  Topological superconductivity is an exotic phase of matter in which the fully
gapped superconducting bulk hosts gapless Majorana surface states protected by
topology. Intercalation of copper, strontium or niobium between the quintuple
layers of the topological insulator Bi$_2$Se$_3$ increases the carrier density
and leads to superconductivity that is suggested to be topological. Here we
study the electronic structure of strontium-intercalated Bi$_2$Se$_3$ using
angle resolved photoemission spectroscopy (ARPES) and Shubnikov-de Haas (SdH)
oscillations. Despite the apparent low Hall number of $\sim2 \times 10
^{19}$cm$^{-3}$, we show that the Fermi surface is shaped as an open cylinder
with a larger carrier density of $\sim 10 ^{20}$cm$^{-3}$. We suggest that
superconductivity in intercalated Bi$_2$Se$_3$ emerges with the appearance of a
quasi-2D open Fermi surface.
