THE STUDY OF ANNULAR DRAINAGE OF THE ANCIENT LAKE AND THE PROSPECTING FOR SUBTLE OIL POOLS IN CRETACEOUS SONGLIAO LACUSTRINE BASIN

The extent of Songliao lacustrine basin in Cretaceous time reached its maximum during the deposition of Qingshankou Formation,covering approx-imately 150,000km².the uplifting of lake bottom at Daqing,Yangda-chengzi,Fuyu,and Suileng separated the basin into two parts:the east depression and the west depression.The rivers on the northern margin and the western margin of the lacustrine basin all entered the ancient lake,forming an annular drainage.the drainage pattern resulted in a coil-like distribution of the sand bars in the shallow and deeper parts of the lake,which after diagenetic compaction,formed anticlinal features of low elevation.As source rocks generally occurred in the central parts of the drainage system,these bars became favourable sites for the formation of subtle oil pools.