Abstract: The source rocks of downfaulted period in Changling sag in Songliao basin are mainly found in Huoshiling formation, Shahezi formation and Yingcheng formation. Because of the effects of tectonic movement in fault-depressed conversion period and the Nenjiang movement, the evolution of the source rocks is of obvious stages in nature. By study on the relationship of present buried depth and source rock maturity of source rocks in Changling oil-gas field, the correlation equations of them in downfaulted and depression periods are established, respectively, conducting the recovery of denudation amount and the calculation of paleogeothermal gradient. The systematic study of the process of hydrocarbon-generation and evolution of source rocks in downfaulted period, it is considered that the source rocks are characterized by both early hydrocarbon generation and secondary hydrocarbon generation. Combined with the analysis of the trapforming time, it is believed that the oil and gas formed in different stages has the reservoir-forming selectivity. In early stage, the stratigraphic trap and lithologic trap may form reservoirs; when entering into high mature-overmature stage, the structural trap may form reservoir, while the source rocks entered into depression period and the secondary hydrocarbon generation stage, all traps in this basin could form reservoirs by the recharging of oil and gas.

Key words: Songliao basin, Changling sag, hydrocarbon-generation process, reservoir-forming trap, paleogeothermal gradient, denudation recovery, downfaulted strata