New Insight into Sequence Stratigraphy and Its Geological Significance in the Yanchang Formation,Longdong Area,Ordos Basin
[Objective]The latest three-dimensional (3D) seismic data displays a large amount of strong reflection seismic event progradation in the Yanchang Formation;this is significantly different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution." This difference provides a new perspective for under-standing the stratigraphic framework of the Yanchang Formation.[Methods]The seismic facies,sequence stratigra-phy,and sedimentary system in the Longdong area of the Ordos Basin are systematically studied through core obser-vation,well seismic calibration,and 3D seismic interpretation combined with seismic inversion technology.[Results]The continuous seismic events of strong reflections in the Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along and across provenance are extremely different. The seismic profiles along provenance can be divided into three types:sub-parallel structures with medium amplitude and medium continuity,progradational reflections with strong amplitude and strong continuity,and disordered structures with low amplitude and low continuity,among which the progradational type are the most evident. The seismic pro-files across provenance can also be divided into three types:parallel structures with strong amplitude and strong con-tinuity,mound structure with medium-strong amplitude and strong continuity,and disordered structures with low am-plitude and low continuity. The lake basin evolution of the Yanchang Formation in the Longdong area with rapid lake transgression and slow lake regression fluctuating processes can be divided into a third-order sequence unit consisting of multi-stage transgressive-regressive (T-R) system tracts. The sequence units prograded and overlapped as they formed wedges toward the center of the lake,developing the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies."[Conclusion]This region differs from the traditional sedimentary model in three aspects. First,the mudstone marker layer has not changed,and the corresponding relationship be-tween individual wells has changed. Second,the extent of the lake in the Yanchang Formation has not changed;how-ever,the evolution of basin filling has changed. Third,the overall distribution of sandstone has not changed,whereas the connectivity between sand layers has changed.
sequence stratigraphytransgressive-regressive (T-R) system tractcondensed sectionprogradationYanchang FormationLongdong area
万方数据
维普
