This paper discusses the theoretical problems about the study of lithologic reservoirs through the application of hydrocarbon reservoir accumulation principle. It is considered that the structural factor of reservoir-forming is the buoyancy factor. The lithology factor is the one that influences reservoir pore structures. The forming process of hydrocarbon reservoirs is a continuous process of oil saturation increase in the volumetric reservoirs. The formation of any lithologic reservoirs is influenced by the structural factors to different extent. Hydrocarbon reservoirs are basically decided by the final formation of structures.