Abstract: Basing on the theory of thermodynamics,the influence of formation pressure on the parameters of hydrocarbons, such as enthalpy, entropy and free enthalpy,and the influence of pressure on the energy distribution in accord with the basical formula of thermodynamics AH=△G-T△S are discussed. It is pointed out that pressure is a negative factor to the pyrolysis of hydrocarbons. The reason for why there is oil still retained in Paleozoic carbonate rocks of the Xiang-Zhong district is also discussed under thermodynamic theory. According to the theoretical consideration of the gasification of oil, as low boiling-point fractions in an oil pool increasingly gasified, the probability of the destruction of the oil pool also increased. Contrarily, there is no phase change within gas pools, only the expansion of gas under heating, hence the influence of temperature and pressure on gas pool is minimized. Therefore, it is easier for gas pools than for oil pools to be presrved.Finally, some cognitions for the prospecting and exploration of areas bearing highly evolved oil and gas are suggested.