Abstract: Caprock sealing effectiveness refers to the time of oil and gas cap closure forming period and matching relationships among a large amount of hydrocarbon expulsion of source rock phase，if the caprock sealing ability formed earlier than the same period or phase and a large amount of hydrocarbon expulsion of source rock, can be closed living source rocks from a large number of oil and gas, oil-gas sealing is effective on time; on the contrary, if the caprock sealing ability formed later than a large amount of hydrocarbon expulsion of source rock, closed the caprock can live amount of oil and gas mainly depends on the size of the time difference between them, the time difference is smaller, caprock can closed living relative amount of oil and gas, the greater the sealing effectiveness of oil and gas on time is relative, the better; the opposite relatively worse. Thus it can be seen that to study the effectiveness of the cap rock oil-gas sealing time must first determine caprock sealing ability formation and the large amount of hydrocarbon expulsion of source rock.