Generation/accumulation of hydrocarbon has been considered as one of the most plausible mechanisms related to abnormal geopressures observed in many conventional and unconventional reservoirs. In this presentation, we demonstrate that hydrocarbon compositions and phase behaviors of the petroleum fluid can be closely related to the observed abnormal geopressures (either overpressure or underpressure) phenomena. Vapor-Liquid Equilibriums (VLE) of various binary hydrocarbon mixtures as functions of temperature and pressure are determined using the Peneloux Soave-Redlich-Kwong (SRK) Equation of State (EoS) method. Our results indicate that (1) Existence of aromatic hydrocarbon compounds such as Benzene, Ethylbenzene, Toluene, and Xylene (BETX) could mix with hydrocarbon gases in porous spaces under typical offshore shallow well conditions. (2) Abnormal geopressures could occur when the hydrocarbon mixture undergoes a phase transformation acrossing the bubble or dew linee, i.e., when the vapor-liquid co-existance zone disappears. And (3) whether if a geopressure is overpressure or underpressure is determined from the critical temperature (Tc) of the petroleum fluid and the reservoir temperature T. When T > Tc, the hydrocarbon mixture will be in the supercritical phase with increasing pressure and an overpressure zone can be formed, whereas for T < Tc, all hydrocarbon will go into a condensed liquid phase to form an underpressure zone. Our interpretation can be applied to both underpressure and overpressure theoretical models. Accurate knowledge of pore pressure is fundamental to any safe and economic well construction. Various interpretation/prediction models have been developed to incorporate geopressure into the reservoir and basin modeling. This work intends to demonstrate that the phase behavior of hydrocarbons plays an important role in pore pressures.