In Reserves Volumetric Calculations there are Two main Approaches (Deterministic and Probabilistic). And Mixed Approach.
In deterministic approach, all minimum values should be taken as Minimum (1P) Case and Vise versa for Most likely (2P) and Maximum (3P) Cases.
I.e. if we have the values of Area; 100, 150 and 250 Acre
And Porosity of 0.13, 0.15 and 0.19 fraction.
And So of 0.5, 0.7 and 0.9 fraction
So, the Low (Minimum or 1P) Case is to use the minimum values for each parameter 100 Acre in Area, 0.13 in porosity and 0.5 in Oil Saturation (So)