Biyang depression is typical with good source rocks and reservoir sands in Paleogene.The first attempt was made to divide seismic sequences

analyse seismic facies and calculate interval velocities of the main reservoir rocks in this area.As a result

seismic sequences are reestablished

some new views of structure is proposed on the basis of the theory and the method of seismic stratigraphy.Based on synthesis of the information of amplitude

frequency

continuity

abundance and reflecting structure

the essential reservoir bed（E

2

h

2

3

）of this depression is divided into eight segments of seismic facies and then transformed into lithological zones.It has been found that the lower velocity abnormal belts varied horizontally in interval velocities are coincident with the known oil-bearing areas

while the higher velocity abnormal belts reflect the increasing content of dolomitic composition

in sedimentary rocks.Finally

the assessment of oil prospects of unknown areas has been proposed according to various data available.