The Oligocene Sha-3 (3) submember in Machang region

Dongpu Depression is a large progressive deltaic deposit.It mainly underwent component-dissolution and dissolved-precipitation during the process of burial diagenesis.The first dissolution oc-curred prior to the peak period of hydrocarbon generation

and the dissolved pores areavailable spaces for hydrocarbon accumulation;the second dissolution happened after thepeak period and prior to the formation of cracking gas

the pores formed may have consid-erable significance for gas accumulation in deep and superdeep strata.