  We show that a three-dimensional (3D) fully gapped type-II superconductor can
feature emergent in-gap Fermi surfaces of Caroli-de Gennes Matricon (CdGM)
quasiparticles in the presence of an Abrikosov vortex lattice. In particular,
these CdGM Fermi surfaces manifest in the emergent 3D band structure enabled by
the intervortex tunneling physics, and their stability is guaranteed by a
$\mathbb{Z}_2$ topological index. By developing an effective analytical theory,
we find that each vortex line carrying a 1D nodal dispersion is a sufficient
condition for the vortex lattice to form CdGM Fermi surfaces. Following this
prediction, in-gap CdGM Fermi surfaces are numerically confirmed in a
microscopic vortex-lattice simulation of a superconducting Dirac semimetal with
an $s$-wave spin-singlet pairing, which is directly applicable to a large class
of type-II superconductors such as LiFeAs. Remarkably, the CdGM Fermi surfaces
persist even when the normal state is deformed to a doped insulator of trivial
band topology. Our work establishes the vortex lattice as a new experimentally
feasible control knob for emergent topological phenomena in conventional
superconductors.
