  We report thermoelectric studies of FeS$_{1-x}$Se$_x$ ($x$ = 0, 0.06)
superconducting single crystals that feature high irreversibility fields and
critical current density $J_c$ comparable to materials with much higher
superconducting critical temperatures ($T_c$'s). The ratio of $T_c$ to the
Fermi temperature $T_F$ is very small indicating weak electronic correlations.
With a slight selenium substitution on sulfur site in FeS both $T_c$/$T_F$ and
the effective mass $m^*$ rise considerably, implying increase in electronic
correlation of the bulk conducting states. The first-principle calculations
show rise of the density of states at the Fermi level in
FeS$_{0.94}$Se$_{0.06}$ when compared to FeS which is related not only to Fe
but also to chalcogen-derived electronic states.
