Volumetric share of petroleum fluids saturating porous reservoir:
s_w + s_o +s_g = 1
where
s_w = \frac{V_w}{V_\phi}
volumetric share of water
s_o = \frac{V_o}{V_\phi}
volumetric share of oil
s_g = \frac{V_g}{V_\phi}
volumetric share of gas