A new quantitative method for evaluating fault sealing properties based on fault connectivity probability is proposed by taking the occurrence of oil and gas in footwall and hangingwall reservoirs of hydrocarbon controlling faults in Chengbei falut step zone of Dagang oilfield as the index of identifying the sealing properties of faults and by taking into consideration many factors that have influence over the properties.Three parameters

i.e.fluid pressure of mudstone

normal stress on fault plane

and clay smear

are regarded as the major factors that influence the sealing properties of faults

and a dimensionless sealing coefficient is defined according to the mechanisms of their influence over the sealing properties of faults.Then typical reservoir profiles crossing faults are selected to calculate sealing coefficients on different parts of the profile and the sealing properties of faults are determined according to the occurrence of oil in the footwall and hanging wall.Furthermore

statistical analysis of the relationship between the sealing coefficients and fault connectivity probability shows that when the coefficient is less than 1.0

the connectivity probability is naught;when the coefficient ranges from 1.0 to 3.5

the relationship between them can be expressed by a quadratic equation;and when the coefficient is more than 3.5

the connectivity probability equals 100%.The connectivity probability contour on a strike projection reveals the sealing properties of different parts on the fault plane.