New cognition and isochronous stratigraphic division scheme of the Yanchang Formation sedimentation in Ordos Basin
The annual oil production of the Triassic Yanchang Formation in the Ordos Basin exceeds 3500×104 t. However, the current division scheme of “marker layer constraints and equal-thickness splitting of oil formations” for this main production layer is challenged in the practice of oil and gas exploration and development. Using the dating data of tuffs in the Chang 7 interval of different sedimentary systems and the high-quality 3D seismic data volume newly collected in the Qingchengbei area of the basin, stratigraphic isotopic chronology studies and high-precision seismic stratigraphic interpretation under the constraints of drilling data were carried out respectively. The zircon dating results show that the Chang 7 interval in the southeast of the basin belongs to the Middle Triassic, and it is suggested that zircon ID-TIMS dating can achieve high-precision stratigraphic correlation of different sedimentary systems. At the same time, 11 prograding bodies were identified from the bottom of Chang 2 interval to the top of Chang 73 interval in Yanchang Formation in North Qingcheng 3D work area, and the evolution of six prograding bodies in Chang 7 interval was described. Aiming at the difficulty of isochronous division of continental strata in the Yanchang Formation, it is proposed to establish high-precision stratigraphic dating “anchor points” for the Chang 7 interval of different provenance systems, and use the elevated frequency seismic data to compare with well logging small layers to “match” and re-divide the high-frequency layers. The sequence “framework” was established, and the isochronous comparison scheme of the Yanchang Formation was established. This study is helpful for summarizing the oil distribution law of Yanchang Formation, and also provides a reference for the sedimentary evolution and stratigraphic isochronous comparison of other continental lacustrine basins.