Abstract: On the basis of the author's practice for gas research, a comprehensive theory of gas diffusion and oil genesis had been offered to solve the problems about geochemical hydrocarbon prospecting. In addition to the derivation of Fick first law equation, it is pointed out that the using of the equation is not enough to solve the problems about the theory of geochemical petroleum exploration. The age of oil and gas generation and the time from the start of HC gas diffusion to the earth surface must be considered. If the time from the start of diffusion to present days is equal to or larger than the time needed from the start of HC gas diffusion to the surface, HC gas wight reach the sureface, and good result of geochemical exploration in this area can be expected. This paper has calculated the cases in several oil and gas fields, such as Daqing and Shengli oil fields. It is demonstrated that good results may be expected in Daqing Area, while poorer results may be exhibited in Shengli Area. On the basis of the theory in this paper, it is predicted prospecting in Kuramay and Nanyang oil fields may be good. Two practical examples are given, one is in northeast-China, the other in northwest.