Reconsideration of Division and Correlation of the Yanchang Formation in the Southwestern Ordos Basin
[Objective]The Yanchang Formation comprises the most important source rock and reservoirs in the Or-dos Basin.Recent 3D seismic exploration shows that progradational reflection is widely developed in the delta-deep-water transition zone of the Yanchang Formation,which means that the previous"thousand-layer pie-like"and near equal-thickness stratigraphic division and correlation scheme would cause stratigraphic diachronistic correlation in clinoform development area.The aims of this study are to explore the establishment of isochronal division and correla-tion of the clinoform pattern for the Yanchang Formation,and to explore its connection and correlation with the tradi-tional stratigraphic division scheme.[Methods]Based on the 3D seismic data,seismic profiles in the study area are carefully interpreted to obtain the shape type,migration superposition relationship and internal structure of the clino-them,guided by the theories of sequence stratigraphy and sedimentology.Well-seismic mutual calibration(i.e.,drill-ing calibration of seismic horizon,seismic calibration of horizontal correlation of drilling profile strata),and con-strained by core facies marks,a new isochronous stratigraphic division and correlation scheme is established for the mid-upper Yanchang Formation:divided into six stages of clinothems(F1-F6)in Qingcheng,Heshui area,and five stages of clinothems(F1-F5)in the Huanxian area.In the topsets,these correspond to the traditionally stratified Chang 7-Chang 1 oil layers.[Results and Discussions]The different stratigraphic divisions are mainly reflected in the foreset and bottomset areas.In the foresets,the strata in each clinothem are generally thicker than suggested by the traditional schemes.In the deep-water area,the newly determined F2 and F3 strata are significantly thinner than the traditional values,while the thicknesses of F5 and F6 are dramatically increased.In plan view,the isochronous stratigraphic units of each clinothem show a NW-SE zonation.In the study area from SW to NE there is an alternating thin-thick-thin NW-SE zonal distribution pattern.Within the clinoform pattern there are isochronous and intrinsic ge-netic relationships of the tracking and correlation relationships between stratigraphic division and sandstone bodies.The connectivity relationship of the sandstone bodies approximates more closely to the actual situation of underground reservoirs,which is conducive to a reasonable well pattern of oilfield development layout,and the determination of water injection horizon.The foresets indicated by the thicker strata continue to advance towards the basin depocenter,indicating the contraction and filling process of the lacustrine basin.[Conclusions]The"natural extension tracking of the topsets,marker bed constraint,and 3-D space closure test"establishes an isochronous correlation in mid-upper Yanchang Formation in the progradational development area in the SW Ordos Basin.It provides significant im-plication for re-understanding the stratigraphic division and correlation in the delta-deepwater areas of the Yanchang Formation,and also provides new insights for a greater understanding of the connectivity and spatial distribution of sandstone bodies,as well as the sedimentary filling process of the basin.
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