Abstract: The cap rocks in the southeastern part of the Songliao Basin are dominated by argillite. The Lower Cretaceous Qingshankou Formation and its upper lacustrine-facies mudstone are good regional cap rocks in the northern part of the area. The upper mudstine of the Quansan(K₁g³) and Quaner(K₁g²) Members is not only the regional cover of the area, but also the critical cover of its underlying gas reservoirs. The breakthrough pressure of the cover is greater than the residual pressure of gas pools and the displacement pressure of reservoir rocks, and the gas-sealing height is greater than the height of gas pools. So, effective sealing is formed. The relationship between the sealing mechanism of cap rocks and the texture of micropores shows that the styles of the cap rocks are dominated by the compound style of microfissures and micropores.