Virtually all wells are logged, but this is after they have been drilled in the hope of finding oil or gas, and there is some evidence that oil or gas exists.  And I have to confess that I simplified this quite a lot, since there are a wide variety of instruments that are now used that can tell with a fair degree of accuracy, what is down there.  However it gets a bit more difficult as we move away from the well, and that may be the subject of a later post.

Generally you have an idea of the geological shape that you are drilling into before you put the well in, but you don't completely know where the oil level is, and some of the papers from Aramco suggest this is still the case, until you physically go down there and look - with an instrument.  This gets even more important when you are trying to steer a bit to hit the rock with the oil in it rather than the water.