We employ graphene saturable absorption for the theoretical demonstration of saturable absorption mirrors based on planar silicon photonic Bragg gratings. Two geometries are investigated, that of a silicon wire grating and a silicon slot grating, showcasing the increased light-matter interaction of the high-confinement slot waveguide. The gratings are designed in the linear regime for single mode operation, low footprint and broadband operation in the near infrared optical communications frequency range. The saturable absorption effect is introduced through the saturation of graphene's interband surface conductivity, and we discuss the necessary biasing conditions and applicability of our CW approach on ps-long pulses. We also rigorously include other, possibly detrimental, nonlinear effects, such as silicon's Kerr effect and two photon absorption, and graphene's Kerr effect. These effects are proven to have a negligible impact on the operation of the graphene saturable absorber mirror, thanks to the much lower power threshold of graphene's saturable absorption. Finally, we calculate the nonlinear reflectance and transmittance of the graphene-enhanced Bragg gratings and demonstrate that they can provide high modulation depths at low saturation powers, both highly valued characteristics of saturable absorber mirrors for mode locking applications in view of next-generation integrated pulsed photonic light sources.