Sealing ability of cap rock in large and medium gas fields mainly depends on the thickness of cap rock

displacement pressure

interior energy of gas reservoir(pressure coefficient)and gas property(flowing viscosity).Cap rock sealing index(CSI)of gas reservoir is directly proportional to gas accumulation efficiency.The accumulation efficiency of China's large and medium gas fields can be classified into three categories:(1)High efficiency gas reservoirs have accumulation efficiency of over 100?10

6

m

3

/(km

2

·Ma).They are mainly distributed in Tarim

Qaidam

Yinggehai-Qiongzhou

Bohai Bay and Songliao basins

of which Tarim

Qaidam and Yinggehai-Qiongzhou basins are in the majority.(2)Moderate efficiency gas reservoirs have accumulation efficiency in the range of 10?10

6

-100?10

6

m

3

/(km

2

·Ma).They are mainly distributed in Sichuan

East China Sea

Songliao

Bohai Bay

Tarim and Turpan-Hami basins

and most of them are in Sichuan basin.(3)Low efficiency gas reservoirs have accumulation efficiency of lower than 10?10

6

m

3

/(km

2

·Ma).They are mainly distributed in Ordos and Sichuan basins.For the formation of large and medium gas fields in China

the CSI value of cap rocks should be over 10

9

m/s.For gas fields with relatively higher accumulation efficiency

however

the CSI value of cap rocks should be raised even higher.