Authigenic illites in sandstone reservoirs form in the presence of flowing K-rich pore water. The entrance of hydrocarbon fluids into sandstone reservoirs brings a halt to the illite-forming processes, and if the last-formed illite can be isolated and its age determined, the maximum age of hydrocarbon entrance can be constrained. K-Ar isotopic chronological analysis has shown that the ages of diagenetic illites in the Carboniferous sandstone reservoir of the Donghetang area and the Triassic condensate reservoir of the Jilake area are 24. 3-31. 8 Ma and 44. 3-49. 2 Ma respectively , which reflects that the former petroleum reservoir formed in the Oligocene, while the latter in the Eocene.