It is usually considered that diagenesis will end by and large as soon as hydrocarbon accumulates in reservoirs.In this paper

a case is presented in which diagenesis still continues after hydrocarbon accumulation.The reservoirs studied are sandstone with a CO

2

gas cap located in the western part of Dongying depression.Diagenesis mainly consists of cementation

replace-ment and solution before oil and gas are accumulated.The cement mineral contains mainly dolomite and calcite

and the latter is a soluble mineral.Cementation of the above dolomite and calcite stops while a large amount of dawsonite associated with kaolinite and primary quartz are produced.Poorly permeable sandstone reservoirs are generated because of the decrease of permea-bility caused by diminished porosity in sandstone filled by dawsonite.The generation of dawsonite is closely related to carbon dioxide.In gas-bearingformation

there is more CO

2

then the amount of dawsonite is large too

asmuch as 35%;in oil-bearing formation

less amount of CO

2

could be found

dawsonite is of less amount too

about 6-8%;in aqueous formation

the amount of CO

2

is the least

the amount of dawsonite is the least too.The poor physic 1 properties of the reservoir caused by diagenesis after the accumulation of oil and gas will not be favourable for the high yielding of oil and gas.However

the effect can be eliminated by acidization and hydraulic fracturing

and production can be increased because dawsonite is the aqueous carbonate.