Based on the base level cycle theory

two long-term base level cycles

i.e.C1(the 1st sub-member of 3rd member of Eogene Hetaoyuan Formation(Eh

3

1

)—the 3rd sub-member of 2nd member of Eogene Hetaoyuan Formation(Eh

2

3

)) and C2(Eh

2

2

-Eh

2

3

)

have been built up through the sequence stratigraphy analysis of 4 well-to-well correlation sections in the study area.Their boundaries might be a breakpoint where base level rises and falls. C1 cycle is vertically a sequence which water body shallows upwardly

and it can further be divided into two medium-term base level cycles

i.e.MSC1 and MSC2

corresponding to the Eh

3

1

and Eh

2

3

deposition. Each medium-term cycles are subdivided into smaller short-term cycles. C2 cycle is vertically not complete and is only composed of one half cycle with a fall in base level (equivalent to Eh

2

2

and Eh

2

1

depositional stages)

which can further be divided into four medium-term base level cycles

i.e.MSC3

MSC4

MSC5 and MSC6. On the basis of the established sequence stratigraphic framework

a more detailed sequence classification will be helpful to study the microfacies of dolomite area in Biyang sag

and to predict the facies belts favorable for development of fractures and dissolved pores closely related to oil and gas accumulation.