Abstract: This paper focuses on the study of fluvial facies isochronous stratigraphic division based on a medium cycle flood surface. Under the guidance of the high resolution sequence stratigraphy theory, based on the flood surface of a middle period cycle, the stratigraphic division of river facies were divided and contrasted according to the elevation difference on the top of channel sand bodies in different periods. Taking the N₂m² oil group in the lower section of the Minghuazhen Formation in the north part of the Bohai Q oilfield as an example, the authors established a set of methods for fluvial facies isochronous stratigraphic division based on the sand body development frequency and elevation difference. According to the comprehensive analysis of seismic facies, logging facies,and sandstone development characteristics, the downcut position of the later channel was identified. As a basis of isochronous stratigraphic division, the original flooding mudstone interface at the end of the medium-term cycle was restored after backfilling the part that had been eroded. According to the frequency characteristics of sand body development, and elevation relationship between a sand body top surface and a medium cycle flood surface, the equal time interface recognition curves were constructed. These curves are used to characterize the concentrated development positon of mudstone and to identify the isochronous interfaces of short or ultra short cycles. Based on the above work, the isochronous stratigraphic division is realized. A case analysis shows that this method can idendify the interface positions between different stratigraphic elements effectively in dividing the N₂m² oil group into six isochronous units, and the sedimentary characteristics of river courses in six periods conform to the sedimentary evolution pattern of the middle term base level rising semi-cycle.

Key words: fluvial facies, isochronous interface, elevation difference, isochronous division