Exploration �� Development ��esearch Institute of Daqing Oilfield Company Ltd�� ��Daqing 163712��Heilongjiang��China

Beier depression is one of the most promising oil-bearing areas in Hailaer Basin with abundance of oil and gas resources�� However��the reservoir forming conditions were complicated due to multi-period tectonic structural transformation�� ��esearches suggested that Beier depression had experienced three-phases in basin forma- tion��and two-period tectonic transformations (by end of the Nantun Formation and by end of Yimin Formation)�� which played key roles in the process of hydrocarbon accumulation�� The source rocks controlled the distribution of oil and gas�� Vertically��there were three sets of reservoir-seal assemblages��while Nantun Formation��with its inher- ent hydrocarbon generation and accumulation conditions��was the major formation for exploration�� There were three hydrocarbon accumulation models��the gently sloping antithetic fault bench��the uplift zone between duplex half- graben fault depressions��and small deep sub-sags which provided good preservation conditions�� Understandings on the controlling factors and rules of hydrocarbon accumulation provided favorable exploration targets of the south of Beixi slape belt the south of Sudeerte structural belt and sags of Beidong zone��