The miscible systems, which are composed of hydrocarbon-water, other gases and mineral salts, are widely distributed in reservoir, with complicated phase transition attributes, and are also significant to the exploration and development of petroleum. After gathering domestic and international data about hydrocarbon-water miscible system and mainly analyzing conditions for occurrence of gaseous formation water and phases transition attributes of water bearing hydrocarbon system, this paper concludes that the regional sudden rising of temperature, reducing of pressure and increasing of hydrocarbon bearing content are all beneficial for the vaporization to the hydrocarbon-water miscible system. The current measured thickness of irreducible water is a fluctuated value, which is dependent on irreducible brine saturation and ineligible for low limit of reserve, and then, the present measured low limit of reserve can be decreased undoubtedly under suitable values of temperature and pressure and content of dissolved hydrocarbon, the actual stable thickness should be about 1.6 nm. The salting out effect, which is caused by vaporization of water, will reduce the permeability of oil/gas field and increase interfacial tension of oil-water, so, it is necessary to inspect the gaseous water change during production.