Architectural Evolution of Sandbodies in an Inland Depression Lacustrine Delta and Response to Base-Level FluctuationsOA

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.