Abstract: In the southwestern margin of Ordos Basin, progradation seismic reflection features of Yanchang Formation are extensively observed in 3D seismic profile. By comprehensively using geological data such as well drilling, logging, and core section, well–seismic data has been combined to conduct stratigraphic correlation and division, and analyze the sedimentary evolution of progradation layer, sand body distribution, and oil and gas distribution laws. The following understanding has been obtained: (1) Multi-stage progradation reflection features are observed in the seventh–third members of Yanchang Formation in the southwestern basin margin, which generally reflect the progradation characteristics of water regression and sand progradation. The deposition units are stacked in a tangential oblique pattern towards the lake basin center. After reconstructing a stratigraphic framework of Yanchang Formation, the traditional method of “isopachous” stratigraphic correlation has been changed; (2) The distribution characteristics of the two types of sand bodies in the southwestern basin margin were mainly controlled by the basin bottom shape. The comprehensive study of logging, seismic, and sedimentary facies shows that the upper and lower sections of the progradation layer in Yanchang Formation respectively controlled the distribution of delta front and gravity flow sand bodies, among which the gravity flow sand bodies are favorable reservoir types for large-scale exploration and development; (3) Based on the “two-wide and one-high” seismic information, the distribution of multi-stage deep-water gravity flow sand bodies in the progradation layer has been finely characterized, effectively identifying the distribution range of high-quality reservoirs, and determining that the slope foot sand-rich belt is a favorable area for increasing tight oil reserves on a large scale. The reservoir prediction method based on seismic progradation reflection structure analysis has important guiding significance for the exploration and development of tight oil with similar deep-water sedimentary backgrounds in China.

Key words: Ordos Basin; Yanchang Formation; progradation characteristics; gravity flow sand body; reservoir prediction