Controls of Base‐Level Cycles on the Sedimentary Architecture of Subaqueous Distributary Channels in a Lacustrine Delta: A Case Study From the Chang 8 Member, Ordos Basin

Base‐level cycles serve as a crucial reference for the stratigraphic sedimentology. In lacustrine basins, high‐frequency base‐level changes contribute to significant diversity and complexity in sequences and sedimentary infilling. The sequences′ infill and facies distribution in lacustrine basins differ greatly from marine deposits. The subaqueous distributary channels are well‐developed lacustrine deltas and serve as important hydrocarbon reservoirs. However, how high‐frequency base‐level cycles control the sedimentary architecture evolution of subaqueous distributary channels in lacustrine basins still requires further investigation. This study investigates the sedimentary architecture of subaqueous distributary channel deposits in the Triassic Yanchang Formation, Baibao area, Ordos Basin. The primary objective is to analyze the constraining effects of base‐level cycles on these sedimentary bodies, which formed under the predominant influence of inertial forces within a lacustrine deltaic system. Based on INPEFA technology, one long‐term, two medium‐term, and eight short‐term base‐level cycles (SSC1–SSC8) were identified within the Chang 8 Member of the Yanchang Formation in the Baibao area. Core and logging analyses reveal that the sedimentary system within the Chang 8 Member is primarily a shallow‐water delta front facies, with four identifiable facies. Subaqueous distributary channels and mouth bars are sandstone‐dominated depositional elements. Within this framework, the MSC2 medium‐term cycle represents a period of lacustrine expansion, dominated by delta front and plain deposits. Conversely, the MSC1 cycle signifies a contraction phase, during which subaqueous distributary channel sandbodies became predominant within the delta front. The evolution of the distributary channels was controlled by short‐term base‐level cycles. The SSC7–SSC8 descending cycle, the channels evolved into isolated, narrow distributary channels. In contrast, the SSC4–SSC6 ascending cycle resulted in less‐connected isolated channels with reduced sandstone accumulation. Finally, during the SSC2–SSC3 descending cycle developed well‐connected channels that form the main sandstone deposits. This evolution demonstrates a clear link between cycle type and reservoir architecture. The observed sedimentary cycles exhibit significant and systematic variations in sandstone body architecture in different base‐level cycles. This study proposed a concept model for allocyclic base‐level controls on the deltaic sandstone. This model provides insights for predicting shallow‐water delta reservoirs in lacustrine basins with high‐frequency base‐level changes.