The fracture caved gas reservoirs of Lower Permian in Sichuan Basin are a kind of dense carbonate monotonic fracture caved reservoirs.The total subsurface space volume of the reservoirs is not large

the formation water body is limited and no areal continuous water exists. The elastic drive energy of formation water in the reservoirs would be negligible

formation water outflow is the result of gas elastic drive rather than the intrusion of edge water or bottom water. On this base

the water-gas reservoir of monotonic fracture cave type could be considered as a whole

and the reservoirs space would remain unchanged.Thus the reserve calculating equation of the reservoir could be deduced:G=(W

p

/B

g

)+G

p

/ P

i

/Z

i

-P/Z/ P

i

/Z

i

.As for those produce water only

this equation could also be used to calculate the gas reserve above formation water after a period of water outflow.