The Macondo Prospect before the Deepwater Horizon floating oil rig arrived

The Macondo Prospect is the name of the reservoir the Deepwater Horizon floating oil rig was attempting to tap when the blowout occurred. It holds an estimated 50 million barrels of recoverable oil plus significant amounts of natural gas. The reservoir is beneath 5,000 feet of water plus 13,000 feet of rock. These numbers are typical for the Gulf of Mexico. The location is about 60 miles South East of the mouth of the Mississippi river. It is 65% owned by BP (British Petroleum.)

Oil is found only where certain, very specific, conditions are met. You must have a source rock containing organic material. The source rock must be "cooked" to convert the organic material into oil. The cooking temperature must be just right. If it is too high, you get gas. If it is too low, no oil gets formed. The oil, because it is lighter, rises until it encounters impermeable rock, where the oil collects in a reservoir. The reservoir is not a pool of oil. The impermeable rock above the reservoir must form a very good seal. Salt is a common seal. There can be no cracks, even a small one, because even a small leak will drain a large reservoir in a million years. But cracks do occur and this explains natural oil seeps. The source rock and the reservoir rock must be permeable so that the oil can easily move around.

The Macondo Prospect

macondo1a (131K)

Archimedes' Principle

Archimedes The beach ball (not to scale) is threatening to bob up out of the water with a certain force. The oil reservoir, like the beach ball, is also trying to bob up. But the force is very roughly a billion times greater.

The first person to think clearly about these kind of things (and publish) was Archimedes, in 250 B.C. Google him. He is interesting. Archimedes' principle states:

"a body fully or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid that body displaces."

The reservoir is the "body" and the "fluid" is the water and rocks where the reservoir is found.

The Macondo Prospect reservoir is estimated to hold 50 million barrels of producible oil. The oil is displacing rock, which has a specific gravity of 2.5 on average. That means rocks are 2.5 times heavier for an equal volume of water. Therefore the oil is "buoyed up" by the weight of 50 times 2.5, or 125 million barrels of water. But the oil has weight which counter balances part of this. Oil typically has a specific gravity of 0.85. So the weight of the oil is equal to the weight of 0.85 times 50 or 42.5 million barrels of water. The difference is 82.5 million, which is a lot of buoyed uppedness. You could also say the oil is trying to punch upward, through the seal.

The oil industry often uses the units of barrels, gallons, feet and pounds. Only one country in the world uses these units, the United States. The reason is tradition and perhaps insularity. It is a bizarre system. Barrels for oil hold 42 gallons. Barrels for water hold 55. Gallons come in two sizes. Miles have 5280 feet. It is hard to think in English units--let alone do science in English units. (Apologies for being a little off point.)