  We present a basic, fast, and flexible planet formation model, called SimAb
(Simulating Abundances), to form giant planets and study their primary
atmospheric composition soon after their formation. In SimAb we introduce
parameters to simplify the assumptions about the complex physics involved in
the formation of a planet. This approach allows us to trace and understand the
influence of complex physical processes on the formed planets. We focus on the
C/O ratio and the metallicity of the planetary atmosphere as an indicator of
their compositions. We show that the initial protoplanet core mass does not
influence the final composition of the planetary atmosphere in the context of
our model. The initial orbital distance affects the C/O ratio due to the
different C/O ratios in the gas phase and the solid phase at different orbital
distances. Additionally, the initial orbital distance together with the amount
of accreted planetesimals cause the planet to have sub-solar or super-solar
metallicity. Furthermore, the C/O ratio is affected by the dust grain fraction
and the planetesimal fraction. Planets that accrete most of their heavy
elements through dust grains will have a C/O ratio close to the solar C/O
ratio, while planets that accrete most of their heavy elements from the
planetesimals in the disk will end up with a C/O ratio closer to the C/O ratio
in the solid phase of the disk. By using the C/O ratio and metallicity together
we can put a lower and upper boundary on the initial orbital distance where
super-solar metallicity planets are formed. We show that planetesimals are the
main source for reaching super-solar metallicity planets. On the other hand,
planets that mainly accrete dust grains will show a more solar composition.
Super-solar metallicity planets that initiate their formation farther than the
CO ice line have a C/O ratio closer to the solar value.
