Open hole (and cased hole) logs can provide data which give a reasonable estimate of oil saturation, but there are variables there which can enhance the apparent oil saturation or even mask it, so that approach without drill stem test or production water data from the formation has its possible problems as well.
The more information, the better.
No method is perfect, there is only a body of information which, properly interpreted, indicates increasing or decreasing probability of production.
There is a danger in assuming that the porosity in any given reservoir remains constant over a larger area (primarily because it usually doesn't) but the attempt is to get baseline data which will allow an estimate of maximum amount of oil present, a minimum amount of oil present, and from that total recoverable oil depends on how efficiently that oil can be recovered.
In the early Bakken wells (Elm Coulee), for instance, the estimated likely recoverable oil was a small fraction of the oil estimated to be in the formation--something on the order of <5%, because of the low permeability of the rock. Not long after that, recoveries for individual wells had eclipsed that number, reached 10% of the estimated oil in place, and were continuing to produce.
Knowing the production history of a field can help with the estimates of what the recovery should be, and what should enhance it. The latter can get pretty wrapped up in the mineralogy of the formation, the pore fluids, and fluid dynamics.
Now we're getting complicated, but was the early production mostly gas? Did the reservoir pressures decline with gas production to the point wells went on pump quickly (no longer flowing to the surface)? Did the wells seem to 'nail up' when the reservoir pressure dropped below a certain pressure?
Did core data indicate there should have been a lot more oil recovered?
All of these bits of information will indicate a probability that production can be enhanced and that there is more oil to recover--sometimes a lot more, but they do not guarantee it can be recovered by any specific means--or at all, just that it should be there.
Note that this will not apply to every oilfield--no two are exactly alike and while some will respond to this sort of enhanced recovery, many will not, sometimes because the initial production strategies were spot on and maximized the recovery.
Most of the targets for this sort of enhanced recovery are fields drilled with vertical wells.
Although converting every other well in a series of parallel laterals might cause enhanced production in a horizontally drilled field, I know so far of no one who has done so in North America.
The evaluation of potential would have to be carried out on a case-by-case basis, as the variables of lithology, pore geometry, geological structure, fluid content, reservoir continuity, and production history all factor in.
Would not some older vertically-drilled fields regain productivity with horizontal drilling?