The rock volume \Omega_R is split into three major components: effective pore volume \Omega_e, shale volume \Omega_{sh} and rock martix \Omega_m:
(1) | \Omega_R = \Omega_e +\Omega_{sh} + \Omega_m |
The usual practice is to use relative volumes:
(2) | \phi_e = \frac{\Omega_e}{\Omega_R}, \quad V_{sh} = \frac{\Omega_{sh}}{\Omega_R}, \quad V_m = \frac{\Omega_m}{\Omega_R} |
which are measured in V/V units (or fracs) and honor the following constraint:
(3) | \phi_e +V_{sh} + V_m = 1 |
The relative effective pore volume \phi_e contains free or connate fluids (water, oil , gas) and called effective porosity.
It corresponds to the air porosity of the dried laboratory cores: \phi_e = V_{\rm air \, core}.