Keep in mind that "pool" is porous rock, not an underground lake.
A simple visualization would be a glass filled with sand, then you pour water into it. The drill a hole through the sand, insert a straw and try to suck the water out. You will never get all the water you poured in.
Tight sands, shale, etc are all the basic concept, but the formation have lower porosity (size of micro-holes) holding the oil. Also they have lower permeability (connection of those holes). By hydraulic fracturing, they put lots of cracks in the formation and sand to hold the cracks slightly open. This allows better flow out.
This is descriptive of what is going on with the Bakken, Eagle Ford, Marcellus and other popular shale currently under drilling and production.
We have far more petroleum trapped in oil shale. The previous was describing how to get oil/gas already separated from the shale or tight sands. This stuff, like what is in the Green River formation has to have the petroleum cooked out of it. That process is called retorting. It isn't really oil; what comes out of that is called Kerogen. It can be made into a synthetic oil that is actually pretty high grade stuff. The industry has done a crappy job distinguishing between the two. Bakken and Eagle Ford are still drilling and pumping. It just takes the horizontal wells and hydraulic fracturing to get enough flow per well to be economical.