Abstract: The dual structure of the complex fault depression in Songliao Basin and its geologic historic construction determined the existence of two sets of petroleum systems and their differences. The deep fault-depression group developed during Late Jurrasic to Early Cretaceous and the particularity of the constructions of its inside sedimentary rock and volcanic rock determined the particularity of the natural gas enrichment conditions. The large thick limnetic facies in Shahezi Formation provided abundant organics for the formation of coal gas. The large-scale volcanic activities during fault-depression period and the strong geothermal field resulted from the huge thick and stable sediment during depression period sped up the transformation of hydrocarbon source rock and resulted in the formation of large amount of abiogenetic gas. The volcanic eruption within several ten thousands square kilometers during the age of Yingcheng Formation led to the formation of the favorable facies belt with the stacked explosive facies and effusive facies, and the medium-acidic volcanic reservoir with pores and fractures developed. The relative independent volcanic rock resulted in the formation of the special trap type under the huge thick seal formation. The early-formed structure provided the trapping conditions for the formation of the primary gas pool. The late structural movement destroyed the primary gas pool, and the secondary gas pool developed. Meanwhile, the deep large fault was connected with the deep inorganic gas sources, as a result, the special CO2 gas reservoir was formed.

Key words: Songliao Basin, gas-bearing system, limnetic facies, volcanic reservoir, special trap type, CO2 gas reservoir