Dave,
  I'm not an engineer, I am a landman. In other words I help contract with landowners to get the property to drill oil wells.
 But, horizontal drilling has made a lot of areas economic, such as the Austin Chalk or the Barnett shale. As was pointed out above, pressure maintainence starts at day one on a lot of reservoirs.
  The majors and the engineers claim that recovery has increased by 50% or more on reservoirs discovered now as opposed to oil fields that were drilled up in the 1920's. I cant prove that but I believe they are correct.
  I feel that the real problem is population growth. There are at least 10 times as many oil consumers in the world as there were 50 years ago. We are looking at a very steep slope because of demand as well as the exhaustion of the big fields. There will still be a lot of oil produced in 20 or 30 years, just not nearly enough to satisfy demand no matter what the price.

  I did not address offshore wells because they are not my area of expertise. However, directional drilling has raised the number of wells that can be drilled by a single rig by a huge factor. Modern rigs can drill 60 or more, while old rigs could get 8 wells or so from a platform. And the fancy new seismic imaging techniques and steerable downhole drill motors can place each well in the best place to produce the maximum amount of oil.
  Also, some reservoirs of heavy crude would not be economic at all without enhanced techniques. Look at Chevron's wells at Bakersfield that require steam to produce, or any of the tar sands that are going to save us all according to the majors.
  I can't be specific about the math or what percentage of oil is produced with "extreme" techniques. I lack the knowlege, but I bet someone around TOD does have the info. I would love to know.