Abstract:  As for the gasgenerating potential at the overmature stage, the authors used the RockEval rock pyrologger and TGMS hightemperature pyrogenating simulated experiment device to carry out a series of experiment and research. By using the coal and carbonaceous mudstone of Jurassic in Junggar Basin and outcrop coal sample of the Permian system and Carboniferous carbonaceous mudstone in Ordos Basin, we carried out the gasgenerating simulation experiment and the temperature is from 300℃ to 800℃. According to the result, all the rocks have generated a lot of gas when the temperature is lower than 600℃(that is Ro＜2.0%～2.5%). When increasing the experiment temperature from 600℃ to 800℃, which means to raise Ro from 2.0%～2.5% to 5.0%, we found that both the coal and the carbonaceous mudstone in the two Basin can generate a great amount of natural gas between 600～800℃. The gas generated from the carboniferous carbonaceous mudstone accounts for 25.38%, 34.39%. The gas generated from the two type source rocks at the overmature stage accounts for 20%～35% of the total generated gas volume. 
In an attempt to verify the above phenomenon, we have cooperated with Taiyuan Institute of Coal Chemistry Chinese Academy of Sciences to carry out the source rock thermogravimetricmassspectrometric thermal simulated experiment and have got very similar result.According to the result of thermogravimetricmassspectrometric experiment on the shale of sea facies and the mud stone of lake facies, the two samples of  Songliao Basin, the organic matter of Ⅱ2 residue gasgenerating when the temperature is between 600～800℃ accounts for 7% of the total generated gas volume or so and the organic matters of Ⅱ1 organic matter accounts for 5%. However, the gasgenerating volume is much smaller than the coalrelated source rock. There is a tendency in which the type of organic matter is worsening and the residue gas volume is reducing at high temperature, which is similar to that experiment result on the abovementioned pyrogenating gas generation rate. When the temperature is between 800～1000℃, the gasgenerating potential disappears, which is consistent with the experiment result of coalrelated source rock. 
Therefore, a large amount of natural gas can be generated at the overmature stage(R_o≥2.5)for coal and carbonaceous mudstone, and the volume can account for 20%,or even more. The low limit of  R_o extend to 5.0% . The generated gas volume stops increasing when the temperature is larger than 800℃, so the simulation temperature 800℃ will be enough for the gasgeneration reflection. 