  The nucleon separation energies and shell gaps in nuclei over the whole
nuclear chart are systematically studied with eight global nuclear mass models.
For unmeasured neutron-rich and super-heavy regions, the uncertainty of the
predictions from these different mass models is still large. The latest version
(WS4) of the Weizs\"acker-Skyrme mass formula, in which the isospin dependence
of model parameters is introduced into the macroscopic-microscopic approach
inspired by the Skyrme energy-density functional, is found to be the most
accurate one in the descriptions of nuclear masses, separation energies and
shell gaps. Based on the predicted shell gaps in nuclei, the possible magic
numbers in super-heavy nuclei region are investigated. In addition to the shell
closures at $N=184, Z=114$, the sub-shell closures at around $N=178, Z=120$
could also play a role for the stability of super-heavy nuclei.
