Kerogen in source rock at the high mature stage usually has a very low hydrogen index (HI), and its Ro value is often more than 2.0% or above, suggesting that it belongs to type I or type II organic matter, which usually contain less than 20 mg/g TOC, even 0 mg/g TOC. Therefore, kerogen of this kind is often regarded as sterile for gasgenerating. This paper analyzes kerogens from various mature stages of kerogens by using elemental analysis and pyrolysis simulation test, finding that gasgenerating potential is multiple of HI. For example, the sample with TOC of 12 mg/g and H/C atomic ratio of 0.46 still has a gas generating of 65 mg/gTOC, which accounts for 15% of total gas generation in a close system. This fact indicates that the HI of mature or overmatured orangic matters can not be used to evaluate gasgenerating potential. Meanwhile, gas generation of kerogen has a good correlation to H/C atomic ratio, with a correlation factor up to 0.96. Calculation by the regression function shows that the minimum H/C atomic ratio for a gasgenerating kerogen is 0.26.