Both simulation experiment on source rocks performed under watered

heated and pressured conditions and measurement of natural evolution sections show that thermal evolution of organic matter abundance in various marine source rocks can be divided into 3 stages

i.e. immature-low mature stage

mature stage and high mature-overmature stage. In immature-low mature stage (R

0

<

0.8%)

TOC and hydrocarbon generating potential change insignificantly with increasing maturity. In mid-late mature stage (R

0

= 0.8%-1.3%)

TOC and hydrocarbon generating potential decrease progressively with increasing maturity

simply due to the persistent generation and expulsion of large amount of hydrocarbons. While in high mature-overmature stage

TOC varies insignificantly again with increasing maturity. The abundance of organic matter lowers significantly from immature-low mature stage to high mature stage

thus the original TOC should be restituted. Comparison of different types of source rock shows that the restitution coefficient is related to hydrocarbon-generating and-expulsing potentials

kerogen types

maturity and TOC

while it has little correlation with lithologies.