Lower Cretaceous in Chagan Depression developed mainly volcanic and clasticrock reservoirs.The clasticrock reservoirs are characterized by low maturities of composition and texture

high contents of rock debris and filling materials

multi-compositon of filling materials and multi-type cemention.The selective soultion of carbonate cement and compacting of volcanic rocks resulted in the development of secondary pores

thus forming available reservoirs in some sections of low porous and low permeable clastic rocks.Volcanic rock reservoirs are mainly formed by overflow basalt

incompletely filled pores

fractures and holes in it and they are favourable reservoir spaces.The distribution of oil layers in the reservoirs is controlled by the sandbody distribution of favourable facies zones and the sections where secondary pore developed.At the same time

high quality oil is an important reason for oil accumulation in reservoirs with poor reservoir physical properties.