Progradational sequence stratigraphic architecture of the Triassic Yanchang Formation and a case study of Qingcheng Oilfield, Ordos Basin, NW China

To address the discrepancies between well and seismic data in stratigraphic correlation of the Triassic Yanchang Formation in the Ordos Basin, NW China, traditional stratigraphic classification schemes, the latest 3D seismic and drilling data, and reservoir sections are thoroughly investigated. Guided by the theory of sequence stratigraphy, the progradational sequence stratigraphic framework of the Yanchang Formation is systematically constructed to elucidate new deposition mechanisms in the depressed lacustrine basin, and it has been successfully applied to the exploration and development practices in the Qingcheng Oilfield. Key findings are obtained in three aspects. First, the seismic progradational reflections, marker tuff beds, and condensed sections of flooding surfaces in the Yanchang Formation are consistent and isochronous. Using flooding surface markers as a reference, a progradational sequence stratigraphic architecture is reconstructed for the middle-upper part of Yanchang Formation, and divided into seven clinoform units (CF1–CF7). Second, progradation predominantly occurs in semi-deep to deep lake environments, with the depositional center not always coinciding with the thickest strata. The lacustrine basin underwent an evolution of “oscillatory regression–progradational infilling- multi-phase superimposition”. Third, the case study of Qingcheng Oilfield reveals that the major pay zones consist of “isochronous but heterochronous” gravity-flow sandstone complexes. Guided by the progradational sequence stratigraphic architecture, horizontal well oil-layer penetration rates remain above 82%. The progradational sequence stratigraphic architecture and associated geological insights are more consistent with the sedimentary infilling mechanisms of large-scale continental depressed lacustrine basins and actual drilling results. The research results provide crucial theoretical and technical support for subsequent refined exploration and development of the Yanchang Formation, and are expected to offer a reference for research and production practice in similar continental lacustrine basins.