Sequence stratigraphic framework and model for the Palaeogene strata in the Chezhen depression

The Palaeogene strata in the Chezhen depression may be divided,on the basis of classical theory of sequence stratigraphy,into one first-order sequence(structural sequence set),three second-order sequences(structural sequence) and nine third-order sequences.The second-order sequences vary in time duration about 7.5 Ma,4.0 Ma and 13.4 Ma,respectively.As for the third-order sequences,the sequences 3 to 5 range in time duration from 1.0 to 1.5 Ma while the sequence 6 has a maximum time duration of about 5.2 Ma.Structurally,the Chezhen depression has gone through the early rifting stage represented by the extensional half-graben filling during the Sha-4 deposition,the intensely rifting stage represented by the intense faulting during the Sha-3 to the early Sha-2 deposition,and the rifting decline stage during the late Sha-2 to the Dongyingian deposition.Three models are presented for the Palaeogene sequences including the structural break type in the dry and evaporatic conditions during the early rifting stage,deep-water depositional break type during the intensely rifting stage,and shallow-water slope type during the rifting decline stage.The systems tracts,sediment filling and sandstone development vary greatly in individual sequence stratigraphic models.The results of research in this study are significant to the selection of petroleum exploration targets.