In recent years, more and more researches have proved that fluids are not only in upper crust but also in lower crust and mantle, and they are mostly composed of H2O and CO2. Because of the particular pressure and temperature conditions in deep Earth, they are under supercritical state and show particular physical and chemical characters that are different from general fluids. They are important transportation media of matter and energy and play important roles in the evolution of Earth. And they also play important roles in the origin and migration of oil and gas. On the other hand, they are related to inorganic origin of oil and gas, and they also produce an important effect in enrichment and migration of petroleum.