The Cretaceous system in Songliao Basin is composed of fluviolacus-trine deposits.Based on hydrodynamic conditions and characteristics offrame work sand body

it is appropriate to divide the sediments of the lakebasin into several major facies

i.e.diluvial

flood plain

distributary plain

delta front

near-shore shallow lake

moderately deep-deep lake and alluvialplain facies.Major target formations for oil and gas exploration are locatedin the distributary plain

delta front and near-shore shallow lake facies.The distributary plain facies may be subdivided into distributary river bed

crevasse fan

natural levee

and interdistributary depression subfacies.Their lithologic assemblage

oryctocoenose

stratification and the shape of sand body are different.The delta front facies is composed of river mouth bar

front sheet and subfluvial channel sands as a framework.According to therelation between the delta extension and the basin axis

the delta facies maybe classified into lobate delta complex and fan delta complex.The former hasa wide zone of fluviatile facies

and the latter has diluvial material flowe ddirectly into the lake.Individual deltas in the lobate delta complex havethe following three types:the lobate

the bird-foot and the sheet.They represent respectively the moderate lake regression

maximum lake regre-ssion and lake transgression periods.They developed in succession on profileand formed a complete delta cycle.After a systematic study of the delta in Songliao Basin

we have pointed out the difference between the sea-warddelta and the lake-ward delta.A near-shore shallow water facies among deltasmay be subdivided into underwater sand bar

shallow lake sand sheet

ostr-acoda or oolitic beach and mudflat subfacies.This is the facies zone wherebiogenic and chemical deposits are most developed in the lake basin

aparticular authigenic mineral—glauconite—is formed.