AbstractWe perform numerical experiments of thermochemical mantle convection in 2‐D spherical annulus geometry to investigate the distribution of post‐perovskite (pPv) with respect to the location of primordial reservoirs of dense material in the lowermost mantle. High core‐mantle boundary temperatures lead to strong anticorrelation between the locations of pPv and large primordial reservoirs, while low values lead to a pPv layer fully covering the outer core. Intermediate values avoid a full pPv layer but allow pPv phase change to occur within the primordial reservoirs. Through interactions between cold downwellings and primordial reservoirs, low viscosity (weak) pPv leads to the formation of long‐lived, thin tails of primordial materials extending laterally at the edges of these reservoirs. Small patches of pPv also form within the primordial reservoir but are short‐lived. If primordial reservoirs are enriched in iron, these patches may provide an explanation for the ultralow‐velocity zones.