The effect of faults in oil and gas reservoirs were examined based on the study of fault sealing and timing effectiveness and their influence factors. This was through comparison of the displacement pressure of faulted rock and of the underlying reservoir, the formation period of fault sealing and the high period of hydrocarbon generation and expulsion in source rocks. A set of study methods on the fault sealing effectiveness and fault sealing timing were built, and they were applied to study of the NP5-2 fault sealing effectiveness and timeliness in the Ed2 mudstone caprock of 9 lines in the Nanpu 5 structure of the Nanpu sag in Bohai Bay Basin. Apart from L6 and L7 where the NP5-2 fault sealing effectiveness is poor, the results indicate that the sealing is good which has favored for gas accumulation and preservation. The formation period of the sealing ability of NP5-2 fault is later than the second high period of hydrocarbon generation and expulsion of Es3 and Es1 source rocks. This meant the fault was not sealed at the right time and so gas from Es3 and Es1 leaked away. This matches with the situation where gas accumulates but not much under the Ed2 mudstone caprock in the 5th structure of the Nanpu sag. It is feasible for this method to study fault sealing effectiveness and timeliness.