Sedimentary Evolution and Controlling Mechanisms of Fluvial–lacustrine Systems in the Triassic Halahatang Formation, Tahe Oilfield, Tarim Basin, China

The Triassic Halahatang Formation in the Tahe Oilfield, Tarim Basin, records a complex transition from fluvial to lacustrine depositional systems, yet its depositional architecture, evolutionary process, and controlling mechanisms remain insufficiently constrained. In this study, core observations, thin-section analysis, well-log interpretation, seismic attributes, heavy-mineral assemblages, palynological data, paleotopographic restoration, and sequence stratigraphic analysis were integrated to clarify sedimentary facies types, provenance directions, sandbody stacking patterns, and depositional evolution. The study area developed braided river, meandering river, and shore–shallow lacustrine systems. During deposition of the lower First Member, abundant sediment supply from the north, steep paleoslope, low base level, and strong hydrodynamic conditions promoted thick, vertically stacked braided-channel sandbodies. In the upper First Member, rising base level, reduced sediment supply, decreased paleoslope gradient, and climatic fluctuation drove a progressive transition to meandering rivers, forming thinner channel sandbodies with improved lateral continuity. During Second Member deposition, continued base-level rise and weakened fluvial energy resulted in widespread shore–shallow lacustrine deposits dominated by mud-rich sediments with locally developed sand bars and flats. The braided-to-meandering transition reflects tectonic–climatic coupling, controlled primarily by base-level rise, sediment-supply reduction, paleoslope decrease, and paleoclimate change. The proposed depositional evolution model provides an improved framework for predicting sandbody distribution and guiding hydrocarbon exploration in the Triassic clastic succession of the Tahe Oilfield.