marine organism that lived during the Jurassic Period. And from what I have seen in recent months from studying many thin sections of core samples through the Arab-A, Arab-B, Arab C, and especially the Arab-D horizons (from two other KSA fields, but not Ghawar), I agree with that statement.

The actual OIL in Ghawar and other oil fields in its vicinity is probably derived from the fats and oils (lipids) that were present in countless marine microorganisms such as zooplankton, which thrived in the Jurassic seas, and the remains of which accumulated over time on the Jurassic seafloor. This marine sediment is now a shale (named the Tuwaiq Mountain Formation) that is older than the Arab A to D formations, and stratigraphically beneath it. Over time, this layer of marine mud and microorganisms was buried and heated at depth in Earth's crust, and in the process the oils in those microorganisms was physically separated, maybe in sort of the same way that heat applied to uncooked bacon (or most other meats) will soon result in the appearance of two phases, a solid (cooked meat) phase and a liquid (oily) phase. The solid remains of the fossils are still in the Tuwaiq Mountain Formation, but the liquid that was driven off by heating has migrated upward in Earth's crust over time, into the porous Arab formations (and other horizons as well), where it was trapped in the pore spaces because further upward migration was not possible owing the presence of thick layers of impermeable anhydrite, an evaporite deposit that formed also during Jurassic time when an enclosed or semi-enclosed sea dried up several times.

The geology of the Persian Gulf oil fields is fascinating, and of varying complexity from field to field, as discussed in laymen's terms in "Twilight in the Desert" by M. Simmons, and, in numerous AAPG publications, in more technical terms.