Abstract:

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.