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

[rosefox]

http://geology.about.com/od/petroleum/a/aa_petroleum.htm

Natural seeps of crude oil are not especially rare. But the oil leaking from the ground is usually a highly degraded substance, close to tar. It was at first used locally as a substitute for pitch or as a crude medicine. When drillers learned to tap petroleum at depth starting in 1859, its virtues began to be discovered, and over the next century oil transformed civilization. Natural gas came into prominence at the same time.

...The conventional petroleum reservoir is in a structural trap—a dome or vault of impermeable rock, formed by folding or faulting of the rock layers or by the rise of salt domes, with permeable rocks beneath it. In those permeable rocks there may be a layer of natural gas on top, with petroleum below. Beneath the oil is usually a layer of rock soaked with water or brine. There are also other unconventional types of reservoirs that are not trapped this way.

The key to a reservoir is sponge-like rock with open space between its grains—porosity. The porosity may have existed from the rock's original sediment; it might also arise as groundwater dissolves pores in the rock or as minerals undergo alteration. One major source of porosity is the transformation of calcite to dolomite, which takes up less space, by fluids rich in magnesium.

Besides porosity, there must be high permeability—the connectedness of pores that allows fluid to move easily through the reservoir rock. Permeability, porosity and geologic structure are all of great interest to petroleum geologists.

Reservoirs may come to be under excessive pressure due to tectonic forces. Modern equipment and practices can handle this pressure, but in the past drilling sometimes produced gushers.

[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.