Engineers have been trying for decades to figure out ways to extract the remaining reserves (usually 30-70% of the original oil-in-place.) Once the reservoir energy is gone, the oil becomes immobile. Pumping salt water and the gas-cap back into the reservoir gets some more oil out and CO2 and polymer floods get even more but it is expensive.
It always goes back to cost. “Clampett oil”, shale oil or any oil.