The isotopic composition of argon in natural gas could be derived possibly from the radiogenic, atmospheric, inherited, and mantle components. If the argon released from the country rocks to the oil-gas pools were derived dominantly from the sedimentary strata, then the amount of radiogenic argon accumulated is a function of time t. A curve of argon accumulated in the sedimentary rocks is shown in Fig. 1. If the degassing rate k of argon out of the country rocks is equivalent to the accumulation rate in the oil-gas field, it could be assumed that the accumulation amount of radiogenic argon at any time in the country rocks can be found from equation (5), and the amount of accumulated argon in gas field will be obtained from equation (6). Then, the amount of argon accumulated at any time t in the field can be calculated from equation (8). The estimation of the age of natural gas reported in this paper indicates that the primitive oil in the Zhongyuan oil-gas field was formed about 21.5 Ma ago.