Tectonoclimatic controls on Paleogene braided river delta evolution and petroleum systems in the Lenghu tectonic belt, northern Qaidam Basin

The Paleogene braided river delta deposits in the Lenghu tectonic belt of the northern Qaidam Basin document an environmental transition critical for understanding regional petroleum systems. Through integration of petrographic (thin-section and heavy mineral analyses), geochemical (trace and rare earth element analyses), and geophysical (well logging and 2D seismic interpretation) methods, this study investigates paleoclimate, paleosalinity, paleoredox conditions, provenance, and paleosedimentary evolution. A transition from humid, freshwater, oxic to semiarid-induced brackish, suboxic conditions is revealed, characterized by increasing salinity, depth, and reducibility, particularly during the deposition of the Xiaganchaigou Formation. The Saishiteng Shan is identified as the primary sediment source, reflecting a continental island-arc setting and supplying intermediate basement igneous rocks that influence reservoir quality. Tectonoclimatically controlled depositional facies shift from braided rivers to lacustrine deltas, characterized by thin-bedded discrete sand bodies and low sand-to-mud ratios. These integrated factors highlight the critical role of tectonoclimatic coupling, aridification-enhanced evaporite seals, and intermediate igneous provenance-driven secondary porosity development in controlling reservoir architecture, source rock potential, and petroleum system evolution within the Qaidam Basin and analogous lacustrine settings.