With the help of the data of the drilling and actually tested analyses, the differences in the hydrocarbon sealing of the mudstone cap rocks between Middle Esl and Ed2 of Dagang oilfield are studied. The achievements show that the differences of the sealing conditions between the two regional cap rocks are the main reason for the hydrocarbon enrichment beneath the mudstone cap rocks in Middle Es1 and Ed2. In detail, both the thickness and area of the former are bigger than the later, and furthermore more stronger microsealing action and combined sealing ability are available.