Would not some older vertically-drilled fields regain productivity with horizontal drilling?
I'll stick my neck out, even and say the more stratigraphically complex the reservoir the more likely there can be significant improvements made in oil recovery. Existing well data can be used to map the structure in the area, and staying within a stratigraphic target is a matter of execution from there.
I have had the privilege of being able to use infield well data on a couple of wells and it made my job significantly easier.
Some of the early horizontal wells I worked were in a field with 160 acre spacing, and we cut a window in existing production casing to drill laterals out of the old wellbores.
This was in a field which had had production for over 30 years and many of the wells had declined to stripper status.
Our first 1500 ft. lateral took a well which had declined below 20 BOPD to 250 BOPD, we came back a year later and drilled another lateral at 90 degrees to the first (after production had declined to about 50 BOPD) and raised production to 200 BOPD and roughly 500 MCF of gas/day.
This was in the early '90s and proved two things:
First, even on a 160 acre spacing, there was plenty of oil left after thirty years...
and Second: the reservoir was anisotropic--that is, the properties in the reservoir were significantly different depending on which direction you drilled through it.
This was a carbonate (limestone) reservoir and proved out a couple of theories a few of us had with respect to carbonates.
The field was subsequently fairly extensively drilled horizontally, with many new wellbores drilled instead of using the 30 year-old wellbores.