Based on the macroscopic and microscopic analyses of mudstone caprocks sealing property of the first and second members of the Nenjiang Formation

the caprock is characterized by big thickness

extensive distribution.The caprock contains mainly micropores with low permeability and big specific surface area

thus forming uncompacted pore fluid superpressure and dense hydrocarbon sealing.Capillary sealing formed in the middle stage of the 3rd member of the Nenjiang Formation with the depth of 65 m;pressure and dense hydrocarbon sealing formed in late Mingshui Formation stage and the depth is 800 m.It is suggested that Qijia Gulong Sag belongs to grade-I sealing area with high capillary sealing ability

the northwestern part of Qiqihar belongs to grade-Ⅲ sealing area and the rest belong to grade-Ⅱ area.