Since the 1950s, hydrocarbon generation has been considered as one of the most important abnormal pressuring mechanisms. Hydrocarbon generation effects on pore-pressure evolution: (1) it increases the pore fluid volume through the conversion of solid kerogen into liquid or gaseous hydrocarbon, and (2) it decreases the sediment permeability because hydrocarbons remain an immiscible phase in pore water. In this paper, the two products of organic matter decomposition, liquid hydrocarbon (oil) and gaseous hydrocarbon (methane), are assumed to occur in sequence as a supplementary phase in pores as a basis for evaluating the effect of this pressuring mechanism. A simplified three-stage first-order kinetic reaction model is applied to simulate the generation of oil and gas. We use a numerical basin model with a two-phase hydrodynamic formulation to calculate the effect of organic maturation on geopressure evolution. Oil generation does not play an important role except when the organic matter content of rocks is relatively large (>5%); however, the generation of gas has a large influence. The results show that beneath a certain depth, oil-to-gas cracking strongly influences overpressuring. The effect of this mechanism becomes more important with increasing organic matter content of rocks. Several environmental conditions, such as kerogen type, temperature gradient, and lithological characteristics, influence to various degrees the effect of organic matter maturation on overpressure development.