Geology question: Why is the oil gushing?

[asterroc]

Okay, someone explain to me, why is the oil gushing out of the well? I picture the oil as being inside an underground lake, with the well being like a straw going down to it. But when you put a straw into a glass of juice, the juice doesn't suddenly start gushing up the straw, it sits inside the glass until you start sucking on the straw. In the case of a juice box, the juice does start gushing out of the juice box, but that's because you're squeezing on the package while holding it to jam in the straw - is there something "squeezing" on the oil in the ground? Or is it heat causing the pressure, and if so then why hasn't all the oil leaked out and stabilized that way?

Comments

[marquiswildbill.livejournal.com]

The gushing comes from a few factors as I understand it, and probably many more. I learned a bit about drilling but most of it was specialized to heavy oil like the Alberta oil sands.
A normal oil resevoir is trapped in a rock formation or similar trap. Because of how oil forms these become pressurized as hydrocarbons break down. Northern gulf oil is particuarly light as crudes go, with a density in the low 0.8 g/mL range, if I recall correctly. Now you put a hole in this trap, and put the pressure of a mile of ocean on top of it. The light material already would float to the surface, and when you add the pressure it's like squeezing a balloon.

[zandperl.livejournal.com]

Ah, so there's a combination of pressure (from both tectonics as rosefox quotes above, and from chemical processes within the reservoir), and simple buoyancy. This is making more sense to me - I didn't see why tectonics would necessarily increase the pressure always as opposed to equal chances of increasing or decreasing pressure.

[marquiswildbill.livejournal.com]

It's amazing how much is involved in oil. The way they get heavy oil out of the sands is remarkable.
Edit: The way they do it is blasting high pressure steam into the tar and then pumping the sludge then separating it from the sand.