Abstract:

 Lack of uniform sequence classification and welltoseismic sequence integration led to inaccuracy of depositional filling characterization and restricted forecasting reservoircaprock assemblages and oilgas distribution. Applying the concept of sedimentary sequence, using core, lithology, log and seismic data, and under the guidance of the principle of combining sequence boundaries with internal features, the author carried out the sequence division of the Paleogene strata in the east slope of Chengdao from the botlom to the top of the slope. The first order sequence is equivalent to a system strata unit, corresponding to tectonic movement, and the sequence boundary is a regional angular unconformity; the second order sequence is equivalent to a series or formation strata unit, corresponding to a tectonic phase in the tectonic movement, and the sequence boundary is a partial unconformity; the third order sequence is equivalent to a member or submember strata unit, corresponding to the secondary tectonic activity during the tectonic phase, and the sequence boundary includes unconformity, overlapping unconformity, transform surfaces of depositional systems and etched surface of gravity flow; the fourth order sequence(system tract)is equivalent to an oil set or a sand set, corresponding to third order sequence base level changes, and the sequence boundary is the first flooding surface and the maximum flooding surface. Six sequences, the SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6, and 15 system tracts were identified. The wellseismic sequence stratigraphic framework, which combined the high vertical resolution of drilling data with the high lateral continuity of seismic data, could reveal the filling and evolution characteristics of sequence stratigraphic framework with highprecision and highresolution. And it could show the cyclicity of sequences and the distributional difference between the system tract and the sedimentary facies. The following events occurred in the east slope of Chengdao in the Paleogene: the planation of uplift; the padding of the slope and depression; the development of overlapping and draping formations; the change of paleotopography from the south being low to the north being low; the change of local source, neighbor source and regional source; and the stacking of multisource deposition systems. Along with these events, the depositional systems changed as follows: fan delta, turbidite fan, semideep lake,  slump turbidite fans, braided river delta, and meandering river floodplain. The sequence stratigraphy and depositional filling characteristics of lowrise slope belt in Paleogene Chengdao are typical for continental rift basin; thus, this study may give some help in solving the problem of insufficient application of sequence stratigraphic classification to oil industry and in forecasting reservoircaprock assemblages and oil distribution.

Key words:  continental rift basin, lowrise, Chengdao slope belt, Paleogene, sequence stratigraphic classification, depositional filling