油藏物理学一油藏烃类的相态 because oil contains gas and formation pressure continuously reduce during the development of reservoir,gas will maybe separate from oil. Hydrocarbon will translate into two-phase,that is oil and gas.So the composition will be transformed Reasons: internal cause:composition; external cause T and P油藏物理学——油藏烃类的相态 because oil contains gas and formation pressure continuously reduce during the development of reservoir, gas will maybe separate from oil. Hydrocarbon will translate into two-phase, that is oil and gas. So the composition will be transformed. Reasons: internal cause：composition； external cause ：T and P