Three petroleum systoms are divided according to high potential surface caprock combination in Songliao Basin.This division is lags behind the present exploration of the basin.So it's necessary to subdivide each petroleum system into more favourable petroleum units.The so called quasi-petroliferous system is actually a kind of hydrocarbon accumulation unit that between the petroliferous system and hydrocarbon pool-forming assemblage.The core of the oil accumulation unit is that it has similar static pool-forming condition such as oil generation-reservoir-caprock and dynamic process of oil migration and accumulation.The middle petroliferous system of Songliao Basin is dominated by oil and gas upward expulsion and lateral migration.This system could be subdivided into three quasi-petroliferous systems according to different hydrocarbon accumulation ways:Daqing Placanticline quasi-petroliferous system of confluent type;Taikang-Yingtai quasi-petroliferous system dominated by lateral migration and accumulation;the south quasi-petroliferous system of divergent accumulation.The lower petroliferous system of the basin is dominated by downward expulsion and vertical migration

and could be subdivided into 4 types of quasi-petroliferous systems:Sanzhao

Qijia

Gulong

Changling subsystems of detained and divergent accumulation;Daqing Placanitcline and Fuxin-Chaoyanggou subsystems of confluent accumulation.