In this paper

we chose several parameters that are closely related with sealing capacity of cap rock

such as cap rock thickness

displacement pressure

pressure coefficient

damage of fault to cap rock and vertical sealing ability.In combination with the concept of sealing effectiveness of cap rock

we proposed a comprehensive evaluation method of cap rock sealing capability.Based on statistics of reserve abundance and cap rock attributes of large tight sandstone gasfields in China

we took the lower limit of each cap rock evaluation parameter necessary for the formation of large gasfield with high reserve abundance as its homogenization standard value.Case study of the regional cap rock of the 5th Member of Xujiahe Formation in Guang'an gas field shows that the cap rock thickness is 30-200 m

displacement pressure is 5.5-9.5 MPa

the pressure coefficient is 1.05-1.50

the fault activity is strong

vertical sealing capacity of the fault is poor

and the sealing effectiveness of cap rock is good.Calculation with the comprehensive evaluation formula obtains a composite sealing capacity evaluation index in the range of 0.04-0.24

indicating that the cap rock of the 5th Member of Xujiahe Formation is capable of sealing gas in reservoirs with ultra-low reserve abundance.The index is high in the west but low in the east as a whole.The play fairways predicted by this method basically coincide with the present exploration results of the 5

th

Member in the Xujiahe Formation.