Abstract: The evolution history of gas diffusion velocity from overpressured sourcerock of Qing-shankou Group (K₁q_n) in Gulong sag has been studied based on the study of formation and evolution of the overpressure. The result shows that abnormal gas-bearing concentration of K₁q_n was formed in the early of K₁n₂ sedimentation period, then the gas began to diffuse upward and downward. Because abnormal gas-bearing concentration decreased to the smallest value, the gas diffusion was the weakest in the middle and late K₁n₄ sedimentation period and in the end of Quaternary sedimentation period. Therefore the gas diffusion of overpressured sourcerock of K₁q_n is divided into three evolution stages. The gas diffusion velocity increases gradually upward and downward in each stage, but the gas diffusion velocity downward is bigger than that upward. It is more favorable for gas in diffusion phase to accumulate in oil layer under K₁q_n than that above K₁q_n. This is the reason that gas accumulates much more in the oil layer under K₁q_n than that above K₁q_n.

Key words: Gulong sag, Qingshankou Group, overpressure sourcerock, gas diffusion