Arid climate disturbance and the development of salinized lacustrine oil shale in the Middle Jurassic Dameigou Formation, Qaidam Basin, northwestern China

The lacustrine Dameigou organic-rich shale of the northern Qaidam Basin is a particularly promising play for recent unconventional oil and gas exploration in China. Deposition was associated with arid intervals during otherwise humid Middle Jurassic, but depositional mechanism organic matter enrichment processes are poorly understood. This study integrates high-resolution inorganic geochemical proxies detailed sedimentary observations samples from Chaiye (CY1) borehole, located Yuqia Depression, to investigate environment development shales. Four major lithofacies, including sandy mudstones, shale, mudstone, argillaceous were identified through microscopic observation mineralogical analyses. Geochemical paleoclimate (CIA, Fe/Mn Mg/Ca) indicate that humidity fluctuations primarily drove lithofacies variation. Other proxies, terrigenous supply (Al2O3, Ti2O regular sterane), lake salinity (Sr/Ba, S/TOC Ga), redox conditions (V/Cr, U/Th Pr/Ph), primary productivity (Baxs, Pxs) climatic variability exerted first-order control on chemical evolution area. During intervals, fresh water precipitation river runoff weakened, leading an increase column stratification. As result, surface became brackish oxygen-rich, promoting growth algae, density stratification bottom waters anoxia, providing favorable preservation. Recycling nutrients such as phosphorus under anoxic further promoted eutrophication high water, which ultimately carbonate minerals.