Thin sandstone and mudstone interbed

deposited in lacustrine river-deltaic enviroment

is the necessary geologic condition of lithologic pool formation in the large

shallow-middle seated depressions

Songliao basin. Since the sedimentary accommodation

caused by the changes of base level

has a great difference in geologic time

the types of sand body have vertically a certain difference. Because the buried depth of reservoir rocks and relative position of reservoir rocks with source rocks are various

the formation condition and distribution of lithologic pools have also a certain difference in different plays. Fault plays an important role in the formation of lithologic pools

which is not only the basic attribute in the formation of composite fault-lithologic trap but also the major migration path of oil and gas into lithologic pools. In the areal extent

types of lithologic pool are controlled by structure position and sand body development. The lithologic pools are mainly developed in the delta-frontal facies belt in the large depressions

whereas the subtle oil and gas pools are mainly distributed in the periphery of the depressions.