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High-quality reservoirs occur in the middle-shallow horizons of the northern Songliao Basin.The distribution and genesis of their anomalously high porosity zones were studied using measured porosities, examination of ordinary thin sections and blue epoxy resin-impregnated thin sections and by SEM. The results show that there are three anomalously high porosity zones at the depth of 450-900 m, 1,300-1,900 m, 2,050-2,350 m respectively, named zone i, zone ii and zone iii from top to bottom. Horizontally, zone i and zone ii are distributed all over the basin except in the Southeastern Uplift, while zone iii is only distributed in the Central Depression. Zone i was formed by meteoric water leaching as well as organic acid dissolution. Zones ii and iii were dominantly formed by organic acid dissolution. Additionally, clay mineral transformation generating H⁺ and hydrocarbon emplacement retarding the cementation of sandstones are also important for the formation of zones ii and iii.