The formation of heavy oil in a reservoir rock is influenced by many factors, including temperature, pressure, and the type and composition of hydrocarbons. Factors such as temperature and pressure can drive hydrocarbons to heavier weights, while hydrocarbon composition and reservoir rock type can also affect oil density and migration. Increased pressure can push hydrocarbons toward heavier molecules, thus accelerating the formation of heavy oil. The presence of heavier and carbon-rich hydrocarbons alone can contribute to the formation of heavy oil. Some reservoir rocks, such as sandstone and limestone, are more suitable for heavy oil accumulation due to their permeability and ability to pass heavier hydrocarbons. The processes of primary and secondary migration of hydrocarbons, in which oil and gas move from the source rock to the reservoir rock and accumulate there, play an important role in the formation of heavy oil too. In this paper, we will discuss about factors related to the formation of heavy oil in a reservoir rock.