Petroleum originates under the floor of a body of water (see the Petroleum Intro page). The petroleum attempts to rise through the "floor", breaking free of weaker rocks and shale. Once it reaches a hardrock bed, however, the crude oil is trapped and remains under the rock wall. In some cases, petroleum does manage to rise all the way to the earth or the floor of a body of water, forming what are known as "surface deposits."

When digging for oil trapped beneath a layer of rock, an oil rig is built. The rig includes a derrick to support the large drill that digs to reach the petroleum reserves below. Once the crude oil supply is reached, the pressure accumulated over the years forces the oil out of its "resting place." Sometimes the pressure is great enough that the oil will spurt out of the ground. Usually, however, a pump must be inserted to withdraw oil. When the price of retrieving oil is greater than the profit that can be gained, the well is said to be depleted.

Sometimes, wells that were thought to be depleted can be tapped by means of advanced recovery techniques. These techniques include pumping water or steam into the oil well both to increase the pressure and to displace some of the actual oil with water.

A new development is the offshore rig, a construction that allows for the retrieval of petroleum buried deep within the ocean.

An offshore oil rig