Seismic geological characteristics and exploration practice of progradation lithologic reservoirs in Yanchang Formation of the Ordos Basin

The upper and middle members of the Yanchang Formation in the Ordos Basin have developed multiple progradation bodies. After the deployment of high-precision 3D seismic, the seismic reflection characteristics of the progradation bodies were clearer, laying a good data foundation for the detailed study of the progradation bodies. This study was the first time to use the 3D seismic data to conduct a detailed analysis of the progradation body structure. Based on the clear understanding of the "three-stage" distribution pattern of the progradation body sand bodies, the sand bodies in the lower slope break zone of the progradation body were in contact with the source rock of the Chang 7 member at the bottom and transformed into mudstone at the top in the middle slope zone, thus having the conditions to form lithologic oil reservoirs. Based on this, the concept of "progradation body lithologic oil reservoirs" was proposed for the first time. This paper focused on the "progradation body lithologic oil reservoirs", through detailed progradation framework interpretation and reservoir prediction, it was clear that the sand bodies in the progradation body were distributed in a flower-like pattern along the slope break, and the multiple flower-like bodies were superimposed along the slope break of the Chang 7 member lake basin, resulting in a northwest to southeast strip-like distribution of the sand bodies on the plane, and the formation of multiple sand belts was controlled by multiple progradation bodies. Compared with the traditional equal-thickness division scheme of the Chang 6 tight oil reservoirs, the "progradation body lithologic oil reservoirs" model was more reasonable and more in line with the isochronous sedimentary geological laws. Recently, the oilfield company has deployed two risk wells which were parallel and perpendicular to the slope break direction for the "progradation body lithologic oil reservoirs". Currently, both well A and well B had achieved exploration breakthrough and obtained high-yield industrial oil flow during the test, confirming that the progradation body lithologic oil reservoirs had a high degree of oil enrichment and great exploration potential. The recognition and exploration breakthrough of the "progradation body lithologic oil reservoirs" were of great significance, changing the traditional understanding of the equal-thickness horizontal distribution of Chang 6 tight oil in the Ordos Basin and providing a new research idea for the exploration and development of tight oil in the Yanchang Formation and the discovery of reserves.