Yes, 40-50% is left but this varies by field geology and history of the field. Conventional recovery (i.e. natural pressure) only gets a small portion. Water flood (i.e. secondary recovery) does another increment.
CO2 (tertiary recovery) removes another increment.
Denbury has some good presentations on their website (well, they used to, I have not checked recently).
Tertiary recovery can be a very cost effective way to recovery oil and what is nice is their is no dry hole risk. The reservoirs have 80+ yrs of production history and you can calculate +/- 10% how much oil was likely in place.
Kinder is minting money from the Yates and SACROC unitized fields.
The Russians have botched a lot of their oil fields. Ever see the pics of the run off? They have depleted all of the pressure trying to produce very heavily.
It takes a lot of upfront effort to attempt a tertiary recovery and it takes a couple of years to turn the corner and start getting good returns.