Sequence Filling Models Under Differential Tectonic Uplift: A Case Study of the Ordovician in Tazhong Area

The Ordovician stratigraphic division and lithofacies paleogeography in the Tazhong area, Tarim Basin, are controversial. Conventional stratigraphic schemes lack well-seismically integrated constraints and fail to achieve isochronous correlation. Previous studies attributed the extensive stratigraphic absence in the inner platform to a single phase of tectonic uplift and erosion, which is inconsistent with actual seismic and geological evidence. To clarify the Ordovician sequence filling, sedimentary evolution and tectonic control under differential subsidence, and support hydrocarbon exploration, this study uses the latest drilling and 3D seismic data, adopts well-seismic integration, residual thickness paleogeomorphology restoration, and seismic facies-facies map method to carry out sequence stratigraphic division, stratigraphic filling reconstruction and lithofacies paleogeographic characterization. The results show that the Middle Ordovician in the study area can be divided into three third-order sequences: SQ1 (Member 1 of Yingshan Formation), SQ2 (Member 2 of Yingshan Formation) and SQ3 (Yijianfang Formation), with four key sequence boundaries (SB1–SB4) identified. Each sequence exhibits distinct onlap, progradation and pinch-out characteristics. The large-scale stratigraphic absence in the inner platform is not caused by a single tectonic uplift and erosion, but by sedimentary differentiation induced by stepwise differential tectonic uplift starting from the Yingshan Formation deposition. The initiation of the North Kunlun Ocean subduction is earlier than previously thought, dating back to the Member 2 of Yingshan Formation. The paleogeomorphology shows a persistent evolution of being high in the south and low in the north, high in the west and low in the east. The Middle Ordovician is featured by a carbonate ramp system with NW–SE trending banded grain shoals, evolving into a rimmed platform system in the Lianglitage Age. Multi-stage tectonic uplifts resulted in multi-stage eogenetic karst within the Middle Ordovician. High-energy grain shoal zones are highly coupled with karst modification areas, constituting high-quality reservoir zones. This study clarifies the co-evolution model of sequence filling, paleogeomorphology and sedimentary facies of the Ordovician in Tazhong under the south-compression and north-extension tectonic setting, providing key geological evidence for fine exploration of Ordovician carbonate reservoirs.