Analyses of data acquired in over 400 wells have reproduced the structures on the bottom boundary of regional caprock (Nenjiang Formation) in the critical period of hydrocarbon migration. Four kinds of ridge like structures

saddle structural lithological ridge

fault-lithological ridge

anticlinal-lithological ridge and lithological ridge

can be identified from the structural map. The spacial array of these ridge-like structures acted as the major hydrocarbon migration pathway system to the western slope zone of Songliao basin. Hydrocarbons from Qijia-Gulong depression would be gathered through this major brancing pathway system

and then migrated northeastward to Fulaerji and westward to Pingyang oilfield

respectively.