Architectural Evolution of Sandbodies in an Inland Depression Lacustrine Delta and Response to Base‐Level Fluctuations

ABSTRACT The Upper Triassic Yanchang Formation in the southeastern Ordos Basin is a depression‐type lacustrine delta system, where strong heterogeneity of delta‐front sandbodies hinders hydrocarbon development. We investigated base‐level controls on sandbody architecture and developed a quantitative model for tight oil reservoirs. Using core, log, and outcrop data, we established a chronostratigraphic framework of 1.5 long‐term and 8 medium‐term base‐level cycles. The results show systematic sandbody evolution: during early base‐level rise, amalgamated multi‐storey channel sands developed; during mid‐to‐late rise, single‐storey channel sands interbedded with mudstones formed; at peak rise, thick mudstones with thin sandstones accumulated; during the falling stage, sheet‐bar sands developed; and during the rapid fall to early rise, mouth bars were widely distributed. Multiple linear regression ( R 2 = 0.398, N = 808, p < 0.001) confirms base‐level change as the primary control on sandbody differentiation. We propose a predictive model for sandbody distribution in such delta fronts, providing a robust framework for exploration in the Ordos Basin and analogous basins worldwide.