The pore diameters of argillaceous caprock decreases as its buried depth increased.Only when the pore diameters are as small as or even smaller than the diameters of oil and gas moleculars

the argillaceous caprock could seal completely every states of hydrocarbon

and reach the sealing critical point.According to this rule

the sealing threshold depths of argillaceous caprock for both oil and gas could be calculated.Then the corresponding threshold time value could be calculated based on bed thickness and the rate of deposition.According to Φ

m

=54.74 exp(-0.000 935 4z)

K=0.136(Φ

e

)

4.4

/S

WB

)

2

and r=√8K/Φ

the critical threshold depth for sealing oil in Sanzhao Region

Songliao Basin should be 2 500-3 000 m

and the critical threshold depth for sealing gas should be 5 000-5 200 m.This does not inclode the sealings of capillary pressure

abnormal pore fluid pressure and dense oil and gas.