There are different ways of classifying reserves and their structure, below are the most popular categories.
See Also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology / Petroleum Reservoir / Hydrocarbon Reserves
[ Russian Reserves Uncertainty Categories ]
References
https://www.spe.org/industry/reserves.php
https://www.spe.org/industry/docs/GuidelinesEvaluationReservesResources_2001.pdf
https://www.spe.org/industry/docs/PRMS-Guide-for-Non-Technical-Users-2007.pdf