Sedimentary Characteristics and Evolution of the High-resolution Sequence Stratigraphic Framework of the Taiyuan Formation in the Hangjinqi Area, Northern Ordos Basin

Abstract

The Ordos Basin is a large superimposed hydrocarbon-bearing basin in China, and further research on the sedimentary characteristics and sedimentary evolution of the sequence framework of target layers is of great theoretical and practical significance for guiding oil and gas exploration. The sedimentary facies and sedimentary evolution of the high-resolution sequence framework of the Carboniferous Taiyuan Formation in the Hangjinqi area have been systematically analyzed for the first time by drilling, logging and seismic data. The results show that four types of sequence interfaces can be identified in the Taiyuan Formation: regional unconformity surfaces, scour surfaces, lithologic–lithofacies transformation surfaces and flooding surfaces. According to the sedimentary response caused by the upward and downward movements of the base level at different levels, the Taiyuan Formation can be divided into 2 long-term cycles (LSC1–LSC2), 4 mid-term cycles (MSC1–MSC4) and 7 short-term cycles (SSC1–SSC7). The long- and mid-term cycles correspond to members T₁ and T₂ and layers T_1-1, T_1-2, T_2-1, and T_2-2, respectively. Long-term cycles are dominated by C₁; mid-term cycles are dominated by C₁ and C₂, followed by A₂; and short-term cycles are dominated by C₁, C₂, A₁ and A₂. Under the high-resolution sequence stratigraphic framework, the Hangjinqi area underwent a transformation of fan delta and tidal flat depositional systems during the Taiyuan Formation sedimentary period. In the MSC1–MSC2 stage, owing to a large-scale paleocontinent, the fan delta sedimentary body, which was limited in scale and scope, developed only in the southeastern corner and gradually transitioned basinward to tidal flat facies. In the MSC3–MSC4 stage, as the paleocontinent continuously decreased and the sedimentary range expanded, fan-delta plain sedimentation began in the study area. Several braided distributary channels with poor connectivity developed on the fan-delta plain, and between them were floodplains and peat swamps.