New insight into sequence stratigraphy and its geological significance of Yanchang Formation in Longdong area， Ordos Basin

Key words: 
• sequence stratigraphy /
• T-R system tract /
• condensed section /
• progradation /
• Yanchang Formation /
• Longdong area

Abstract: [Objective] The latest 3D seismic data displays a large number of strong reflections seismic events progradation in Yanchang Formation, which is quite different from the traditional stratigraphic scheme of "flat rise flat fall and equal thickness distribution". This difference provides a new perspective for re-understanding the stratigraphic framework of Yanchang Formation. [Methods] The seismic facies, sequence stratigraphy and sedimentary system in Longdong area of Ordos Basin are systematically studied through core observation, well seismic calibration and 3D seismic interpretation combined with seismic inversion technology. [Results] The continuous seismic events of strong reflections in Yanchang Formation can indicate the deposition of condensed layers during lake flooding. The seismic facies along provenance and across provenance are quite different. The seismic profiles along the 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 most obvious. The seismic profiles across the provenance can be divided into three types: parallel structures with strong amplitude and strong continuity, mound structure with medium-strong amplitude and strong continuity, disordered structures with low amplitude and low continuity. The lake basin evolution of Yanchang Formation in Longdong area with some 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 prograde and overlapped as the form of wedges toward the center of the lake, and develop the sedimentary sequence of "slope rich in mudstone and both top-set and bottom-set rich in sand bodies". [Conclusion] The conclusion of this paper is quite different from the traditional sedimentary model, which is manifested in three aspects. First, the mudstone marker layer has not changed and the corresponding relationship between individual wells has changed. Second, the extent of lake in Yanchang Formation has not changed, however, the evolution process of basin filling has changed. Third, the overall distribution of sandstone has not changed, while the connectivity between sand layers has changed.