@wikipediaTotal volume of oil in petroleum reservoir
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 Oil Reserves (OR) is often split into Drainable Oil and Non-drainable Oil:
OR = Drainable Oil + Non-drainable Oil
The Non-drainable Oil is a result of deviation from the current Field Development Plan (FDP) or mistakes in FDP itself, for example, the overestimation of OR.
See Also
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Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Oil In Place (OIP)
[ Hydrocarbon Reserves ] [ Gas reserves ]