ANALYSIS OF SEQUENCE STRATIGRAPHY OF THE 3RD MEMBER OF SHAHEJIE FORMATION IN THE BOXING SUB-DEPRESSION,JIYANG DEPRESSION

The authors carried out the analysis of sequence stratigraphy of the 3rd member of Shahejie Formation in western Boxing sub-depression with high-resolution sequence stratigraphy with base level as the reference surface.Five sequence boundaries and four flooding surfaces are recognized in the 3rd member of Shahejie Formation. According to these surfaces,the member is divided into four long-term base level cycles. Moreover,there are eight medium-scale base level cycles through recognizing inner turnaround surfaces of the long-term base level cycles. High-resolution sequence stratigraphic framework is constructed through correlations among wells and seismic profiles. Characters and depositional evolutions of each sequence are analyzed in the sequence framework. Based on the above analysis,sequence stratigraphic models for braided river delta to turbidite fan are constructed.Braided river deltas and turbidite fans have the upgrading characters; the former are distributed in slope areas and the latter are generally distributed in depression areas.Braided river deltas are concentrated in the falling hemi-cycles of base level and turbidite fans in the rising hemi-cycles.The scales of turbidite fans are relative to lake flooding surfaces. We draw a conclusion that turbidite fan is the main type of reservoir to lithologic traps.The favorite members to lithologic traps are the rising hemi-cycles of MSC5,MSC6,MSC7 and MSC8;and the favorite areas to lithologic traps are the depressions at the break of the south slope of Boxing sub-depression.