Part of current Oil In Place (OIP) which can be recovered to the surface under specific FDP.
The maximum volume of Oil Reserves for a newly explored field is called Estimated Ultimate Oil Recovery (EUOR).
The current OR is often split into Drainable Oil and Non-drainable Oil:
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Oil In Place (OIP)
[ Hydrocarbon Reserves ] [ Gas reserves ]
[ Estimated Ultimate Oil Recovery (EUOR) ]
[ Drainable Oil ][ Non-drainable Oil]