  We present a study of entropy transport in Bi2Se3 at low temperatures and
high magnetic fields. In the zero-temperature limit, the magnitude of the
Seebeck coefficient quantitatively tracks the Fermi temperature of the 3D Fermi
surface at \Gamma-point as the carrier concentration changes by two orders of
magnitude (10$^{17}$ to 10$^{19}$cm$^{-3}$). In high magnetic fields, the
Nernst response displays giant quantum oscillations indicating that this
feature is not exclusive to compensated semi-metals. A comprehensive analysis
of the Landau Level spectrum firmly establishes a large $g$-factor in this
material and a substantial decrease of the Fermi energy with increasing
magnetic field across the quantum limit. Thus, the presence of bulk carriers
significantly affects the spectrum of the intensively debated surface states in
Bi2Se3 and related materials.
