The boundary faults of basin are generally synsedimentary faults

which are characterized as having relatively short period of activity with intermittency and relatively long stratum deposition.The two factors together determine the stratigraphic distribution of fault depression basins.This paper analyzed the theoretical model of synsedimentary faults.Results show that the thickness of the gentle slope zones of the fault depression is equivalent to that of the footwall of the boundary fault

and at the footwall

the strata corresponding to the upper wall are usually absent.The thickness of strata in the gentle slope zone can replace that of the footwall of the boundary fault

and a virtual footwall thickness framework can be established

then the boundary faults can be studied quantitatively by means of displacement-distance graphs.Fulongquan fault depression locating in the southeast of Songliao Basin

a half-graben basin with faulting in the east and overlapping in the west

develops four boundary faults controlling four sub-sags in turn

and the activity intensity and inversion degree of these boundary faults are different.It has experienced three main structural movement

fault depression

depression

and inversion

corresponding to the three active phases of boundary faults

intense activity

inactivity

and inverting activity.The structural inverdion wich happened in the end of the Cretaceous

functions to reactivate the boundary faults and form hydrocarbon migration paths;hence the deep hydrocarbons migrated upward to the shallower traps

and the secondary hydrocarbon reserviors are thus produced.The quantitarive study about the inverted rate of each bondary fault

combined with the study of hydrocarbon accumulation patterns in Fulongquan fault depression

indicates that the areas with mrdium inverted rate are the most conducive in the generation and preservation of secondary hydrocarbon reservoirs.