I'm no expert on the question. But isn't it a bit easier to say than to do? The US went through various EOR techniques after what was considered a peak in production rate in 1970. But until recently (40 years after this "peak"), the production rate was decreasing, even with the more recent discoveries and exploitation of offshore production.
Apples and oranges. Recovery factors in the conventional reservoirs were already much higher (averaging primary recoveries around 35%). The tendency in those reservoirs is to drill enough wells to get them to a plateau rate (usually governed by amount of pipeline capacity available, reservoir considerations etc) and gradually let them decline over time. Water or natural gas injection might be considered during the plateau period if there is too much pressure decline. It isn't until sometime after the wells have already declined considerably that full on water injection or EOR would normally be considered. Hence the fields will never reattain their peaks, they do increase in production for sometime but not to the peak rate. In the shales it is a bit of a different story because the recovery is so small the issue is not as much the problem of residual oil saturation do to wettability changes or adverse mobility it is mainly due to the fact the fractures which are created can only access a certain amount of the formation. Imagine a swimming pool size of shale that has high porosities which allows for lots of storage of oil that has been generated in-situ but with nano-darcies of permeability (that's 10*9 smaller than most conventional reservoirs). You hammer on the rock with a big sledge and create a big fracture across the width of the pool with a bunch of subsidiary fractures that fan out for a metre or so around the large fracture. What that does is it allows all of the oil which has direct access to the big fracture (via smaller fractures or microcracks) to be extracted. Yet the vast majority of that swimming pool of rock is untouched. So in the case of shales or marls the issue is that there needs to be more of the reservoir intersected where permeability can be increased. The technology is different and the economics are different.