Extraction Rates

For most of my life I assumed that when you drilled for oil, you hit a pocket and sucked it all up until the well was dry. For most of the twentieth century we were a hell of a lot lazier. Most of the time a well was drilled and the oil was under enough pressure that it would flow out on its own. This primary extraction is not very effective however. It will only recover 5-10% of a deposit.
To get more we need to employ secondary recovery methods. This mostly involves pumping huge amounts of water down an abandoned well. This will raise back up the pressure of a deposit and will keep the oil flowing through other wells. It's good for recovering another 30% or so. Better, but still not great.
So then we move into the enhanced recovery methods. The first of which is steam injection. This warms up the oil which lowers the viscosity and makes it flow easier. It is especially effective in recovering heavy oil. The other relatively new method is CO2 injection. At sufficent depth, the gas will permeate the oil and will both increase the pressure of the deposit and lower the viscosity. These techniques will add another ten or so percent recovery to a standard field and more to the heavy oil fields.
Even with all of these techniques, oil recovery is still little better than 50%. It goes to show just how much oil we still have sitting in the earth.