Abstract: Carbonate rock is dominated by cementation and metasomatism, in which little primary pore occur even at shallow depth. When entering perfect enclosed compacting fluid environment, the small-scale pressure solution could be formed due to deep burying and geothermal rise with faint diagenesis. After reaching the hydrocarbon generation threshold, low abundant organic matter may generate a certain amount of hydrocarbons, but carbonate rock may not dewater a lot like shale. The hydrocarbon migration in it is absent from both "carrier" and abnormal pressure as a drive force that creates fractures. So the hydrocarbon in it is difficult to expulse effectively. It may be dispersed in disconnected intercrystalline pores or form crystal-wrapped and inclusion organic matter combined with minerals. So hydrocarbon percentage in carbonate rock is commonly higher than shale. It is concluded that carbonate rock is almost impossible to be as effective source rock for its original low abundance of organic matter and post hydrocarbon-expulsion difficulty.

Key words: carbonate rock, shale, diagenesis, hydrocarbon expulsion, effective source rock