The evaluation of underground gas storage（UGS） tightness is a critical aspect of the construction and operation phases. The Liuzhuang UGS in the Subei Basin

characterized by a fault-controlled fault nose structure and edge water UGS

underwent a comprehensive evaluation of its sealing performance. This assessment focused on both cap rocks and faults

incorporating rock mechanics experiments and theoretical analysis. The evaluation analyzed the sealing strength of the cap rock from macroscopic

microscopic

and mechanical perspectives. It was determined that the thick cap rocks are primarily mudstone from the upper part of the second member and the lower part of the third member of the Funing Formation. These cap rocks are widespread throughout the area with stable regional distribution. Their lithology is favorable

containing over 30% clay minerals. The cap rock's mechanical properties

including a Young's modulus generally below 20 GPa and a brittleness index typically under 45%

contribute to its effective sealing capabilities. The predominant clay minerals in the cap rock are illite-montmorillonite mixed layers

which easily expand in water

enhancing the oil and gas seal effectiveness. Furthermore

the fault analysis focused on both lateral and vertical sealing capabilities. The Liu① fault

characterized by significant fault throw and steep dip angles

demonstrated robust vertical sealing obtained through the combination of the positive pressure method on the fault plane and regional experiences with overlying strata depths. The lateral seal of the fault was substantiated using both qualitative methods

like sand-mud docking

and quantitative assessments

including the shale smear factor. These methodologies confirmed that the Liuzhuang reservoir-controlling fault possesses strong lateral sealing properties.