A source rock refers to a sequence of mudstone (or shale) interbedded with permeable strata, which may include sandstone with favorable porosity and permeability, or carbonate rocks exhibiting similar properties, such as marl, algal limestone, and dolomite. This chapter analyzes the forces driving the primary migration of petroleum within the lithological assemblages of source rocks, identifying the key mechanisms responsible for this process. A mathematical model is developed to describe petroleum’s primary migration, alongside a quantitative method for calculating the effective oil expulsion zone thickness within source rocks. By comparing oil expulsion efficiencies across different sandstone-mudstone assemblages, the primary factors controlling petroleum’s primary migration in source rocks are clarified. This research offers valuable insights into the mechanisms of petroleum generation and accumulation in source rocks and provides a robust basis for objectively assessing petroleum resources in subsurface geological environments.