Abstract: A series of thin-layer mudstone caprock reservoirs have been found in the last two years in the south-central Liaodong Bay,Bohai offshore area.This paper comprehensively summarizes the formation mechanisms and characteristics of thin-layer mudstone caprock,and then analyzes the relationship between thin-layer mudstone caprock and hydrocarbon accumulation.Through systemic research,the following important conclusions are obtained.Under favorable trap shape condition,thin-layer mudstone caprocks have good sealing effect.Mudstone quality has obvious effect on reservoir height.But mudstone thickness has no relation with reservoir height.Reservoir height raises great along with the better trap shape under the similar sealing condition.The result of this research can broaden exploration thought for the future: In reservoir development area,large and medium oil and gas fields can be found as long as traps are in good condition.